Export of internal Abseil changes

--
f34cd235a12ad0ee1fea3a1ee5a427272dc2b285 by Abseil Team <absl-team@google.com>:

Migrates uses of deprecated map types to recommended types.

PiperOrigin-RevId: 309945156

--
e3410a47ad32c0775b6911610bc47b22938decad by Matthew Brown <matthewbr@google.com>:

Internal Change

PiperOrigin-RevId: 309856021

--
a58cfa25e0bb59e7fa9647ac1aae65eaccff0086 by Greg Falcon <gfalcon@google.com>:

Internal change.

PiperOrigin-RevId: 309804612

--
cdc5ec310035fbe25f496bda283fe655d94d7769 by Mark Barolak <mbar@google.com>:

Standardize the header comments for friend functions in cord.h

PiperOrigin-RevId: 309779073

--
fe61602701be795e54477b0fdbf5ffc1df12a6b7 by Samuel Benzaquen <sbenza@google.com>:

Implement %f natively for any input.
It evaluates the input at runtime and allocates stack space accordingly.

This removes a potential fallback into snprintf, improves performance, and removes all memory allocations in this formatting path.

PiperOrigin-RevId: 309752501

--
79e2a24f3f959e8b06ddf1d440bbabbd5f89b5b7 by Greg Falcon <gfalcon@google.com>:

Add a Cord::swap() method.  Many other Abseil types already provide this, but it was missing here.

We already provided a two-argument free function form of `swap()`, but that API is better suited for generic code.  The swap member function is a better API when the types are known.

PiperOrigin-RevId: 309751740

--
85cdf60024f153fb4fcb7fe68ed2b14b9faf119d by Derek Mauro <dmauro@google.com>:

Cleanup uses of "linker initialized" SpinLocks

PiperOrigin-RevId: 309581867

--
9e5443bfcec4b94056b13c75326576e987ab88fb by Matt Kulukundis <kfm@google.com>:

Clarify intended mixing properties of `absl::Hash`

PiperOrigin-RevId: 309520174

--
a0630f0827b67f217aaeae68a448fe4c1101e17d by Greg Falcon <gfalcon@google.com>:

Comment out a test in Emscripten to sidestep `long double` issues.

PiperOrigin-RevId: 309482953
GitOrigin-RevId: f34cd235a12ad0ee1fea3a1ee5a427272dc2b285
Change-Id: Icce0c9d547117374d596b9d684e4054ddd118669

11 files changed, 1014 insertions, 154 deletions

diff --git a/absl/strings/internal/str_format/arg.cc b/absl/strings/internal/str_format/arg.cc
index a112071c43a7..964f25f7da64 100644
--- a/absl/strings/internal/str_format/arg.cc
+++ b/absl/strings/internal/str_format/arg.cc
@@ -167,24 +167,26 @@ class IntDigits {
 // Note: 'o' conversions do not have a base indicator, it's just that
 // the '#' flag is specified to modify the precision for 'o' conversions.
 string_view BaseIndicator(const IntDigits &as_digits,
-                          const ConversionSpec conv) {
+                          const FormatConversionSpecImpl conv) {
   // always show 0x for %p.
-  bool alt = conv.has_alt_flag() || conv.conversion_char() == ConversionChar::p;
-  bool hex = (conv.conversion_char() == FormatConversionChar::x ||
-              conv.conversion_char() == FormatConversionChar::X ||
-              conv.conversion_char() == FormatConversionChar::p);
+  bool alt = conv.has_alt_flag() ||
+             conv.conversion_char() == FormatConversionCharInternal::p;
+  bool hex = (conv.conversion_char() == FormatConversionCharInternal::x ||
+              conv.conversion_char() == FormatConversionCharInternal::X ||
+              conv.conversion_char() == FormatConversionCharInternal::p);
   // From the POSIX description of '#' flag:
   //   "For x or X conversion specifiers, a non-zero result shall have
   //   0x (or 0X) prefixed to it."
   if (alt && hex && !as_digits.without_neg_or_zero().empty()) {
-    return conv.conversion_char() == FormatConversionChar::X ? "0X" : "0x";
+    return conv.conversion_char() == FormatConversionCharInternal::X ? "0X"
+                                                                     : "0x";
   }
   return {};
 }
 
-string_view SignColumn(bool neg, const ConversionSpec conv) {
-  if (conv.conversion_char() == FormatConversionChar::d ||
-      conv.conversion_char() == FormatConversionChar::i) {
+string_view SignColumn(bool neg, const FormatConversionSpecImpl conv) {
+  if (conv.conversion_char() == FormatConversionCharInternal::d ||
+      conv.conversion_char() == FormatConversionCharInternal::i) {
     if (neg) return "-";
     if (conv.has_show_pos_flag()) return "+";
     if (conv.has_sign_col_flag()) return " ";
@@ -192,7 +194,7 @@ string_view SignColumn(bool neg, const ConversionSpec conv) {
   return {};
 }
 
-bool ConvertCharImpl(unsigned char v, const ConversionSpec conv,
+bool ConvertCharImpl(unsigned char v, const FormatConversionSpecImpl conv,
                      FormatSinkImpl *sink) {
   size_t fill = 0;
   if (conv.width() >= 0) fill = conv.width();
@@ -204,7 +206,8 @@ bool ConvertCharImpl(unsigned char v, const ConversionSpec conv,
 }
 
 bool ConvertIntImplInnerSlow(const IntDigits &as_digits,
-                             const ConversionSpec conv, FormatSinkImpl *sink) {
+                             const FormatConversionSpecImpl conv,
+                             FormatSinkImpl *sink) {
   // Print as a sequence of Substrings:
   //   [left_spaces][sign][base_indicator][zeroes][formatted][right_spaces]
   size_t fill = 0;
@@ -224,7 +227,8 @@ bool ConvertIntImplInnerSlow(const IntDigits &as_digits,
   if (!precision_specified)
     precision = 1;
 
-  if (conv.has_alt_flag() && conv.conversion_char() == ConversionChar::o) {
+  if (conv.has_alt_flag() &&
+      conv.conversion_char() == FormatConversionCharInternal::o) {
     // From POSIX description of the '#' (alt) flag:
     //   "For o conversion, it increases the precision (if necessary) to
     //   force the first digit of the result to be zero."
@@ -258,42 +262,43 @@ bool ConvertIntImplInnerSlow(const IntDigits &as_digits,
 }
 
 template <typename T>
-bool ConvertIntArg(T v, const ConversionSpec conv, FormatSinkImpl *sink) {
+bool ConvertIntArg(T v, const FormatConversionSpecImpl conv,
+                   FormatSinkImpl *sink) {
   using U = typename MakeUnsigned<T>::type;
   IntDigits as_digits;
 
   switch (conv.conversion_char()) {
-    case FormatConversionChar::c:
+    case FormatConversionCharInternal::c:
       return ConvertCharImpl(static_cast<unsigned char>(v), conv, sink);
 
-    case FormatConversionChar::o:
+    case FormatConversionCharInternal::o:
       as_digits.PrintAsOct(static_cast<U>(v));
       break;
 
-    case FormatConversionChar::x:
+    case FormatConversionCharInternal::x:
       as_digits.PrintAsHexLower(static_cast<U>(v));
       break;
-    case FormatConversionChar::X:
+    case FormatConversionCharInternal::X:
       as_digits.PrintAsHexUpper(static_cast<U>(v));
       break;
 
-    case FormatConversionChar::u:
+    case FormatConversionCharInternal::u:
       as_digits.PrintAsDec(static_cast<U>(v));
       break;
 
-    case FormatConversionChar::d:
-    case FormatConversionChar::i:
+    case FormatConversionCharInternal::d:
+    case FormatConversionCharInternal::i:
       as_digits.PrintAsDec(v);
       break;
 
-    case FormatConversionChar::a:
-    case FormatConversionChar::e:
-    case FormatConversionChar::f:
-    case FormatConversionChar::g:
-    case FormatConversionChar::A:
-    case FormatConversionChar::E:
-    case FormatConversionChar::F:
-    case FormatConversionChar::G:
+    case FormatConversionCharInternal::a:
+    case FormatConversionCharInternal::e:
+    case FormatConversionCharInternal::f:
+    case FormatConversionCharInternal::g:
+    case FormatConversionCharInternal::A:
+    case FormatConversionCharInternal::E:
+    case FormatConversionCharInternal::F:
+    case FormatConversionCharInternal::G:
       return ConvertFloatImpl(static_cast<double>(v), conv, sink);
 
     default:
@@ -308,12 +313,13 @@ bool ConvertIntArg(T v, const ConversionSpec conv, FormatSinkImpl *sink) {
 }
 
 template <typename T>
-bool ConvertFloatArg(T v, const ConversionSpec conv, FormatSinkImpl *sink) {
+bool ConvertFloatArg(T v, const FormatConversionSpecImpl conv,
+                     FormatSinkImpl *sink) {
   return FormatConversionCharIsFloat(conv.conversion_char()) &&
          ConvertFloatImpl(v, conv, sink);
 }
 
-inline bool ConvertStringArg(string_view v, const ConversionSpec conv,
+inline bool ConvertStringArg(string_view v, const FormatConversionSpecImpl conv,
                              FormatSinkImpl *sink) {
   if (conv.conversion_char() != FormatConversionCharInternal::s) return false;
   if (conv.is_basic()) {
@@ -328,19 +334,20 @@ inline bool ConvertStringArg(string_view v, const ConversionSpec conv,
 
 // ==================== Strings ====================
 StringConvertResult FormatConvertImpl(const std::string &v,
-                                      const ConversionSpec conv,
+                                      const FormatConversionSpecImpl conv,
                                       FormatSinkImpl *sink) {
   return {ConvertStringArg(v, conv, sink)};
 }
 
-StringConvertResult FormatConvertImpl(string_view v, const ConversionSpec conv,
+StringConvertResult FormatConvertImpl(string_view v,
+                                      const FormatConversionSpecImpl conv,
                                       FormatSinkImpl *sink) {
   return {ConvertStringArg(v, conv, sink)};
 }
 
