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

2 files changed, 15 insertions, 1 deletions

diff --git a/absl/strings/internal/cord_internal.cc b/absl/strings/internal/cord_internal.cc
index 35f4d235..59f2e4d9 100644
--- a/absl/strings/internal/cord_internal.cc
+++ b/absl/strings/internal/cord_internal.cc
@@ -24,7 +24,10 @@ namespace absl {
 ABSL_NAMESPACE_BEGIN
 namespace cord_internal {
 
-ABSL_CONST_INIT std::atomic<bool> cord_ring_buffer_enabled(false);
+ABSL_CONST_INIT std::atomic<bool> cord_ring_buffer_enabled(
+    kCordEnableRingBufferDefault);
+ABSL_CONST_INIT std::atomic<bool> shallow_subcords_enabled(
+    kCordShallowSubcordsDefault);
 
 void CordRep::Destroy(CordRep* rep) {
   assert(rep != nullptr);
diff --git a/absl/strings/internal/cord_internal.h b/absl/strings/internal/cord_internal.h
index f1af8e6e..57e7046a 100644
--- a/absl/strings/internal/cord_internal.h
+++ b/absl/strings/internal/cord_internal.h
@@ -31,12 +31,23 @@ namespace absl {
 ABSL_NAMESPACE_BEGIN
 namespace cord_internal {
 
+// Default feature enable states for cord ring buffers
+enum CordFeatureDefaults {
+  kCordEnableRingBufferDefault = false,
+  kCordShallowSubcordsDefault = false
+};
+
 extern std::atomic<bool> cord_ring_buffer_enabled;
+extern std::atomic<bool> shallow_subcords_enabled;
 
 inline void enable_cord_ring_buffer(bool enable) {
   cord_ring_buffer_enabled.store(enable, std::memory_order_relaxed);
 }
 
+inline void enable_shallow_subcords(bool enable) {
+  shallow_subcords_enabled.store(enable, std::memory_order_relaxed);
+}
+
 enum Constants {
   // The inlined size to use with absl::InlinedVector.
   //