Export of internal Abseil changes

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0bfa836596a9c787a2f0bdc283011dd1f6810c6e by Benjamin Barenblat <bbaren@google.com>:

Ignore missing CPU frequency on more architectures

Linux on MIPS, PA-RISC, RISC-V, and SystemZ doesn’t expose the nominal
CPU frequency via /sys, so don’t worry if `NominalCPUFrequency` returns
1.0 on those platforms.

Some POWER machines expose the CPU frequency; others do not. Since we
can’t predict which type of machine the tests will run on, simply
disable testing for `NominalCPUFrequency` on POWER.

PiperOrigin-RevId: 347079873

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492b6834ed4a07cbc3abccd846f7e37d8c556ee5 by Benjamin Barenblat <bbaren@google.com>:

Use ABSL_HAVE_THREAD_LOCAL macro instead of copying code

Reduce code duplication by checking the ABSL_HAVE_THREAD_LOCAL macro
instead of copying code from base/config.h.

PiperOrigin-RevId: 347079561

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8d656efce4da9cb032094377e58493d98427a536 by Abseil Team <absl-team@google.com>:

Rollback

PiperOrigin-RevId: 347078779

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221bc69ec6dd7e2777ffcff6942584f979ef6382 by Abseil Team <absl-team@google.com>:

Add flag for 'shallow subcord' feature for experimental ring buffer rollout

There is a potential trade-off of CPU cost vs over-sharing cord data for subcord of large cords. This flag allows making subcords shallow for ringbuffers (with a potential larger waste of referenced source cords), which allows us to make subcord fast for this apps that do no persist (unmodified / plain copied) sub cords.

This change also introduces constants for the default settings, intended to keep the internal cord settings concistent with external flags.

PiperOrigin-RevId: 347053271

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00a56c24293566734009f6bf2169a83fb37a35ba by Abseil Team <absl-team@google.com>:

Revert the usage of variant<> in Cord iterator and reader.

The introduction of the variant may lead to some missed compiler optimizations.

PiperOrigin-RevId: 347053041

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c7b7b5ed7e3ab46b1e75b80f1a7de0bda26c8f70 by Chris Kennelly <ckennelly@google.com>:

Release library for integer power-of-2 functions and bit counting.

PiperOrigin-RevId: 347035065

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5a035c0d9840b251967f9e7039fc6a4e01dd52f3 by Abseil Team <absl-team@google.com>:

Restructure Cord::ChunkIterator for future ring buffer support.

PiperOrigin-RevId: 346890054
GitOrigin-RevId: 0bfa836596a9c787a2f0bdc283011dd1f6810c6e
Change-Id: I3a58e2a44cb4c6f2116c43e2a4ccbc319d3ccecf

1 files changed, 27 insertions, 3 deletions

diff --git a/absl/strings/cord.h b/absl/strings/cord.h
index b9cdc435..5fc61a73 100644
--- a/absl/strings/cord.h
+++ b/absl/strings/cord.h
@@ -362,6 +362,8 @@ class Cord {
     friend class CharIterator;
 
    private:
+    using Stack = absl::InlinedVector<absl::cord_internal::CordRep*, 4>;
+
     // Constructs a `begin()` iterator from `cord`.
     explicit ChunkIterator(const Cord* cord);
 
@@ -370,6 +372,10 @@ class Cord {
     void RemoveChunkPrefix(size_t n);
     Cord AdvanceAndReadBytes(size_t n);
     void AdvanceBytes(size_t n);
+
+    // Stack specific operator++
+    ChunkIterator& AdvanceStack();
+
     // Iterates `n` bytes, where `n` is expected to be greater than or equal to
     // `current_chunk_.size()`.
     void AdvanceBytesSlowPath(size_t n);
@@ -383,8 +389,10 @@ class Cord {
     absl::cord_internal::CordRep* current_leaf_ = nullptr;
     // The number of bytes left in the `Cord` over which we are iterating.
     size_t bytes_remaining_ = 0;
-    absl::InlinedVector<absl::cord_internal::CordRep*, 4>
-        stack_of_right_children_;
+    // Context of this chunk iterator, can be one of:
+    // - monostate: iterator holds only one chunk or is empty.
+    // - Stack    : iterator holds a concat / substring tree
+    absl::variant<absl::monostate, Stack> context_;
   };
 
   // Cord::ChunkIterator::chunk_begin()
@@ -1100,13 +1108,29 @@ inline Cord::ChunkIterator::ChunkIterator(const Cord* cord)
     : bytes_remaining_(cord->size()) {
   if (cord->empty()) return;
   if (cord->contents_.is_tree()) {
-    stack_of_right_children_.push_back(cord->contents_.tree());
+    Stack& stack_of_right_children = context_.emplace<Stack>();
+    stack_of_right_children.push_back(cord->contents_.tree());
     operator++();
   } else {
     current_chunk_ = absl::string_view(cord->contents_.data(), cord->size());
   }
 }
 
+inline Cord::ChunkIterator& Cord::ChunkIterator::operator++() {
+  ABSL_HARDENING_ASSERT(bytes_remaining_ > 0 &&
+                        "Attempted to iterate past `end()`");
+  assert(bytes_remaining_ >= current_chunk_.size());
+  bytes_remaining_ -= current_chunk_.size();
+  if (bytes_remaining_ > 0) {
+    if (absl::holds_alternative<Stack>(context_)) {
+      return AdvanceStack();
+    }
+  } else {
+    current_chunk_ = {};
+  }
+  return *this;
+}
+
 inline Cord::ChunkIterator Cord::ChunkIterator::operator++(int) {
   ChunkIterator tmp(*this);
   operator++();