 ArgConvertResult<FormatConversionCharSetUnion(
     FormatConversionCharSetInternal::s, FormatConversionCharSetInternal::p)>
-FormatConvertImpl(const char *v, const ConversionSpec conv,
+FormatConvertImpl(const char *v, const FormatConversionSpecImpl conv,
                   FormatSinkImpl *sink) {
   if (conv.conversion_char() == FormatConversionCharInternal::p)
     return {FormatConvertImpl(VoidPtr(v), conv, sink).value};
@@ -358,7 +365,7 @@ FormatConvertImpl(const char *v, const ConversionSpec conv,
 
 // ==================== Raw pointers ====================
 ArgConvertResult<FormatConversionCharSetInternal::p> FormatConvertImpl(
-    VoidPtr v, const ConversionSpec conv, FormatSinkImpl *sink) {
+    VoidPtr v, const FormatConversionSpecImpl conv, FormatSinkImpl *sink) {
   if (conv.conversion_char() != FormatConversionCharInternal::p) return {false};
   if (!v.value) {
     sink->Append("(nil)");
@@ -370,82 +377,87 @@ ArgConvertResult<FormatConversionCharSetInternal::p> FormatConvertImpl(
 }
 
 // ==================== Floats ====================
-FloatingConvertResult FormatConvertImpl(float v, const ConversionSpec conv,
+FloatingConvertResult FormatConvertImpl(float v,
+                                        const FormatConversionSpecImpl conv,
                                         FormatSinkImpl *sink) {
   return {ConvertFloatArg(v, conv, sink)};
 }
-FloatingConvertResult FormatConvertImpl(double v, const ConversionSpec conv,
+FloatingConvertResult FormatConvertImpl(double v,
+                                        const FormatConversionSpecImpl conv,
                                         FormatSinkImpl *sink) {
   return {ConvertFloatArg(v, conv, sink)};
 }
 FloatingConvertResult FormatConvertImpl(long double v,
-                                        const ConversionSpec conv,
+                                        const FormatConversionSpecImpl conv,
                                         FormatSinkImpl *sink) {
   return {ConvertFloatArg(v, conv, sink)};
 }
 
 // ==================== Chars ====================
-IntegralConvertResult FormatConvertImpl(char v, const ConversionSpec conv,
+IntegralConvertResult FormatConvertImpl(char v,
+                                        const FormatConversionSpecImpl conv,
                                         FormatSinkImpl *sink) {
   return {ConvertIntArg(v, conv, sink)};
 }
 IntegralConvertResult FormatConvertImpl(signed char v,
-                                        const ConversionSpec conv,
+                                        const FormatConversionSpecImpl conv,
                                         FormatSinkImpl *sink) {
   return {ConvertIntArg(v, conv, sink)};
 }
 IntegralConvertResult FormatConvertImpl(unsigned char v,
-                                        const ConversionSpec conv,
+                                        const FormatConversionSpecImpl conv,
                                         FormatSinkImpl *sink) {
   return {ConvertIntArg(v, conv, sink)};
 }
 
 // ==================== Ints ====================
 IntegralConvertResult FormatConvertImpl(short v,  // NOLINT
-                                        const ConversionSpec conv,
+                                        const FormatConversionSpecImpl conv,
                                         FormatSinkImpl *sink) {
   return {ConvertIntArg(v, conv, sink)};
 }
 IntegralConvertResult FormatConvertImpl(unsigned short v,  // NOLINT
-                                        const ConversionSpec conv,
+                                        const FormatConversionSpecImpl conv,
                                         FormatSinkImpl *sink) {
   return {ConvertIntArg(v, conv, sink)};
 }
-IntegralConvertResult FormatConvertImpl(int v, const ConversionSpec conv,
+IntegralConvertResult FormatConvertImpl(int v,
+                                        const FormatConversionSpecImpl conv,
                                         FormatSinkImpl *sink) {
   return {ConvertIntArg(v, conv, sink)};
 }
-IntegralConvertResult FormatConvertImpl(unsigned v, const ConversionSpec conv,
+IntegralConvertResult FormatConvertImpl(unsigned v,
+                                        const FormatConversionSpecImpl conv,
                                         FormatSinkImpl *sink) {
   return {ConvertIntArg(v, conv, sink)};
 }
 IntegralConvertResult FormatConvertImpl(long v,  // NOLINT
-                                        const ConversionSpec conv,
+                                        const FormatConversionSpecImpl conv,
                                         FormatSinkImpl *sink) {
   return {ConvertIntArg(v, conv, sink)};
 }
 IntegralConvertResult FormatConvertImpl(unsigned long v,  // NOLINT
-                                        const ConversionSpec conv,
+                                        const FormatConversionSpecImpl conv,
                                         FormatSinkImpl *sink) {
   return {ConvertIntArg(v, conv, sink)};
 }
 IntegralConvertResult FormatConvertImpl(long long v,  // NOLINT
-                                        const ConversionSpec conv,
+                                        const FormatConversionSpecImpl conv,
                                         FormatSinkImpl *sink) {
   return {ConvertIntArg(v, conv, sink)};
 }
 IntegralConvertResult FormatConvertImpl(unsigned long long v,  // NOLINT
-                                        const ConversionSpec conv,
+                                        const FormatConversionSpecImpl conv,
                                         FormatSinkImpl *sink) {
   return {ConvertIntArg(v, conv, sink)};
 }
 IntegralConvertResult FormatConvertImpl(absl::int128 v,
-                                        const ConversionSpec conv,
+                                        const FormatConversionSpecImpl conv,
                                         FormatSinkImpl *sink) {
   return {ConvertIntArg(v, conv, sink)};
 }
 IntegralConvertResult FormatConvertImpl(absl::uint128 v,
-                                        const ConversionSpec conv,
+                                        const FormatConversionSpecImpl conv,
                                         FormatSinkImpl *sink) {
   return {ConvertIntArg(v, conv, sink)};
 }
diff --git a/absl/strings/internal/str_format/arg.h b/absl/strings/internal/str_format/arg.h
index f4ac940ab43f..9a1e5ef276e8 100644
--- a/absl/strings/internal/str_format/arg.h
+++ b/absl/strings/internal/str_format/arg.h
@@ -67,20 +67,24 @@ constexpr FormatConversionCharSet ExtractCharSet(ArgConvertResult<C>) {
 using StringConvertResult =
     ArgConvertResult<FormatConversionCharSetInternal::s>;
 ArgConvertResult<FormatConversionCharSetInternal::p> FormatConvertImpl(
-    VoidPtr v, ConversionSpec conv, FormatSinkImpl* sink);
+    VoidPtr v, FormatConversionSpecImpl conv, FormatSinkImpl* sink);
 
 // Strings.
-StringConvertResult FormatConvertImpl(const std::string& v, ConversionSpec conv,
+StringConvertResult FormatConvertImpl(const std::string& v,
+                                      FormatConversionSpecImpl conv,
                                       FormatSinkImpl* sink);
-StringConvertResult FormatConvertImpl(string_view v, ConversionSpec conv,
+StringConvertResult FormatConvertImpl(string_view v,
+                                      FormatConversionSpecImpl conv,
                                       FormatSinkImpl* sink);
 ArgConvertResult<FormatConversionCharSetUnion(
     FormatConversionCharSetInternal::s, FormatConversionCharSetInternal::p)>
-FormatConvertImpl(const char* v, ConversionSpec conv, FormatSinkImpl* sink);
+FormatConvertImpl(const char* v, const FormatConversionSpecImpl conv,
+                  FormatSinkImpl* sink);
+
 template <class AbslCord, typename std::enable_if<std::is_same<
                               AbslCord, absl::Cord>::value>::type* = nullptr>
 StringConvertResult FormatConvertImpl(const AbslCord& value,
-                                      ConversionSpec conv,
+                                      FormatConversionSpecImpl conv,
                                       FormatSinkImpl* sink) {
   if (conv.conversion_char() != FormatConversionCharInternal::s) {
     return {false};
@@ -127,50 +131,55 @@ using FloatingConvertResult =
     ArgConvertResult<FormatConversionCharSetInternal::kFloating>;
 
 // Floats.
-FloatingConvertResult FormatConvertImpl(float v, ConversionSpec conv,
+FloatingConvertResult FormatConvertImpl(float v, FormatConversionSpecImpl conv,
                                         FormatSinkImpl* sink);
-FloatingConvertResult FormatConvertImpl(double v, ConversionSpec conv,
+FloatingConvertResult FormatConvertImpl(double v, FormatConversionSpecImpl conv,
                                         FormatSinkImpl* sink);
-FloatingConvertResult FormatConvertImpl(long double v, ConversionSpec conv,
+FloatingConvertResult FormatConvertImpl(long double v,
+                                        FormatConversionSpecImpl conv,
                                         FormatSinkImpl* sink);
 
 // Chars.
-IntegralConvertResult FormatConvertImpl(char v, ConversionSpec conv,
+IntegralConvertResult FormatConvertImpl(char v, FormatConversionSpecImpl conv,
                                         FormatSinkImpl* sink);
-IntegralConvertResult FormatConvertImpl(signed char v, ConversionSpec conv,
+IntegralConvertResult FormatConvertImpl(signed char v,
+                                        FormatConversionSpecImpl conv,
                                         FormatSinkImpl* sink);
-IntegralConvertResult FormatConvertImpl(unsigned char v, ConversionSpec conv,
+IntegralConvertResult FormatConvertImpl(unsigned char v,
+                                        FormatConversionSpecImpl conv,
                                         FormatSinkImpl* sink);
 
