Export of internal Abseil changes

--
1620e8ffaa93ef24510ca60c7fff2a07248ac9f6 by Abseil Team <absl-team@google.com>:

Update comment.

PiperOrigin-RevId: 382858259

--
20db116f28469149d10e0f7f8b976cb903dd4879 by Gennadiy Rozental <rogeeff@google.com>:

Add benchmark running on multiple flags.

Update size_tester to include cost of absl::GetFlag call.
Add size_tester invocation for bool flag.

New benchmark better represent GetFlag usage.

PiperOrigin-RevId: 382820341

--
2e097ad3811c4e329f75b98877a5e74c1d3d84fd by Abseil Team <absl-team@google.com>:

Avoid 64x64->128 multiplication in absl::Hash's mix on AArch64

On AArch64, calculating a 128-bit product is inefficient, because it requires a sequence of two instructions to calculate the upper and lower halves of the result. So calculate a 64-bit product instead.

Making MultType 64-bits means the upper 32 bits of the result do not participate in shift/xor, but the add/multiply gives us sufficient mixing.

PiperOrigin-RevId: 382625931

--
f3ae3f32cb53168c8dc91b766f2932dc87cec503 by Abseil Team <absl-team@google.com>:

Remove homegrown Round implementation

absl/time/duration.cc defined a Round implementation to accommodate old
versions of MSVC that lacked std::round(long double). Abseil no longer
supports those MSVCs, so we don’t need the homegrown implementation
anymore. Remove it, and replace calls to it with std::rint.

PiperOrigin-RevId: 382605191

--
a13631c91bf5478289e1a512ce215c85501a26f7 by Martijn Vels <mvels@google.com>:

Move the Consume() conversion functions out of cord_rep_ring into cord_rep_consume.

This makes these functions generic, so we can repurpose these for the new Btree conversion functions.

PiperOrigin-RevId: 382594902

--
7394c737500c2d8371fcf913b21ad1b321ba499d by Benjamin Barenblat <bbaren@google.com>:

Remove homegrown Round implementation

absl/time/duration.cc defined a Round implementation to accommodate old
versions of MSVC that lacked std::round(long double). Abseil no longer
supports those MSVCs, so we don’t need the homegrown implementation
anymore. Remove it, and replace calls to it with std::rint.

PiperOrigin-RevId: 382569900

--
d72a761f43dc5c9b9510c3a1363177ed26646b5d by Abseil Team <absl-team@google.com>:

Prefer `getentropy` for Emscripten.

It needs a different header, so I've separated it out from the GLIBC
check above.

PiperOrigin-RevId: 382332475

--
74e261dbb467741b2ddd8b490e04c531fdd2f559 by Martijn Vels <mvels@google.com>:

Add BTREE tag for CordRepNode implementing a Btree cord.

This change only forward declared the CordRepBtree class (not implemented yet) and defines the enum value BTREE. While RING and BTREE should never co-exist, we define a new value for BTREE so as not to make transitioning between RING and BTREE harder than it needs to be. This changes shifts the FLAT value / computation from FLAT = 4 to FLAT =5

PiperOrigin-RevId: 382326710
GitOrigin-RevId: 1620e8ffaa93ef24510ca60c7fff2a07248ac9f6
Change-Id: Ia8f99dde3874808f56062bd37ab3e63764099734

13 files changed, 537 insertions, 157 deletions

diff --git a/CMake/AbseilDll.cmake b/CMake/AbseilDll.cmake
index 8ee4120f..78ace82b 100644
--- a/CMake/AbseilDll.cmake
+++ b/CMake/AbseilDll.cmake
@@ -203,6 +203,8 @@ set(ABSL_INTERNAL_DLL_FILES
   "strings/internal/charconv_parse.h"
   "strings/internal/cord_internal.cc"
   "strings/internal/cord_internal.h"
+  "strings/internal/cord_rep_consume.h"
+  "strings/internal/cord_rep_consume.cc"
   "strings/internal/cord_rep_flat.h"
   "strings/internal/cord_rep_ring.cc"
   "strings/internal/cord_rep_ring.h"
diff --git a/absl/flags/flag_benchmark.cc b/absl/flags/flag_benchmark.cc
index 57584f85..fc572d9c 100644
--- a/absl/flags/flag_benchmark.cc
+++ b/absl/flags/flag_benchmark.cc
@@ -101,7 +101,39 @@ std::string AbslUnparseFlag(const UDT&) { return ""; }
   A(AbslDuration)            \
   A(UDT)
 
