Export of internal Abseil changes.

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fe689b30f9a3a614e8a577997cc340043d01c2f1 by Abseil Team <absl-team@google.com>:

Change arm32 linux backtrace to use the generic implementation.

PiperOrigin-RevId: 218386158

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86f8678d055d32edc989e6a4d1dc49c3a15cd350 by Abseil Team <absl-team@google.com>:

Update documentation on SHARED_LOCKS_REQUIRED and EXCLUSIVE_LOCKS_REQUIRED so that users know the differences between them.

PiperOrigin-RevId: 218365545

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22947b48ce4a1ba71ad4794f762235dac3a1df12 by Greg Falcon <gfalcon@google.com>:

The from_chars implementation incorrectly assumed `uint32_t` was `unsigned int`.

`strings_internal::BigUnsigned` had `uint32_t` and `uint64_t` constructors; when both of these types differ from `unsigned int`, `BigUnsigned(1u)` is ambiguous (neither conversion is better).

Fix this by removing the `uint32_t` constructor.  When the `uint64_t` constructor is called with a literal or type that is 32 bits or narrower, the compiler is smart enough to optimize away the two-word case, so this fix is free.

PiperOrigin-RevId: 218346935

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7201ab430bb90ca0e30b102915d02564f61353eb by Abseil Team <absl-team@google.com>:

Fix formatting errors discovered during merge conflict.

PiperOrigin-RevId: 218229891
GitOrigin-RevId: fe689b30f9a3a614e8a577997cc340043d01c2f1
Change-Id: I5d382482ad227d48ffe57b243ce11b1eb44a1314

4 files changed, 18 insertions, 13 deletions

diff --git a/absl/base/thread_annotations.h b/absl/base/thread_annotations.h
index fbb2797b825f..2241ace4a478 100644
--- a/absl/base/thread_annotations.h
+++ b/absl/base/thread_annotations.h
@@ -108,13 +108,23 @@
 // The mutex is expected to be held both on entry to, and exit from, the
 // function.
 //
+// An exclusive lock allows read-write access to the guarded data member(s), and
+// only one thread can acquire a lock exclusively at any one time. A shared lock
+// allows read-only access, and any number of threads can acquire a shared lock
+// concurrently.
+//
+// Generally, non-const methods should be annotated with
+// EXCLUSIVE_LOCKS_REQUIRED, while const methods should be annotated with
+// SHARED_LOCKS_REQUIRED.
+//
 // Example:
 //
 //   Mutex mu1, mu2;
 //   int a GUARDED_BY(mu1);
 //   int b GUARDED_BY(mu2);
 //
-//   void foo() EXCLUSIVE_LOCKS_REQUIRED(mu1, mu2) { ... };
+//   void foo() EXCLUSIVE_LOCKS_REQUIRED(mu1, mu2) { ... }
+//   void bar() const SHARED_LOCKS_REQUIRED(mu1, mu2) { ... }
 #define EXCLUSIVE_LOCKS_REQUIRED(...) \
   THREAD_ANNOTATION_ATTRIBUTE__(exclusive_locks_required(__VA_ARGS__))
 
diff --git a/absl/debugging/internal/stacktrace_config.h b/absl/debugging/internal/stacktrace_config.h
index dd713da8c0c7..578e4968b009 100644
--- a/absl/debugging/internal/stacktrace_config.h
+++ b/absl/debugging/internal/stacktrace_config.h
@@ -41,8 +41,9 @@
 #define ABSL_STACKTRACE_INL_HEADER \
     "absl/debugging/internal/stacktrace_aarch64-inl.inc"
 # elif defined(__arm__)
+// Note: When using glibc this may require -funwind-tables to function properly.
 #define ABSL_STACKTRACE_INL_HEADER \
-    "absl/debugging/internal/stacktrace_arm-inl.inc"
+  "absl/debugging/internal/stacktrace_generic-inl.inc"
 # else
 #define ABSL_STACKTRACE_INL_HEADER \
    "absl/debugging/internal/stacktrace_unimplemented-inl.inc"
diff --git a/absl/debugging/internal/stacktrace_generic-inl.inc b/absl/debugging/internal/stacktrace_generic-inl.inc
index 2c9ca410c608..08b87bd6ae39 100644
--- a/absl/debugging/internal/stacktrace_generic-inl.inc
+++ b/absl/debugging/internal/stacktrace_generic-inl.inc
@@ -19,6 +19,7 @@
 template <bool IS_STACK_FRAMES, bool IS_WITH_CONTEXT>
 static int UnwindImpl(void** result, int* sizes, int max_depth, int skip_count,
                       const void *ucp, int *min_dropped_frames) {
+  static_cast<void>(ucp);  // Unused.
   static const int kStackLength = 64;
   void * stack[kStackLength];
   int size;
diff --git a/absl/strings/internal/charconv_bigint.h b/absl/strings/internal/charconv_bigint.h
index aa70af2c2894..5c579437e521 100644
--- a/absl/strings/internal/charconv_bigint.h
+++ b/absl/strings/internal/charconv_bigint.h
@@ -57,17 +57,10 @@ class BigUnsigned {
                 "unsupported max_words value");
 
   BigUnsigned() : size_(0), words_{} {}
-  explicit BigUnsigned(uint32_t v) : size_(v > 0 ? 1 : 0), words_{v} {}
-  explicit BigUnsigned(uint64_t v)
-      : size_(0),
-        words_{static_cast<uint32_t>(v & 0xffffffff),
-               static_cast<uint32_t>(v >> 32)} {
-    if (words_[1]) {
-      size_ = 2;
-    } else if (words_[0]) {
-      size_ = 1;
-    }
-  }
+  explicit constexpr BigUnsigned(uint64_t v)
+      : size_((v >> 32) ? 2 : v ? 1 : 0),
+        words_{static_cast<uint32_t>(v & 0xffffffffu),
+               static_cast<uint32_t>(v >> 32)} {}
 
   // Constructs a BigUnsigned from the given string_view containing a decimal
   // value.  If the input std::string is not a decimal integer, constructs a 0