 // Ints.
 IntegralConvertResult FormatConvertImpl(short v,  // NOLINT
-                                        ConversionSpec conv,
+                                        FormatConversionSpecImpl conv,
                                         FormatSinkImpl* sink);
 IntegralConvertResult FormatConvertImpl(unsigned short v,  // NOLINT
-                                        ConversionSpec conv,
+                                        FormatConversionSpecImpl conv,
                                         FormatSinkImpl* sink);
-IntegralConvertResult FormatConvertImpl(int v, ConversionSpec conv,
+IntegralConvertResult FormatConvertImpl(int v, FormatConversionSpecImpl conv,
                                         FormatSinkImpl* sink);
-IntegralConvertResult FormatConvertImpl(unsigned v, ConversionSpec conv,
+IntegralConvertResult FormatConvertImpl(unsigned v,
+                                        FormatConversionSpecImpl conv,
                                         FormatSinkImpl* sink);
 IntegralConvertResult FormatConvertImpl(long v,  // NOLINT
-                                        ConversionSpec conv,
+                                        FormatConversionSpecImpl conv,
                                         FormatSinkImpl* sink);
 IntegralConvertResult FormatConvertImpl(unsigned long v,  // NOLINT
-                                        ConversionSpec conv,
+                                        FormatConversionSpecImpl conv,
                                         FormatSinkImpl* sink);
 IntegralConvertResult FormatConvertImpl(long long v,  // NOLINT
-                                        ConversionSpec conv,
+                                        FormatConversionSpecImpl conv,
                                         FormatSinkImpl* sink);
 IntegralConvertResult FormatConvertImpl(unsigned long long v,  // NOLINT
-                                        ConversionSpec conv,
+                                        FormatConversionSpecImpl conv,
                                         FormatSinkImpl* sink);
-IntegralConvertResult FormatConvertImpl(int128 v, ConversionSpec conv,
+IntegralConvertResult FormatConvertImpl(int128 v, FormatConversionSpecImpl conv,
                                         FormatSinkImpl* sink);
-IntegralConvertResult FormatConvertImpl(uint128 v, ConversionSpec conv,
+IntegralConvertResult FormatConvertImpl(uint128 v,
+                                        FormatConversionSpecImpl conv,
                                         FormatSinkImpl* sink);
 template <typename T, enable_if_t<std::is_same<T, bool>::value, int> = 0>
-IntegralConvertResult FormatConvertImpl(T v, ConversionSpec conv,
+IntegralConvertResult FormatConvertImpl(T v, FormatConversionSpecImpl conv,
                                         FormatSinkImpl* sink) {
   return FormatConvertImpl(static_cast<int>(v), conv, sink);
 }
@@ -181,11 +190,11 @@ template <typename T>
 typename std::enable_if<std::is_enum<T>::value &&
                             !HasUserDefinedConvert<T>::value,
                         IntegralConvertResult>::type
-FormatConvertImpl(T v, ConversionSpec conv, FormatSinkImpl* sink);
+FormatConvertImpl(T v, FormatConversionSpecImpl conv, FormatSinkImpl* sink);
 
 template <typename T>
 StringConvertResult FormatConvertImpl(const StreamedWrapper<T>& v,
-                                      ConversionSpec conv,
+                                      FormatConversionSpecImpl conv,
                                       FormatSinkImpl* out) {
   std::ostringstream oss;
   oss << v.v_;
@@ -198,7 +207,8 @@ StringConvertResult FormatConvertImpl(const StreamedWrapper<T>& v,
 struct FormatCountCaptureHelper {
   template <class T = int>
   static ArgConvertResult<FormatConversionCharSetInternal::n> ConvertHelper(
-      const FormatCountCapture& v, ConversionSpec conv, FormatSinkImpl* sink) {
+      const FormatCountCapture& v, FormatConversionSpecImpl conv,
+      FormatSinkImpl* sink) {
     const absl::enable_if_t<sizeof(T) != 0, FormatCountCapture>& v2 = v;
 
     if (conv.conversion_char() !=
@@ -212,7 +222,8 @@ struct FormatCountCaptureHelper {
 
 template <class T = int>
 ArgConvertResult<FormatConversionCharSetInternal::n> FormatConvertImpl(
-    const FormatCountCapture& v, ConversionSpec conv, FormatSinkImpl* sink) {
+    const FormatCountCapture& v, FormatConversionSpecImpl conv,
+    FormatSinkImpl* sink) {
   return FormatCountCaptureHelper::ConvertHelper(v, conv, sink);
 }
 
@@ -221,13 +232,13 @@ ArgConvertResult<FormatConversionCharSetInternal::n> FormatConvertImpl(
 struct FormatArgImplFriend {
   template <typename Arg>
   static bool ToInt(Arg arg, int* out) {
-    // A value initialized ConversionSpec has a `none` conv, which tells the
-    // dispatcher to run the `int` conversion.
+    // A value initialized FormatConversionSpecImpl has a `none` conv, which
+    // tells the dispatcher to run the `int` conversion.
     return arg.dispatcher_(arg.data_, {}, out);
   }
 
   template <typename Arg>
-  static bool Convert(Arg arg, str_format_internal::ConversionSpec conv,
+  static bool Convert(Arg arg, FormatConversionSpecImpl conv,
                       FormatSinkImpl* out) {
     return arg.dispatcher_(arg.data_, conv, out);
   }
@@ -251,7 +262,7 @@ class FormatArgImpl {
     char buf[kInlinedSpace];
   };
 
-  using Dispatcher = bool (*)(Data, ConversionSpec, void* out);
+  using Dispatcher = bool (*)(Data, FormatConversionSpecImpl, void* out);
 
   template <typename T>
   struct store_by_value
@@ -393,7 +404,7 @@ class FormatArgImpl {
   }
 
   template <typename T>
-  static bool Dispatch(Data arg, ConversionSpec spec, void* out) {
+  static bool Dispatch(Data arg, FormatConversionSpecImpl spec, void* out) {
     // A `none` conv indicates that we want the `int` conversion.
     if (ABSL_PREDICT_FALSE(spec.conversion_char() ==
                            FormatConversionCharInternal::kNone)) {
@@ -410,8 +421,9 @@ class FormatArgImpl {
   Dispatcher dispatcher_;
 };
 
-#define ABSL_INTERNAL_FORMAT_DISPATCH_INSTANTIATE_(T, E) \
-  E template bool FormatArgImpl::Dispatch<T>(Data, ConversionSpec, void*)
+#define ABSL_INTERNAL_FORMAT_DISPATCH_INSTANTIATE_(T, E)                     \
+  E template bool FormatArgImpl::Dispatch<T>(Data, FormatConversionSpecImpl, \
+                                             void*)
 
 #define ABSL_INTERNAL_FORMAT_DISPATCH_OVERLOADS_EXPAND_(...)                   \
   ABSL_INTERNAL_FORMAT_DISPATCH_INSTANTIATE_(str_format_internal::VoidPtr,     \
diff --git a/absl/strings/internal/str_format/arg_test.cc b/absl/strings/internal/str_format/arg_test.cc
index 8d30d8b8ce34..37e5b7545f84 100644
--- a/absl/strings/internal/str_format/arg_test.cc
+++ b/absl/strings/internal/str_format/arg_test.cc
@@ -95,8 +95,9 @@ TEST_F(FormatArgImplTest, OtherPtrDecayToVoidPtr) {
 TEST_F(FormatArgImplTest, WorksWithCharArraysOfUnknownSize) {
   std::string s;
   FormatSinkImpl sink(&s);
-  ConversionSpec conv;
-  FormatConversionSpecImplFriend::SetConversionChar(ConversionChar::s, &conv);
+  FormatConversionSpecImpl conv;
+  FormatConversionSpecImplFriend::SetConversionChar(FormatConversionChar::s,
+                                                    &conv);
   FormatConversionSpecImplFriend::SetFlags(Flags(), &conv);
   FormatConversionSpecImplFriend::SetWidth(-1, &conv);
   FormatConversionSpecImplFriend::SetPrecision(-1, &conv);
diff --git a/absl/strings/internal/str_format/bind.h b/absl/strings/internal/str_format/bind.h
index 05105d8d5c9c..585246e77e56 100644
--- a/absl/strings/internal/str_format/bind.h
+++ b/absl/strings/internal/str_format/bind.h
@@ -19,7 +19,7 @@ class UntypedFormatSpec;
 
 namespace str_format_internal {
 
-class BoundConversion : public ConversionSpec {
+class BoundConversion : public FormatConversionSpecImpl {
  public:
   const FormatArgImpl* arg() const { return arg_; }
   void set_arg(const FormatArgImpl* a) { arg_ = a; }
@@ -119,7 +119,7 @@ class FormatSpecTemplate
 
 #endif  // ABSL_INTERNAL_ENABLE_FORMAT_CHECKER
 
-  template <Conv... C,
+  template <FormatConversionCharSet... C,
             typename = typename std::enable_if<
                 AllOf(sizeof...(C) == sizeof...(Args), Contains(Args,
                                                                 C)...)>::type>
@@ -190,7 +190,8 @@ class StreamedWrapper {
  private:
   template <typename S>
   friend ArgConvertResult<FormatConversionCharSetInternal::s> FormatConvertImpl(
-      const StreamedWrapper<S>& v, ConversionSpec conv, FormatSinkImpl* out);
+      const StreamedWrapper<S>& v, FormatConversionSpecImpl conv,
+      FormatSinkImpl* out);
   const T& v_;
 };
 
diff --git a/absl/strings/internal/str_format/checker_test.cc b/absl/strings/internal/str_format/checker_test.cc
index 49a24b4026c3..233481747bfd 100644
--- a/absl/strings/internal/str_format/checker_test.cc
+++ b/absl/strings/internal/str_format/checker_test.cc
@@ -24,7 +24,7 @@ std::string ConvToString(FormatConversionCharSet conv) {
 }
 
 TEST(StrFormatChecker, ArgumentToConv) {
-  Conv conv = ArgumentToConv<std::string>();
+  FormatConversionCharSet conv = ArgumentToConv<std::string>();
   EXPECT_EQ(ConvToString(conv), "s");
 
   conv = ArgumentToConv<const char*>();
diff --git a/absl/strings/internal/str_format/convert_test.cc b/absl/strings/internal/str_format/convert_test.cc
index cbcd7caf4665..dd167f76e68f 100644
--- a/absl/strings/internal/str_format/convert_test.cc
+++ b/absl/strings/internal/str_format/convert_test.cc
@@ -1,14 +1,18 @@
 #include <errno.h>
 #include <stdarg.h>
 #include <stdio.h>
+
 #include <cctype>
 #include <cmath>
+#include <limits>
 #include <string>
+#include <thread>  // NOLINT
 
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
 #include "absl/base/internal/raw_logging.h"
 #include "absl/strings/internal/str_format/bind.h"
+#include "absl/types/optional.h"
 
 namespace absl {
 ABSL_NAMESPACE_BEGIN
@@ -57,7 +61,7 @@ std::string Esc(const T &v) {
   return oss.str();
 }
 
