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

1 files changed, 4 insertions, 11 deletions

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