-#define FLAG_DEF(T) ABSL_FLAG(T, T##_flag, {}, "");
+#define REPLICATE_0(A, T, name, index) A(T, name, index)
+#define REPLICATE_1(A, T, name, index) \
+  REPLICATE_0(A, T, name, index##0) REPLICATE_0(A, T, name, index##1)
+#define REPLICATE_2(A, T, name, index) \
+  REPLICATE_1(A, T, name, index##0) REPLICATE_1(A, T, name, index##1)
+#define REPLICATE_3(A, T, name, index) \
+  REPLICATE_2(A, T, name, index##0) REPLICATE_2(A, T, name, index##1)
+#define REPLICATE_4(A, T, name, index) \
+  REPLICATE_3(A, T, name, index##0) REPLICATE_3(A, T, name, index##1)
+#define REPLICATE_5(A, T, name, index) \
+  REPLICATE_4(A, T, name, index##0) REPLICATE_4(A, T, name, index##1)
+#define REPLICATE_6(A, T, name, index) \
+  REPLICATE_5(A, T, name, index##0) REPLICATE_5(A, T, name, index##1)
+#define REPLICATE_7(A, T, name, index) \
+  REPLICATE_6(A, T, name, index##0) REPLICATE_6(A, T, name, index##1)
+#define REPLICATE_8(A, T, name, index) \
+  REPLICATE_7(A, T, name, index##0) REPLICATE_7(A, T, name, index##1)
+#define REPLICATE_9(A, T, name, index) \
+  REPLICATE_8(A, T, name, index##0) REPLICATE_8(A, T, name, index##1)
+#if defined(_MSC_VER)
+#define REPLICATE(A, T, name) \
+  REPLICATE_7(A, T, name, 0) REPLICATE_7(A, T, name, 1)
+#define SINGLE_FLAG(T) FLAGS_##T##_flag_00000000
+#else
+#define REPLICATE(A, T, name) \
+  REPLICATE_9(A, T, name, 0) REPLICATE_9(A, T, name, 1)
+#define SINGLE_FLAG(T) FLAGS_##T##_flag_0000000000
+#endif
+#define REPLICATE_ALL(A, T, name) \
+  REPLICATE_9(A, T, name, 0) REPLICATE_9(A, T, name, 1)
+
+#define COUNT(T, name, index) +1
+constexpr size_t kNumFlags = 0 REPLICATE(COUNT, _, _);
 
 #if defined(__clang__) && defined(__linux__)
 // Force the flags used for benchmarks into a separate ELF section.
@@ -110,38 +142,87 @@ std::string AbslUnparseFlag(const UDT&) { return ""; }
 // benchmark results more reproducible across unrelated code changes.
 #pragma clang section data = ".benchmark_flags"
 #endif
+#define DEFINE_FLAG(T, name, index) ABSL_FLAG(T, name##_##index, {}, "");
+#define FLAG_DEF(T) REPLICATE(DEFINE_FLAG, T, T##_flag);
 BENCHMARKED_TYPES(FLAG_DEF)
 #if defined(__clang__) && defined(__linux__)
 #pragma clang section data = ""
 #endif
 // Register thousands of flags to bloat up the size of the registry.
 // This mimics real life production binaries.
-#define DEFINE_FLAG_0(name) ABSL_FLAG(int, name, 0, "");
-#define DEFINE_FLAG_1(name) DEFINE_FLAG_0(name##0) DEFINE_FLAG_0(name##1)
-#define DEFINE_FLAG_2(name) DEFINE_FLAG_1(name##0) DEFINE_FLAG_1(name##1)
-#define DEFINE_FLAG_3(name) DEFINE_FLAG_2(name##0) DEFINE_FLAG_2(name##1)
-#define DEFINE_FLAG_4(name) DEFINE_FLAG_3(name##0) DEFINE_FLAG_3(name##1)
-#define DEFINE_FLAG_5(name) DEFINE_FLAG_4(name##0) DEFINE_FLAG_4(name##1)
-#define DEFINE_FLAG_6(name) DEFINE_FLAG_5(name##0) DEFINE_FLAG_5(name##1)
-#define DEFINE_FLAG_7(name) DEFINE_FLAG_6(name##0) DEFINE_FLAG_6(name##1)
-#define DEFINE_FLAG_8(name) DEFINE_FLAG_7(name##0) DEFINE_FLAG_7(name##1)
-#define DEFINE_FLAG_9(name) DEFINE_FLAG_8(name##0) DEFINE_FLAG_8(name##1)
-#define DEFINE_FLAG_10(name) DEFINE_FLAG_9(name##0) DEFINE_FLAG_9(name##1)
-#define DEFINE_FLAG_11(name) DEFINE_FLAG_10(name##0) DEFINE_FLAG_10(name##1)
-#define DEFINE_FLAG_12(name) DEFINE_FLAG_11(name##0) DEFINE_FLAG_11(name##1)
-DEFINE_FLAG_12(bloat_flag_);
+#define BLOAT_FLAG(_unused1, _unused2, index) \
+  ABSL_FLAG(int, bloat_flag_##index, 0, "");
+REPLICATE_ALL(BLOAT_FLAG, _, _)
 