-void StrAppend(std::string *dst, const char *format, va_list ap) {
+void StrAppendV(std::string *dst, const char *format, va_list ap) {
   // First try with a small fixed size buffer
   static const int kSpaceLength = 1024;
   char space[kSpaceLength];
@@ -98,11 +102,18 @@ void StrAppend(std::string *dst, const char *format, va_list ap) {
   delete[] buf;
 }
 
+void StrAppend(std::string *out, const char *format, ...) {
+  va_list ap;
+  va_start(ap, format);
+  StrAppendV(out, format, ap);
+  va_end(ap);
+}
+
 std::string StrPrint(const char *format, ...) {
   va_list ap;
   va_start(ap, format);
   std::string result;
-  StrAppend(&result, format, ap);
+  StrAppendV(&result, format, ap);
   va_end(ap);
   return result;
 }
@@ -471,8 +482,8 @@ TEST_F(FormatConvertTest, Float) {
 #endif  // _MSC_VER
 
   const char *const kFormats[] = {
-      "%",  "%.3",  "%8.5",   "%9",   "%.60", "%.30",   "%03",    "%+",
-      "% ", "%-10", "%#15.3", "%#.0", "%.0",  "%1$*2$", "%1$.*2$"};
+      "%",  "%.3", "%8.5", "%500",   "%.5000", "%.60", "%.30",   "%03",
+      "%+", "% ",  "%-10", "%#15.3", "%#.0",   "%.0",  "%1$*2$", "%1$.*2$"};
 
   std::vector<double> doubles = {0.0,
                                  -0.0,
@@ -489,11 +500,6 @@ TEST_F(FormatConvertTest, Float) {
                                  std::numeric_limits<double>::infinity(),
                                  -std::numeric_limits<double>::infinity()};
 
-#ifndef __APPLE__
-  // Apple formats NaN differently (+nan) vs. (nan)
-  doubles.push_back(std::nan(""));
-#endif
-
   // Some regression tests.
   doubles.push_back(0.99999999999999989);
 
@@ -512,43 +518,204 @@ TEST_F(FormatConvertTest, Float) {
     }
   }
 
+  // Workaround libc bug.
+  // https://sourceware.org/bugzilla/show_bug.cgi?id=22142
+  const bool gcc_bug_22142 =
+      StrPrint("%f", std::numeric_limits<double>::max()) !=
+      "1797693134862315708145274237317043567980705675258449965989174768031"
+      "5726078002853876058955863276687817154045895351438246423432132688946"
+      "4182768467546703537516986049910576551282076245490090389328944075868"
+      "5084551339423045832369032229481658085593321233482747978262041447231"
+      "68738177180919299881250404026184124858368.000000";
+
+  if (!gcc_bug_22142) {
+    for (int exp = -300; exp <= 300; ++exp) {
+      const double all_ones_mantissa = 0x1fffffffffffff;
+      doubles.push_back(std::ldexp(all_ones_mantissa, exp));
+    }
+  }
+
+  if (gcc_bug_22142) {
+    for (auto &d : doubles) {
+      using L = std::numeric_limits<double>;
+      double d2 = std::abs(d);
+      if (d2 == L::max() || d2 == L::min() || d2 == L::denorm_min()) {
+        d = 0;
+      }
+    }
+  }
+
+  // Remove duplicates to speed up the logic below.
+  std::sort(doubles.begin(), doubles.end());
+  doubles.erase(std::unique(doubles.begin(), doubles.end()), doubles.end());
+
+#ifndef __APPLE__
+  // Apple formats NaN differently (+nan) vs. (nan)
+  doubles.push_back(std::nan(""));
+#endif
+
+  // Reserve the space to ensure we don't allocate memory in the output itself.
+  std::string str_format_result;
+  str_format_result.reserve(1 << 20);
+  std::string string_printf_result;
+  string_printf_result.reserve(1 << 20);
+
   for (const char *fmt : kFormats) {
     for (char f : {'f', 'F',  //
                    'g', 'G',  //
                    'a', 'A',  //
                    'e', 'E'}) {
       std::string fmt_str = std::string(fmt) + f;
+
+      if (fmt == absl::string_view("%.5000") && f != 'f' && f != 'F') {
+        // This particular test takes way too long with snprintf.
+        // Disable for the case we are not implementing natively.
+        continue;
+      }
+
       for (double d : doubles) {
         int i = -10;
         FormatArgImpl args[2] = {FormatArgImpl(d), FormatArgImpl(i)};
         UntypedFormatSpecImpl format(fmt_str);
-        // We use ASSERT_EQ here because failures are usually correlated and a
-        // bug would print way too many failed expectations causing the test to
-        // time out.
-        ASSERT_EQ(StrPrint(fmt_str.c_str(), d, i),
-                  FormatPack(format, absl::MakeSpan(args)))
-            << fmt_str << " " << StrPrint("%.18g", d) << " "
-            << StrPrint("%.999f", d);
+
+        string_printf_result.clear();
+        StrAppend(&string_printf_result, fmt_str.c_str(), d, i);
+        str_format_result.clear();
+
+        {
+          AppendPack(&str_format_result, format, absl::MakeSpan(args));
+        }
+
+        if (string_printf_result != str_format_result) {
+          // We use ASSERT_EQ here because failures are usually correlated and a
+          // bug would print way too many failed expectations causing the test
+          // to time out.
+          ASSERT_EQ(string_printf_result, str_format_result)
+              << fmt_str << " " << StrPrint("%.18g", d) << " "
+              << StrPrint("%a", d) << " " << StrPrint("%.1080f", d);
+        }
       }
     }
   }
 }
 
+TEST_F(FormatConvertTest, FloatRound) {
+  std::string s;
+  const auto format = [&](const char *fmt, double d) -> std::string & {
+    s.clear();
+    FormatArgImpl args[1] = {FormatArgImpl(d)};
+    AppendPack(&s, UntypedFormatSpecImpl(fmt), absl::MakeSpan(args));
+#if !defined(_MSC_VER)
+    // MSVC has a different rounding policy than us so we can't test our
+    // implementation against the native one there.
+    EXPECT_EQ(StrPrint(fmt, d), s);
+#endif  // _MSC_VER
+
+    return s;
+  };
+  // All of these values have to be exactly represented.
+  // Otherwise we might not be testing what we think we are testing.
+
+  // These values can fit in a 64bit "fast" representation.
+  const double exact_value = 0.00000000000005684341886080801486968994140625;
+  assert(exact_value == std::pow(2, -44));
+  // Round up at a 5xx.
+  EXPECT_EQ(format("%.13f", exact_value), "0.0000000000001");
+  // Round up at a >5
+  EXPECT_EQ(format("%.14f", exact_value), "0.00000000000006");
+  // Round down at a <5
+  EXPECT_EQ(format("%.16f", exact_value), "0.0000000000000568");
+  // Nine handling
+  EXPECT_EQ(format("%.35f", exact_value),
+            "0.00000000000005684341886080801486969");
+  EXPECT_EQ(format("%.36f", exact_value),
+            "0.000000000000056843418860808014869690");
+  // Round down the last nine.
+  EXPECT_EQ(format("%.37f", exact_value),
+            "0.0000000000000568434188608080148696899");
+  EXPECT_EQ(format("%.10f", 0.000003814697265625), "0.0000038147");
+  // Round up the last nine
+  EXPECT_EQ(format("%.11f", 0.000003814697265625), "0.00000381470");
+  EXPECT_EQ(format("%.12f", 0.000003814697265625), "0.000003814697");
+
+  // Round to even (down)
+  EXPECT_EQ(format("%.43f", exact_value),
+            "0.0000000000000568434188608080148696899414062");
+  // Exact
+  EXPECT_EQ(format("%.44f", exact_value),
+            "0.00000000000005684341886080801486968994140625");
+  // Round to even (up), let make the last digits 75 instead of 25
+  EXPECT_EQ(format("%.43f", exact_value + std::pow(2, -43)),
+            "0.0000000000001705302565824240446090698242188");
+  // Exact, just to check.
+  EXPECT_EQ(format("%.44f", exact_value + std::pow(2, -43)),
+            "0.00000000000017053025658242404460906982421875");
+
+  // This value has to be small enough that it won't fit in the uint128
+  // representation for printing.
+  const double small_exact_value =
+      0.000000000000000000000000000000000000752316384526264005099991383822237233803945956334136013765601092018187046051025390625;  // NOLINT
+  assert(small_exact_value == std::pow(2, -120));
+  // Round up at a 5xx.
+  EXPECT_EQ(format("%.37f", small_exact_value),
+            "0.0000000000000000000000000000000000008");
+  // Round down at a <5
+  EXPECT_EQ(format("%.38f", small_exact_value),
+            "0.00000000000000000000000000000000000075");
+  // Round up at a >5
+  EXPECT_EQ(format("%.41f", small_exact_value),
+            "0.00000000000000000000000000000000000075232");
+  // Nine handling
+  EXPECT_EQ(format("%.55f", small_exact_value),
+            "0.0000000000000000000000000000000000007523163845262640051");
+  EXPECT_EQ(format("%.56f", small_exact_value),
+            "0.00000000000000000000000000000000000075231638452626400510");
+  EXPECT_EQ(format("%.57f", small_exact_value),
+            "0.000000000000000000000000000000000000752316384526264005100");
+  EXPECT_EQ(format("%.58f", small_exact_value),
+            "0.0000000000000000000000000000000000007523163845262640051000");
+  // Round down the last nine
+  EXPECT_EQ(format("%.59f", small_exact_value),
+            "0.00000000000000000000000000000000000075231638452626400509999");
+  // Round up the last nine
+  EXPECT_EQ(format("%.79f", small_exact_value),
+            "0.000000000000000000000000000000000000"
+            "7523163845262640050999913838222372338039460");
+
+  // Round to even (down)
+  EXPECT_EQ(format("%.119f", small_exact_value),
+            "0.000000000000000000000000000000000000"
+            "75231638452626400509999138382223723380"
+            "394595633413601376560109201818704605102539062");
+  // Exact
+  EXPECT_EQ(format("%.120f", small_exact_value),
+            "0.000000000000000000000000000000000000"
+            "75231638452626400509999138382223723380"
+            "3945956334136013765601092018187046051025390625");
+  // Round to even (up), let make the last digits 75 instead of 25
+  EXPECT_EQ(format("%.119f", small_exact_value + std::pow(2, -119)),
+            "0.000000000000000000000000000000000002"
+            "25694915357879201529997415146671170141"
+            "183786900240804129680327605456113815307617188");
+  // Exact, just to check.
+  EXPECT_EQ(format("%.120f", small_exact_value + std::pow(2, -119)),
+            "0.000000000000000000000000000000000002"
+            "25694915357879201529997415146671170141"
+            "1837869002408041296803276054561138153076171875");
+}
+
 TEST_F(FormatConvertTest, LongDouble) {
-  const char *const kFormats[] = {"%",    "%.3", "%8.5", "%9",
+#ifdef _MSC_VER
+  // MSVC has a different rounding policy than us so we can't test our
+  // implementation against the native one there.
+  return;
+#endif  // _MSC_VER
+  const char *const kFormats[] = {"%",    "%.3", "%8.5", "%9",  "%.5000",
                                   "%.60", "%+",  "% ",   "%-10"};
 
