Export of internal Abseil changes

--
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

--
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

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

Rollback

PiperOrigin-RevId: 347078779

--
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

--
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

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

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

PiperOrigin-RevId: 347035065

--
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

5 files changed, 70 insertions, 25 deletions

diff --git a/absl/strings/CMakeLists.txt b/absl/strings/CMakeLists.txt
index 54c10151..6cdab257 100644
--- a/absl/strings/CMakeLists.txt
+++ b/absl/strings/CMakeLists.txt
@@ -56,7 +56,7 @@ absl_cc_library(
   DEPS
     absl::strings_internal
     absl::base
-    absl::bits
+    absl::internal_bits
     absl::config
     absl::core_headers
     absl::endian
@@ -406,7 +406,7 @@ absl_cc_library(
   COPTS
     ${ABSL_DEFAULT_COPTS}
   DEPS
-    absl::bits
+    absl::internal_bits
     absl::strings
     absl::config
     absl::core_headers
diff --git a/absl/strings/cord.cc b/absl/strings/cord.cc
index f475042c..f87f4882 100644
--- a/absl/strings/cord.cc
+++ b/absl/strings/cord.cc
@@ -1298,26 +1298,23 @@ void Cord::CopyToArraySlowPath(char* dst) const {
   }
 }
 
-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 (stack_of_right_children_.empty()) {
+Cord::ChunkIterator& Cord::ChunkIterator::AdvanceStack() {
+  assert(absl::holds_alternative<Stack>(context_));
+  auto& stack_of_right_children = absl::get<Stack>(context_);
+  if (stack_of_right_children.empty()) {
     assert(!current_chunk_.empty());  // Called on invalid iterator.
     // We have reached the end of the Cord.
     return *this;
   }
 
   // Process the next node on the stack.
-  CordRep* node = stack_of_right_children_.back();
-  stack_of_right_children_.pop_back();
+  CordRep* node = stack_of_right_children.back();
+  stack_of_right_children.pop_back();
 
   // Walk down the left branches until we hit a non-CONCAT node. Save the
   // right children to the stack for subsequent traversal.
   while (node->tag == CONCAT) {
-    stack_of_right_children_.push_back(node->concat()->right);
+    stack_of_right_children.push_back(node->concat()->right);
     node = node->concat()->left;
   }
 
@@ -1360,6 +1357,8 @@ Cord Cord::ChunkIterator::AdvanceAndReadBytes(size_t n) {
     }
     return subcord;
   }
+  assert(absl::holds_alternative<Stack>(context_));
+  auto& stack_of_right_children = absl::get<Stack>(context_);
   if (n < current_chunk_.size()) {
     // Range to read is a proper subrange of the current chunk.
     assert(current_leaf_ != nullptr);
@@ -1388,13 +1387,13 @@ Cord Cord::ChunkIterator::AdvanceAndReadBytes(size_t n) {
   // Process the next node(s) on the stack, reading whole subtrees depending on
   // their length and how many bytes we are advancing.
   CordRep* node = nullptr;
-  while (!stack_of_right_children_.empty()) {
-    node = stack_of_right_children_.back();
-    stack_of_right_children_.pop_back();
+  while (!stack_of_right_children.empty()) {
+    node = stack_of_right_children.back();
+    stack_of_right_children.pop_back();
     if (node->length > n) break;
     // TODO(qrczak): This might unnecessarily recreate existing concat nodes.
     // Avoiding that would need pretty complicated logic (instead of
-    // current_leaf_, keep current_subtree_ which points to the highest node
+    // current_leaf, keep current_subtree_ which points to the highest node
     // such that the current leaf can be found on the path of left children
     // starting from current_subtree_; delay creating subnode while node is
     // below current_subtree_; find the proper node along the path of left
@@ -1419,7 +1418,7 @@ Cord Cord::ChunkIterator::AdvanceAndReadBytes(size_t n) {
   while (node->tag == CONCAT) {
     if (node->concat()->left->length > n) {
       // Push right, descend left.
-      stack_of_right_children_.push_back(node->concat()->right);
+      stack_of_right_children.push_back(node->concat()->right);
       node = node->concat()->left;
     } else {
       // Read left, descend right.
@@ -1462,12 +1461,20 @@ void Cord::ChunkIterator::AdvanceBytesSlowPath(size_t n) {
   n -= current_chunk_.size();
   bytes_remaining_ -= current_chunk_.size();
 
+  if (!absl::holds_alternative<Stack>(context_)) {
+    // We have reached the end of the Cord.
+    assert(bytes_remaining_ == 0);
+    return;
+  }
+
   // Process the next node(s) on the stack, skipping whole subtrees depending on
   // their length and how many bytes we are advancing.
   CordRep* node = nullptr;
-  while (!stack_of_right_children_.empty()) {
-    node = stack_of_right_children_.back();
-    stack_of_right_children_.pop_back();
+  assert(absl::holds_alternative<Stack>(context_));
+  auto& stack_of_right_children = absl::get<Stack>(context_);
+  while (!stack_of_right_children.empty()) {
+    node = stack_of_right_children.back();
+    stack_of_right_children.pop_back();
     if (node->length > n) break;
     n -= node->length;
     bytes_remaining_ -= node->length;
@@ -1485,7 +1492,7 @@ void Cord::ChunkIterator::AdvanceBytesSlowPath(size_t n) {
   while (node->tag == CONCAT) {
     if (node->concat()->left->length > n) {
       // Push right, descend left.
-      stack_of_right_children_.push_back(node->concat()->right);
+      stack_of_right_children.push_back(node->concat()->right);
       node = node->concat()->left;
     } else {
       // Skip left, descend right.
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++();
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.
   //