 namespace {
 
-#define BM_GetFlag(T)                                            \
-  void BM_GetFlag_##T(benchmark::State& state) {                 \
-    for (auto _ : state) {                                       \
-      benchmark::DoNotOptimize(absl::GetFlag(FLAGS_##T##_flag)); \
-    }                                                            \
-  }                                                              \
-  BENCHMARK(BM_GetFlag_##T)->ThreadRange(1, 16);
+#define FLAG_PTR(T, name, index) &FLAGS_##name##_##index,
+#define FLAG_PTR_ARR(T)                              \
+  static constexpr absl::Flag<T>* FlagPtrs_##T[] = { \
+      REPLICATE(FLAG_PTR, T, T##_flag)};
+BENCHMARKED_TYPES(FLAG_PTR_ARR)
+
+#define BM_SingleGetFlag(T)                                    \
+  void BM_SingleGetFlag_##T(benchmark::State& state) {         \
+    for (auto _ : state) {                                     \
+      benchmark::DoNotOptimize(absl::GetFlag(SINGLE_FLAG(T))); \
+    }                                                          \
+  }                                                            \
+  BENCHMARK(BM_SingleGetFlag_##T)->ThreadRange(1, 16);
+
+BENCHMARKED_TYPES(BM_SingleGetFlag)
+
+template <typename T>
+struct Accumulator {
+  using type = T;
+};
+template <>
+struct Accumulator<String> {
+  using type = size_t;
+};
+template <>
+struct Accumulator<VectorOfStrings> {
+  using type = size_t;
+};
+template <>
+struct Accumulator<OptionalInt> {
+  using type = bool;
+};
+template <>
+struct Accumulator<OptionalString> {
+  using type = bool;
+};
+template <>
+struct Accumulator<UDT> {
+  using type = bool;
+};
+
+template <typename T>
+void Accumulate(typename Accumulator<T>::type& a, const T& f) {
+  a += f;
+}
+void Accumulate(bool& a, bool f) { a = a || f; }
+void Accumulate(size_t& a, const std::string& f) { a += f.size(); }
+void Accumulate(size_t& a, const std::vector<std::string>& f) { a += f.size(); }
+void Accumulate(bool& a, const OptionalInt& f) { a |= f.has_value(); }
+void Accumulate(bool& a, const OptionalString& f) { a |= f.has_value(); }
+void Accumulate(bool& a, const UDT& f) {
+  a |= reinterpret_cast<int64_t>(&f) & 0x1;
+}
+
+#define BM_ManyGetFlag(T)                            \
+  void BM_ManyGetFlag_##T(benchmark::State& state) { \
+    Accumulator<T>::type res = {};                   \
+    while (state.KeepRunningBatch(kNumFlags)) {      \
+      for (auto* flag_ptr : FlagPtrs_##T) {          \
+        Accumulate(res, absl::GetFlag(*flag_ptr));   \
+      }                                              \
+    }                                                \
+    benchmark::DoNotOptimize(res);                   \
+  }                                                  \
+  BENCHMARK(BM_ManyGetFlag_##T)->ThreadRange(1, 8);
 
-BENCHMARKED_TYPES(BM_GetFlag)
+BENCHMARKED_TYPES(BM_ManyGetFlag)
 
 void BM_ThreadedFindCommandLineFlag(benchmark::State& state) {
   char dummy[] = "dummy";
@@ -150,17 +231,18 @@ void BM_ThreadedFindCommandLineFlag(benchmark::State& state) {
   // is finalized.
   absl::ParseCommandLine(1, argv);
 