-  // This value is not representable in double, but it is in long double that
-  // uses the extended format.
-  // This is to verify that we are not truncating the value mistakenly through a
-  // double.
-  long double very_precise = 10000000000000000.25L;
-
   std::vector<long double> doubles = {
       0.0,
       -0.0,
-      very_precise,
-      1 / very_precise,
       std::numeric_limits<long double>::max(),
       -std::numeric_limits<long double>::max(),
       std::numeric_limits<long double>::min(),
@@ -556,22 +723,44 @@ TEST_F(FormatConvertTest, LongDouble) {
       std::numeric_limits<long double>::infinity(),
       -std::numeric_limits<long double>::infinity()};
 
+  for (long double base : {1.L, 12.L, 123.L, 1234.L, 12345.L, 123456.L,
+                           1234567.L, 12345678.L, 123456789.L, 1234567890.L,
+                           12345678901.L, 123456789012.L, 1234567890123.L,
+                           // This value is not representable in double, but it
+                           // is in long double that uses the extended format.
+                           // This is to verify that we are not truncating the
+                           // value mistakenly through a double.
+                           10000000000000000.25L}) {
+    for (int exp : {-1000, -500, 0, 500, 1000}) {
+      for (int sign : {1, -1}) {
+        doubles.push_back(sign * std::ldexp(base, exp));
+        doubles.push_back(sign / std::ldexp(base, exp));
+      }
+    }
+  }
+
   for (const char *fmt : kFormats) {
     for (char f : {'f', 'F',  //
                    'g', 'G',  //
                    'a', 'A',  //
                    'e', 'E'}) {
       std::string fmt_str = std::string(fmt) + 'L' + f;
+
+      if (fmt == absl::string_view("%.5000") && f != 'f' && f != 'F') {
+        // This particular test takes way too long with snprintf.
+        // Disable for the case we are not implementing natively.
+        continue;
+      }
+
       for (auto d : doubles) {
         FormatArgImpl arg(d);
         UntypedFormatSpecImpl format(fmt_str);
         // We use ASSERT_EQ here because failures are usually correlated and a
         // bug would print way too many failed expectations causing the test to
         // time out.
-        ASSERT_EQ(StrPrint(fmt_str.c_str(), d),
-                  FormatPack(format, {&arg, 1}))
+        ASSERT_EQ(StrPrint(fmt_str.c_str(), d), FormatPack(format, {&arg, 1}))
             << fmt_str << " " << StrPrint("%.18Lg", d) << " "
-            << StrPrint("%.999Lf", d);
+            << StrPrint("%La", d) << " " << StrPrint("%.1080Lf", d);
       }
     }
   }
diff --git a/absl/strings/internal/str_format/extension.h b/absl/strings/internal/str_format/extension.h
index f0cffe1e6d6c..33903df0be39 100644
--- a/absl/strings/internal/str_format/extension.h
+++ b/absl/strings/internal/str_format/extension.h
@@ -411,11 +411,6 @@ inline size_t Excess(size_t used, size_t capacity) {
   return used < capacity ? capacity - used : 0;
 }
 
-// Type alias for use during migration.
-using ConversionChar = FormatConversionChar;
-using ConversionSpec = FormatConversionSpecImpl;
-using Conv = FormatConversionCharSet;
-
 class FormatConversionSpec {
  public:
   // Width and precison are not specified, no flags are set.
diff --git a/absl/strings/internal/str_format/float_conversion.cc b/absl/strings/internal/str_format/float_conversion.cc
index d6858cfffd95..cdccc86f551c 100644
--- a/absl/strings/internal/str_format/float_conversion.cc
+++ b/absl/strings/internal/str_format/float_conversion.cc
@@ -1,12 +1,22 @@
 #include "absl/strings/internal/str_format/float_conversion.h"
 
 #include <string.h>
+
 #include <algorithm>
 #include <cassert>
 #include <cmath>
+#include <limits>
 #include <string>
 
+#include "absl/base/attributes.h"
 #include "absl/base/config.h"
+#include "absl/base/internal/bits.h"
+#include "absl/base/optimization.h"
+#include "absl/functional/function_ref.h"
+#include "absl/meta/type_traits.h"
+#include "absl/numeric/int128.h"
+#include "absl/types/optional.h"
+#include "absl/types/span.h"
 
 namespace absl {
 ABSL_NAMESPACE_BEGIN
@@ -14,13 +24,640 @@ namespace str_format_internal {
 
 namespace {
 
-char *CopyStringTo(string_view v, char *out) {
+// The code below wants to avoid heap allocations.
+// To do so it needs to allocate memory on the stack.
+// `StackArray` will allocate memory on the stack in the form of a uint32_t
+// array and call the provided callback with said memory.
+// It will allocate memory in increments of 512 bytes. We could allocate the
+// largest needed unconditionally, but that is more than we need in most of
+// cases. This way we use less stack in the common cases.
+class StackArray {
+  using Func = absl::FunctionRef<void(absl::Span<uint32_t>)>;
+  static constexpr size_t kStep = 512 / sizeof(uint32_t);
+  // 5 steps is 2560 bytes, which is enough to hold a long double with the
+  // largest/smallest exponents.
+  // The operations below will static_assert their particular maximum.
+  static constexpr size_t kNumSteps = 5;
+
+  // We do not want this function to be inlined.
+  // Otherwise the caller will allocate the stack space unnecessarily for all
+  // the variants even though it only calls one.
+  template <size_t steps>
+  ABSL_ATTRIBUTE_NOINLINE static void RunWithCapacityImpl(Func f) {
+    uint32_t values[steps * kStep]{};
+    f(absl::MakeSpan(values));
+  }
+
+ public:
+  static constexpr size_t kMaxCapacity = kStep * kNumSteps;
+
+  static void RunWithCapacity(size_t capacity, Func f) {
+    assert(capacity <= kMaxCapacity);
+    const size_t step = (capacity + kStep - 1) / kStep;
+    assert(step <= kNumSteps);
+    switch (step) {
+      case 1:
+        return RunWithCapacityImpl<1>(f);
+      case 2:
+        return RunWithCapacityImpl<2>(f);
+      case 3:
+        return RunWithCapacityImpl<3>(f);
+      case 4:
+        return RunWithCapacityImpl<4>(f);
+      case 5:
+        return RunWithCapacityImpl<5>(f);
+    }
+
+    assert(false && "Invalid capacity");
+  }
+};
+
+// Calculates `10 * (*v) + carry` and stores the result in `*v` and returns
+// the carry.
+template <typename Int>
+inline Int MultiplyBy10WithCarry(Int *v, Int carry) {
+  using BiggerInt = absl::conditional_t<sizeof(Int) == 4, uint64_t, uint128>;
+  BiggerInt tmp = 10 * static_cast<BiggerInt>(*v) + carry;
+  *v = static_cast<Int>(tmp);
+  return static_cast<Int>(tmp >> (sizeof(Int) * 8));
+}
+
+// Calculates `(2^64 * carry + *v) / 10`.
+// Stores the quotient in `*v` and returns the remainder.
+// Requires: `0 <= carry <= 9`
+inline uint64_t DivideBy10WithCarry(uint64_t *v, uint64_t carry) {
+  constexpr uint64_t divisor = 10;
+  // 2^64 / divisor = chunk_quotient + chunk_remainder / divisor
+  constexpr uint64_t chunk_quotient = (uint64_t{1} << 63) / (divisor / 2);
+  constexpr uint64_t chunk_remainder = uint64_t{} - chunk_quotient * divisor;
+
+  const uint64_t mod = *v % divisor;
+  const uint64_t next_carry = chunk_remainder * carry + mod;
+  *v = *v / divisor + carry * chunk_quotient + next_carry / divisor;
+  return next_carry % divisor;
+}
+
+// Generates the decimal representation for an integer of the form `v * 2^exp`,
+// where `v` and `exp` are both positive integers.
+// It generates the digits from the left (ie the most significant digit first)
+// to allow for direct printing into the sink.
+//
+// Requires `0 <= exp` and `exp <= numeric_limits<long double>::max_exponent`.
+class BinaryToDecimal {
+  static constexpr int ChunksNeeded(int exp) {
+    // We will left shift a uint128 by `exp` bits, so we need `128+exp` total
+    // bits. Round up to 32.
+    // See constructor for details about adding `10%` to the value.
+    return (128 + exp + 31) / 32 * 11 / 10;
+  }
+
+ public:
+  // Run the conversion for `v * 2^exp` and call `f(binary_to_decimal)`.
+  // This function will allocate enough stack space to perform the conversion.
+  static void RunConversion(uint128 v, int exp,
+                            absl::FunctionRef<void(BinaryToDecimal)> f) {
+    assert(exp > 0);
+    assert(exp <= std::numeric_limits<long double>::max_exponent);
+    static_assert(
+        StackArray::kMaxCapacity >=
+            ChunksNeeded(std::numeric_limits<long double>::max_exponent),
+        "");
+
+    StackArray::RunWithCapacity(
+        ChunksNeeded(exp),
+        [=](absl::Span<uint32_t> input) { f(BinaryToDecimal(input, v, exp)); });
+  }
+
+  int TotalDigits() const {
+    return static_cast<int>((decimal_end_ - decimal_start_) * kDigitsPerChunk +
+                            CurrentDigits().size());
+  }
+
+  // See the current block of digits.
+  absl::string_view CurrentDigits() const {
+    return absl::string_view(digits_ + kDigitsPerChunk - size_, size_);
+  }
+
+  // Advance the current view of digits.
+  // Returns `false` when no more digits are available.
+  bool AdvanceDigits() {
+    if (decimal_start_ >= decimal_end_) return false;
+
+    uint32_t w = data_[decimal_start_++];
+    for (size_ = 0; size_ < kDigitsPerChunk; w /= 10) {
+      digits_[kDigitsPerChunk - ++size_] = w % 10 + '0';
+    }
+    return true;
+  }
+
+ private:
+  BinaryToDecimal(absl::Span<uint32_t> data, uint128 v, int exp) : data_(data) {
+    // We need to print the digits directly into the sink object without
+    // buffering them all first. To do this we need two things:
+    // - to know the total number of digits to do padding when necessary
+    // - to generate the decimal digits from the left.
+    //
+    // In order to do this, we do a two pass conversion.
+    // On the first pass we convert the binary representation of the value into
+    // a decimal representation in which each uint32_t chunk holds up to 9
+    // decimal digits.  In the second pass we take each decimal-holding-uint32_t
+    // value and generate the ascii decimal digits into `digits_`.
+    //
+    // The binary and decimal representations actually share the same memory
+    // region. As we go converting the chunks from binary to decimal we free
+    // them up and reuse them for the decimal representation. One caveat is that
+    // the decimal representation is around 7% less efficient in space than the
+    // binary one. We allocate an extra 10% memory to account for this. See
+    // ChunksNeeded for this calculation.
+    int chunk_index = exp / 32;
+    decimal_start_ = decimal_end_ = ChunksNeeded(exp);
+    const int offset = exp % 32;
+    // Left shift v by exp bits.
+    data_[chunk_index] = static_cast<uint32_t>(v << offset);
+    for (v >>= (32 - offset); v; v >>= 32)
+      data_[++chunk_index] = static_cast<uint32_t>(v);
+
+    while (chunk_index >= 0) {
+      // While we have more than one chunk available, go in steps of 1e9.
+      // `data_[chunk_index]` holds the highest non-zero binary chunk, so keep
+      // the variable updated.
+      uint32_t carry = 0;
+      for (int i = chunk_index; i >= 0; --i) {
+        uint64_t tmp = uint64_t{data_[i]} + (uint64_t{carry} << 32);
+        data_[i] = static_cast<uint32_t>(tmp / uint64_t{1000000000});
+        carry = static_cast<uint32_t>(tmp % uint64_t{1000000000});
+      }
+
+      // If the highest chunk is now empty, remove it from view.
+      if (data_[chunk_index] == 0) --chunk_index;
+
+      --decimal_start_;
+      assert(decimal_start_ != chunk_index);
+      data_[decimal_start_] = carry;
+    }
+
+    // Fill the first set of digits. The first chunk might not be complete, so
+    // handle differently.
+    for (uint32_t first = data_[decimal_start_++]; first != 0; first /= 10) {
+      digits_[kDigitsPerChunk - ++size_] = first % 10 + '0';
+    }
+  }
+
+ private:
+  static constexpr size_t kDigitsPerChunk = 9;
+
+  int decimal_start_;
+  int decimal_end_;
+
+  char digits_[kDigitsPerChunk];
+  int size_ = 0;
+
+  absl::Span<uint32_t> data_;
+};
+
+// Converts a value of the form `x * 2^-exp` into a sequence of decimal digits.
+// Requires `-exp < 0` and
+// `-exp >= limits<long double>::min_exponent - limits<long double>::digits`.
+class FractionalDigitGenerator {
+ public:
+  // Run the conversion for `v * 2^exp` and call `f(generator)`.
+  // This function will allocate enough stack space to perform the conversion.
+  static void RunConversion(
+      uint128 v, int exp, absl::FunctionRef<void(FractionalDigitGenerator)> f) {
+    assert(-exp < 0);
+    assert(-exp >= std::numeric_limits<long double>::min_exponent - 128);
+    static_assert(
+        StackArray::kMaxCapacity >=
+            (128 - std::numeric_limits<long double>::min_exponent + 31) / 32,
+        "");
+    StackArray::RunWithCapacity((exp + 31) / 32,
+                                [=](absl::Span<uint32_t> input) {
+                                  f(FractionalDigitGenerator(input, v, exp));
+                                });
+  }
+
+  // Returns true if there are any more non-zero digits left.
+  bool HasMoreDigits() const { return next_digit_ != 0 || chunk_index_ >= 0; }
+
+  // Returns true if the remainder digits are greater than 5000...
+  bool IsGreaterThanHalf() const {
+    return next_digit_ > 5 || (next_digit_ == 5 && chunk_index_ >= 0);
+  }
+  // Returns true if the remainder digits are exactly 5000...
+  bool IsExactlyHalf() const { return next_digit_ == 5 && chunk_index_ < 0; }
+
+  struct Digits {
+    int digit_before_nine;
+    int num_nines;
+  };
+
+  // Get the next set of digits.
+  // They are composed by a non-9 digit followed by a runs of zero or more 9s.
+  Digits GetDigits() {
+    Digits digits{next_digit_, 0};
+
+    next_digit_ = GetOneDigit();
+    while (next_digit_ == 9) {
+      ++digits.num_nines;
+      next_digit_ = GetOneDigit();
+    }
+
+    return digits;
+  }
+
+ private:
+  // Return the next digit.
+  int GetOneDigit() {
+    if (chunk_index_ < 0) return 0;
+
+    uint32_t carry = 0;
+    for (int i = chunk_index_; i >= 0; --i) {
+      carry = MultiplyBy10WithCarry(&data_[i], carry);
+    }
+    // If the lowest chunk is now empty, remove it from view.
+    if (data_[chunk_index_] == 0) --chunk_index_;
+    return carry;
+  }
+
+  FractionalDigitGenerator(absl::Span<uint32_t> data, uint128 v, int exp)
+      : chunk_index_(exp / 32), data_(data) {
+    const int offset = exp % 32;
+    // Right shift `v` by `exp` bits.
+    data_[chunk_index_] = static_cast<uint32_t>(v << (32 - offset));
+    v >>= offset;
+    // Make sure we don't overflow the data. We already calculated that
+    // non-zero bits fit, so we might not have space for leading zero bits.
+    for (int pos = chunk_index_; v; v >>= 32)
+      data_[--pos] = static_cast<uint32_t>(v);
+
+    // Fill next_digit_, as GetDigits expects it to be populated always.
+    next_digit_ = GetOneDigit();
+  }
+
+  int next_digit_;
+  int chunk_index_;
+  absl::Span<uint32_t> data_;
+};
+
+// Count the number of leading zero bits.
+int LeadingZeros(uint64_t v) { return base_internal::CountLeadingZeros64(v); }
+int LeadingZeros(uint128 v) {
+  auto high = static_cast<uint64_t>(v >> 64);
+  auto low = static_cast<uint64_t>(v);
+  return high != 0 ? base_internal::CountLeadingZeros64(high)
+                   : 64 + base_internal::CountLeadingZeros64(low);
+}
+
+// Round up the text digits starting at `p`.
+// The buffer must have an extra digit that is known to not need rounding.
+// This is done below by having an extra '0' digit on the left.
+void RoundUp(char *p) {
+  while (*p == '9' || *p == '.') {
+    if (*p == '9') *p = '0';
+    --p;
+  }
+  ++*p;
+}
+
+// Check the previous digit and round up or down to follow the round-to-even
+// policy.
+void RoundToEven(char *p) {
+  if (*p == '.') --p;
+  if (*p % 2 == 1) RoundUp(p);
+}
+
+// Simple integral decimal digit printing for values that fit in 64-bits.
+// Returns the pointer to the last written digit.
+char *PrintIntegralDigitsFromRightFast(uint64_t v, char *p) {
+  do {
+    *--p = DivideBy10WithCarry(&v, 0) + '0';
+  } while (v != 0);
+  return p;
+}
+
+// Simple integral decimal digit printing for values that fit in 128-bits.
+// Returns the pointer to the last written digit.
+char *PrintIntegralDigitsFromRightFast(uint128 v, char *p) {
+  auto high = static_cast<uint64_t>(v >> 64);
+  auto low = static_cast<uint64_t>(v);
+
+  while (high != 0) {
+    uint64_t carry = DivideBy10WithCarry(&high, 0);
+    carry = DivideBy10WithCarry(&low, carry);
+    *--p = carry + '0';
+  }
+  return PrintIntegralDigitsFromRightFast(low, p);
+}
+
+// Simple fractional decimal digit printing for values that fir in 64-bits after
+// shifting.
+// Performs rounding if necessary to fit within `precision`.
+// Returns the pointer to one after the last character written.
+char *PrintFractionalDigitsFast(uint64_t v, char *start, int exp,
+                                int precision) {
+  char *p = start;
+  v <<= (64 - exp);
+  while (precision > 0) {
+    if (!v) return p;
+    *p++ = MultiplyBy10WithCarry(&v, uint64_t{0}) + '0';
+    --precision;
+  }
+
+  // We need to round.
+  if (v < 0x8000000000000000) {
+    // We round down, so nothing to do.
+  } else if (v > 0x8000000000000000) {
+    // We round up.
+    RoundUp(p - 1);
+  } else {
+    RoundToEven(p - 1);
+  }
+
+  assert(precision == 0);
+  // Precision can only be zero here.
+  return p;
+}
+
+// Simple fractional decimal digit printing for values that fir in 128-bits
+// after shifting.
+// Performs rounding if necessary to fit within `precision`.
+// Returns the pointer to one after the last character written.
+char *PrintFractionalDigitsFast(uint128 v, char *start, int exp,
+                                int precision) {
+  char *p = start;
+  v <<= (128 - exp);
+  auto high = static_cast<uint64_t>(v >> 64);
+  auto low = static_cast<uint64_t>(v);
+
+  // While we have digits to print and `low` is not empty, do the long
+  // multiplication.
+  while (precision > 0 && low != 0) {
+    uint64_t carry = MultiplyBy10WithCarry(&low, uint64_t{0});
+    carry = MultiplyBy10WithCarry(&high, carry);
+
+    *p++ = carry + '0';
+    --precision;
+  }
+
+  // Now `low` is empty, so use a faster approach for the rest of the digits.
+  // This block is pretty much the same as the main loop for the 64-bit case
+  // above.
+  while (precision > 0) {
+    if (!high) return p;
+    *p++ = MultiplyBy10WithCarry(&high, uint64_t{0}) + '0';
+    --precision;
+  }
+
+  // We need to round.
+  if (high < 0x8000000000000000) {
+    // We round down, so nothing to do.
+  } else if (high > 0x8000000000000000 || low != 0) {
+    // We round up.
+    RoundUp(p - 1);
+  } else {
+    RoundToEven(p - 1);
+  }
+
+  assert(precision == 0);
+  // Precision can only be zero here.
+  return p;
+}
+
+struct FormatState {
+  char sign_char;
+  int precision;
+  const FormatConversionSpecImpl &conv;
+  FormatSinkImpl *sink;
+
+  // In `alt` mode (flag #) we keep the `.` even if there are no fractional
+  // digits. In non-alt mode, we strip it.
+  bool ShouldPrintDot() const { return precision != 0 || conv.has_alt_flag(); }
+};
+
+struct Padding {
+  int left_spaces;
+  int zeros;
+  int right_spaces;
+};
+
+Padding ExtraWidthToPadding(int total_size, const FormatState &state) {
+  int missing_chars = std::max(state.conv.width() - total_size, 0);
+  if (state.conv.has_left_flag()) {
+    return {0, 0, missing_chars};
+  } else if (state.conv.has_zero_flag()) {
+    return {0, missing_chars, 0};
+  } else {
+    return {missing_chars, 0, 0};
+  }
+}
+
+void FinalPrint(absl::string_view data, int trailing_zeros,
+                const FormatState &state) {
+  if (state.conv.width() < 0) {
+    // No width specified. Fast-path.
+    if (state.sign_char != '\0') state.sink->Append(1, state.sign_char);
+    state.sink->Append(data);
+    state.sink->Append(trailing_zeros, '0');
+    return;
+  }
+
+  auto padding =
+      ExtraWidthToPadding((state.sign_char != '\0' ? 1 : 0) +
+                              static_cast<int>(data.size()) + trailing_zeros,
+                          state);
+
+  state.sink->Append(padding.left_spaces, ' ');
+  if (state.sign_char != '\0') state.sink->Append(1, state.sign_char);
+  state.sink->Append(padding.zeros, '0');
+  state.sink->Append(data);
+  state.sink->Append(trailing_zeros, '0');
+  state.sink->Append(padding.right_spaces, ' ');
+}
+
+// Fastpath %f formatter for when the shifted value fits in a simple integral
+// type.
+// Prints `v*2^exp` with the options from `state`.
+template <typename Int>
+void FormatFFast(Int v, int exp, const FormatState &state) {
+  constexpr int input_bits = sizeof(Int) * 8;
+
+  static constexpr size_t integral_size =
+      /* in case we need to round up an extra digit */ 1 +
+      /* decimal digits for uint128 */ 40 + 1;
+  char buffer[integral_size + /* . */ 1 + /* max digits uint128 */ 128];
+  buffer[integral_size] = '.';
+  char *const integral_digits_end = buffer + integral_size;
+  char *integral_digits_start;
+  char *const fractional_digits_start = buffer + integral_size + 1;
+  char *fractional_digits_end = fractional_digits_start;
+
+  if (exp >= 0) {
+    const int total_bits = input_bits - LeadingZeros(v) + exp;
+    integral_digits_start =
+        total_bits <= 64
+            ? PrintIntegralDigitsFromRightFast(static_cast<uint64_t>(v) << exp,
+                                               integral_digits_end)
+            : PrintIntegralDigitsFromRightFast(static_cast<uint128>(v) << exp,
+                                               integral_digits_end);
+  } else {
+    exp = -exp;
+
+    integral_digits_start = PrintIntegralDigitsFromRightFast(
+        exp < input_bits ? v >> exp : 0, integral_digits_end);
+    // PrintFractionalDigits may pull a carried 1 all the way up through the
+    // integral portion.
+    integral_digits_start[-1] = '0';
+
+    fractional_digits_end =
+        exp <= 64 ? PrintFractionalDigitsFast(v, fractional_digits_start, exp,
+                                              state.precision)
+                  : PrintFractionalDigitsFast(static_cast<uint128>(v),
+                                              fractional_digits_start, exp,
+                                              state.precision);
+    // There was a carry, so include the first digit too.
+    if (integral_digits_start[-1] != '0') --integral_digits_start;
+  }
+
+  size_t size = fractional_digits_end - integral_digits_start;
+
+  // In `alt` mode (flag #) we keep the `.` even if there are no fractional
+  // digits. In non-alt mode, we strip it.
+  if (!state.ShouldPrintDot()) --size;
+  FinalPrint(absl::string_view(integral_digits_start, size),
+             static_cast<int>(state.precision - (fractional_digits_end -
+                                                 fractional_digits_start)),
+             state);
+}
+
+// Slow %f formatter for when the shifted value does not fit in a uint128, and
+// `exp > 0`.
+// Prints `v*2^exp` with the options from `state`.
+// This one is guaranteed to not have fractional digits, so we don't have to
+// worry about anything after the `.`.
+void FormatFPositiveExpSlow(uint128 v, int exp, const FormatState &state) {
+  BinaryToDecimal::RunConversion(v, exp, [&](BinaryToDecimal btd) {
+    const int total_digits =
+        btd.TotalDigits() + (state.ShouldPrintDot() ? state.precision + 1 : 0);
+
+    const auto padding = ExtraWidthToPadding(
+        total_digits + (state.sign_char != '\0' ? 1 : 0), state);
+
+    state.sink->Append(padding.left_spaces, ' ');
+    if (state.sign_char != '\0') state.sink->Append(1, state.sign_char);
+    state.sink->Append(padding.zeros, '0');
+
+    do {
+      state.sink->Append(btd.CurrentDigits());
+    } while (btd.AdvanceDigits());
+
+    if (state.ShouldPrintDot()) state.sink->Append(1, '.');
+    state.sink->Append(state.precision, '0');
+    state.sink->Append(padding.right_spaces, ' ');
+  });
+}
+
+// Slow %f formatter for when the shifted value does not fit in a uint128, and
+// `exp < 0`.
+// Prints `v*2^exp` with the options from `state`.
+// This one is guaranteed to be < 1.0, so we don't have to worry about integral
+// digits.
+void FormatFNegativeExpSlow(uint128 v, int exp, const FormatState &state) {
+  const int total_digits =
+      /* 0 */ 1 + (state.ShouldPrintDot() ? state.precision + 1 : 0);
+  auto padding =
+      ExtraWidthToPadding(total_digits + (state.sign_char ? 1 : 0), state);
+  padding.zeros += 1;
+  state.sink->Append(padding.left_spaces, ' ');
+  if (state.sign_char != '\0') state.sink->Append(1, state.sign_char);
+  state.sink->Append(padding.zeros, '0');
+
+  if (state.ShouldPrintDot()) state.sink->Append(1, '.');
+
+  // Print digits
+  int digits_to_go = state.precision;
+
+  FractionalDigitGenerator::RunConversion(
+      v, exp, [&](FractionalDigitGenerator digit_gen) {
+        // There are no digits to print here.
+        if (state.precision == 0) return;
+
+        // We go one digit at a time, while keeping track of runs of nines.
+        // The runs of nines are used to perform rounding when necessary.
+
+        while (digits_to_go > 0 && digit_gen.HasMoreDigits()) {
+          auto digits = digit_gen.GetDigits();
+
+          // Now we have a digit and a run of nines.
+          // See if we can print them all.
+          if (digits.num_nines + 1 < digits_to_go) {
+            // We don't have to round yet, so print them.
+            state.sink->Append(1, digits.digit_before_nine + '0');
+            state.sink->Append(digits.num_nines, '9');
+            digits_to_go -= digits.num_nines + 1;
+
+          } else {
+            // We can't print all the nines, see where we have to truncate.
+
+            bool round_up = false;
+            if (digits.num_nines + 1 > digits_to_go) {
+              // We round up at a nine. No need to print them.
+              round_up = true;
+            } else {
+              // We can fit all the nines, but truncate just after it.
+              if (digit_gen.IsGreaterThanHalf()) {
+                round_up = true;
+              } else if (digit_gen.IsExactlyHalf()) {
+                // Round to even
+                round_up =
+                    digits.num_nines != 0 || digits.digit_before_nine % 2 == 1;
+              }
+            }
+
+            if (round_up) {
+              state.sink->Append(1, digits.digit_before_nine + '1');
+              --digits_to_go;
+              // The rest will be zeros.
+            } else {
+              state.sink->Append(1, digits.digit_before_nine + '0');
+              state.sink->Append(digits_to_go - 1, '9');
+              digits_to_go = 0;
+            }
+            return;
+          }
+        }
+      });
+
+  state.sink->Append(digits_to_go, '0');
+  state.sink->Append(padding.right_spaces, ' ');
+}
+
+template <typename Int>
+void FormatF(Int mantissa, int exp, const FormatState &state) {
+  if (exp >= 0) {
+    const int total_bits = sizeof(Int) * 8 - LeadingZeros(mantissa) + exp;
+
+    // Fallback to the slow stack-based approach if we can't do it in a 64 or
+    // 128 bit state.
+    if (ABSL_PREDICT_FALSE(total_bits > 128)) {
+      return FormatFPositiveExpSlow(mantissa, exp, state);
+    }
+  } else {
+    // Fallback to the slow stack-based approach if we can't do it in a 64 or
+    // 128 bit state.
+    if (ABSL_PREDICT_FALSE(exp < -128)) {
+      return FormatFNegativeExpSlow(mantissa, -exp, state);
+    }
+  }
+  return FormatFFast(mantissa, exp, state);
+}
+
+char *CopyStringTo(absl::string_view v, char *out) {
   std::memcpy(out, v.data(), v.size());
   return out + v.size();
 }
 
 template <typename Float>
-bool FallbackToSnprintf(const Float v, const ConversionSpec &conv,
+bool FallbackToSnprintf(const Float v, const FormatConversionSpecImpl &conv,
                         FormatSinkImpl *sink) {
   int w = conv.width() >= 0 ? conv.width() : 0;
   int p = conv.precision() >= 0 ? conv.precision() : -1;
@@ -38,12 +675,12 @@ bool FallbackToSnprintf(const Float v, const ConversionSpec &conv,
     assert(fp < fmt + sizeof(fmt));
   }
   std::string space(512, '\0');
-  string_view result;
+  absl::string_view result;
   while (true) {
     int n = snprintf(&space[0], space.size(), fmt, w, p, v);
     if (n < 0) return false;
     if (static_cast<size_t>(n) < space.size()) {
-      result = string_view(space.data(), n);
+      result = absl::string_view(space.data(), n);
       break;
     }
     space.resize(n + 1);
@@ -96,9 +733,10 @@ enum class FormatStyle { Fixed, Precision };
 // Otherwise, return false.
 template <typename Float>
 bool ConvertNonNumericFloats(char sign_char, Float v,
-                             const ConversionSpec &conv, FormatSinkImpl *sink) {
+                             const FormatConversionSpecImpl &conv,
+                             FormatSinkImpl *sink) {
   char text[4], *ptr = text;
-  if (sign_char) *ptr++ = sign_char;
+  if (sign_char != '\0') *ptr++ = sign_char;
   if (std::isnan(v)) {
     ptr = std::copy_n(
         FormatConversionCharIsUpper(conv.conversion_char()) ? "NAN" : "nan", 3,
@@ -172,7 +810,12 @@ constexpr bool CanFitMantissa() {
 
 template <typename Float>
 struct Decomposed {
-  Float mantissa;
+  using MantissaType =
+      absl::conditional_t<std::is_same<long double, Float>::value, uint128,
+                          uint64_t>;
+  static_assert(std::numeric_limits<Float>::digits <= sizeof(MantissaType) * 8,
+                "");
+  MantissaType mantissa;
   int exponent;
 };
 
@@ -183,7 +826,8 @@ Decomposed<Float> Decompose(Float v) {
   Float m = std::frexp(v, &exp);
   m = std::ldexp(m, std::numeric_limits<Float>::digits);
   exp -= std::numeric_limits<Float>::digits;
-  return {m, exp};
+
+  return {static_cast<typename Decomposed<Float>::MantissaType>(m), exp};
 }
 
 // Print 'digits' as decimal.
@@ -352,8 +996,9 @@ bool FloatToBuffer(Decomposed<Float> decomposed, int precision, Buffer *out,
   return false;
 }
 
-void WriteBufferToSink(char sign_char, string_view str,
-                       const ConversionSpec &conv, FormatSinkImpl *sink) {
+void WriteBufferToSink(char sign_char, absl::string_view str,
+                       const FormatConversionSpecImpl &conv,
+                       FormatSinkImpl *sink) {
   int left_spaces = 0, zeros = 0, right_spaces = 0;
   int missing_chars =
       conv.width() >= 0 ? std::max(conv.width() - static_cast<int>(str.size()) -
@@ -369,14 +1014,14 @@ void WriteBufferToSink(char sign_char, string_view str,
   }
 
   sink->Append(left_spaces, ' ');
-  if (sign_char) sink->Append(1, sign_char);
+  if (sign_char != '\0') sink->Append(1, sign_char);
   sink->Append(zeros, '0');
   sink->Append(str);
   sink->Append(right_spaces, ' ');
 }
 
 template <typename Float>
-bool FloatToSink(const Float v, const ConversionSpec &conv,
+bool FloatToSink(const Float v, const FormatConversionSpecImpl &conv,
                  FormatSinkImpl *sink) {
   // Print the sign or the sign column.
   Float abs_v = v;
@@ -407,11 +1052,9 @@ bool FloatToSink(const Float v, const ConversionSpec &conv,
 
   if (c == FormatConversionCharInternal::f ||
       c == FormatConversionCharInternal::F) {
-    if (!FloatToBuffer<FormatStyle::Fixed>(decomposed, precision, &buffer,
-                                           nullptr)) {
-      return FallbackToSnprintf(v, conv, sink);
-    }
-    if (!conv.has_alt_flag() && buffer.back() == '.') buffer.pop_back();
+    FormatF(decomposed.mantissa, decomposed.exponent,
+            {sign_char, precision, conv, sink});
+    return true;
   } else if (c == FormatConversionCharInternal::e ||
              c == FormatConversionCharInternal::E) {
     if (!FloatToBuffer<FormatStyle::Precision>(decomposed, precision, &buffer,
@@ -462,25 +1105,32 @@ bool FloatToSink(const Float v, const ConversionSpec &conv,
   }
 
   WriteBufferToSink(sign_char,
-                    string_view(buffer.begin, buffer.end - buffer.begin), conv,
-                    sink);
+                    absl::string_view(buffer.begin, buffer.end - buffer.begin),
+                    conv, sink);
 
   return true;
 }
 
 }  // namespace
 
-bool ConvertFloatImpl(long double v, const ConversionSpec &conv,
+bool ConvertFloatImpl(long double v, const FormatConversionSpecImpl &conv,
                       FormatSinkImpl *sink) {
+  if (std::numeric_limits<long double>::digits ==
+      2 * std::numeric_limits<double>::digits) {
+    // This is the `double-double` representation of `long double`.
+    // We do not handle it natively. Fallback to snprintf.
+    return FallbackToSnprintf(v, conv, sink);
+  }
+
   return FloatToSink(v, conv, sink);
 }
 
-bool ConvertFloatImpl(float v, const ConversionSpec &conv,
+bool ConvertFloatImpl(float v, const FormatConversionSpecImpl &conv,
                       FormatSinkImpl *sink) {
   return FloatToSink(v, conv, sink);
 }
 
-bool ConvertFloatImpl(double v, const ConversionSpec &conv,
+bool ConvertFloatImpl(double v, const FormatConversionSpecImpl &conv,
                       FormatSinkImpl *sink) {
   return FloatToSink(v, conv, sink);
 }
diff --git a/absl/strings/internal/str_format/float_conversion.h b/absl/strings/internal/str_format/float_conversion.h
index 49a6a63630ee..e78bc19106ff 100644
--- a/absl/strings/internal/str_format/float_conversion.h
+++ b/absl/strings/internal/str_format/float_conversion.h
@@ -7,13 +7,13 @@ namespace absl {
 ABSL_NAMESPACE_BEGIN
 namespace str_format_internal {
 
-bool ConvertFloatImpl(float v, const ConversionSpec &conv,
+bool ConvertFloatImpl(float v, const FormatConversionSpecImpl &conv,
                       FormatSinkImpl *sink);
 
-bool ConvertFloatImpl(double v, const ConversionSpec &conv,
+bool ConvertFloatImpl(double v, const FormatConversionSpecImpl &conv,
                       FormatSinkImpl *sink);
 
-bool ConvertFloatImpl(long double v, const ConversionSpec &conv,
+bool ConvertFloatImpl(long double v, const FormatConversionSpecImpl &conv,
                       FormatSinkImpl *sink);
 
 }  // namespace str_format_internal
diff --git a/absl/strings/internal/str_format/parser.h b/absl/strings/internal/str_format/parser.h
index fd2dc9704507..fffed04fa072 100644
--- a/absl/strings/internal/str_format/parser.h
+++ b/absl/strings/internal/str_format/parser.h
@@ -83,7 +83,7 @@ const char* ConsumeUnboundConversion(const char* p, const char* end,
 // conversions.
 class ConvTag {
  public:
-  constexpr ConvTag(ConversionChar conversion_char)  // NOLINT
+  constexpr ConvTag(FormatConversionChar conversion_char)  // NOLINT
       : tag_(static_cast<int8_t>(conversion_char)) {}
   // We invert the length modifiers to make them negative so that we can easily
   // test for them.
@@ -94,9 +94,9 @@ class ConvTag {
 
   bool is_conv() const { return tag_ >= 0; }
   bool is_length() const { return tag_ < 0 && tag_ != -128; }
-  ConversionChar as_conv() const {
+  FormatConversionChar as_conv() const {
     assert(is_conv());
-    return static_cast<ConversionChar>(tag_);
+    return static_cast<FormatConversionChar>(tag_);
   }
   LengthMod as_length() const {
     assert(is_length());
@@ -282,7 +282,7 @@ class ParsedFormatBase {
 // This is the only API that allows the user to pass a runtime specified format
 // string. These factory functions will return NULL if the format does not match
 // the conversions requested by the user.
-template <str_format_internal::Conv... C>
+template <FormatConversionCharSet... C>
 class ExtendedParsedFormat : public str_format_internal::ParsedFormatBase {
  public:
   explicit ExtendedParsedFormat(string_view format)
diff --git a/absl/strings/internal/str_format/parser_test.cc b/absl/strings/internal/str_format/parser_test.cc
index 26f5bec6bcff..dae2d20f593e 100644
--- a/absl/strings/internal/str_format/parser_test.cc
+++ b/absl/strings/internal/str_format/parser_test.cc
@@ -41,7 +41,7 @@ TEST(LengthModTest, Names) {
 
 TEST(ConversionCharTest, Names) {
   struct Expectation {
-    ConversionChar id;
+    FormatConversionChar id;
     char name;
   };
   // clang-format off
@@ -57,7 +57,7 @@ TEST(ConversionCharTest, Names) {
   // clang-format on
   for (auto e : kExpect) {
     SCOPED_TRACE(e.name);
-    ConversionChar v = e.id;
+    FormatConversionChar v = e.id;
     EXPECT_EQ(e.name, FormatConversionCharToChar(v));
   }
 }
@@ -368,7 +368,7 @@ TEST_F(ParsedFormatTest, ValueSemantics) {
 
 struct ExpectParse {
   const char* in;
-  std::initializer_list<Conv> conv_set;
+  std::initializer_list<FormatConversionCharSet> conv_set;
   const char* out;
 };