-  for (auto s : state) {
-    benchmark::DoNotOptimize(
-        absl::FindCommandLineFlag("bloat_flag_010101010101"));
+  while (state.KeepRunningBatch(kNumFlags)) {
+    for (auto* flag_ptr : FlagPtrs_bool) {
+      benchmark::DoNotOptimize(absl::FindCommandLineFlag(flag_ptr->Name()));
+    }
   }
 }
 BENCHMARK(BM_ThreadedFindCommandLineFlag)->ThreadRange(1, 16);
 
 }  // namespace
 
-#define InvokeGetFlag(T)                                               \
-  T AbslInvokeGetFlag##T() { return absl::GetFlag(FLAGS_##T##_flag); } \
+#define InvokeGetFlag(T)                                             \
+  T AbslInvokeGetFlag##T() { return absl::GetFlag(SINGLE_FLAG(T)); } \
   int odr##T = (benchmark::DoNotOptimize(AbslInvokeGetFlag##T), 1);
 
 BENCHMARKED_TYPES(InvokeGetFlag)
diff --git a/absl/hash/internal/hash.h b/absl/hash/internal/hash.h
index 69dbbc6b..90627e00 100644
--- a/absl/hash/internal/hash.h
+++ b/absl/hash/internal/hash.h
@@ -856,8 +856,15 @@ class ABSL_DLL MixingHashState : public HashStateBase<MixingHashState> {
   }
 
   ABSL_ATTRIBUTE_ALWAYS_INLINE static uint64_t Mix(uint64_t state, uint64_t v) {
+#if defined(__aarch64__)
+    // On AArch64, calculating a 128-bit product is inefficient, because it
+    // requires a sequence of two instructions to calculate the upper and lower
+    // halves of the result.
+    using MultType = uint64_t;
+#else
     using MultType =
         absl::conditional_t<sizeof(size_t) == 4, uint64_t, uint128>;
+#endif
     // We do the addition in 64-bit space to make sure the 128-bit
     // multiplication is fast. If we were to do it as MultType the compiler has
     // to assume that the high word is non-zero and needs to perform 2
diff --git a/absl/random/internal/seed_material.cc b/absl/random/internal/seed_material.cc
index 7c1d9efa..c03cad85 100644
--- a/absl/random/internal/seed_material.cc
+++ b/absl/random/internal/seed_material.cc
@@ -57,6 +57,12 @@
 #define ABSL_RANDOM_USE_GET_ENTROPY 1
 #endif
 
+#if defined(__EMSCRIPTEN__)
+#include <sys/random.h>
+// Emscripten has getentropy, but it resides in a different header.
+#define ABSL_RANDOM_USE_GET_ENTROPY 1
+#endif
+
 #if defined(ABSL_RANDOM_USE_BCRYPT)
 #include <bcrypt.h>
 
diff --git a/absl/strings/BUILD.bazel b/absl/strings/BUILD.bazel
index 1cb5b3e5..b70aac3e 100644
--- a/absl/strings/BUILD.bazel
+++ b/absl/strings/BUILD.bazel
@@ -269,10 +269,12 @@ cc_library(
     name = "cord_internal",
     srcs = [
         "internal/cord_internal.cc",
+        "internal/cord_rep_consume.cc",
         "internal/cord_rep_ring.cc",
     ],
     hdrs = [
         "internal/cord_internal.h",
+        "internal/cord_rep_consume.h",
         "internal/cord_rep_flat.h",
         "internal/cord_rep_ring.h",
         "internal/cord_rep_ring_reader.h",
@@ -292,6 +294,7 @@ cc_library(
         "//absl/container:compressed_tuple",
         "//absl/container:inlined_vector",
         "//absl/container:layout",
+        "//absl/functional:function_ref",
         "//absl/meta:type_traits",
     ],
 )
@@ -650,6 +653,22 @@ cc_test(
 )
 
 cc_test(
+    name = "cord_rep_consume_test",
+    size = "medium",
+    srcs = ["internal/cord_rep_consume_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    visibility = ["//visibility:private"],
+    deps = [
+        ":cord_internal",
+        ":strings",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/debugging:leak_check",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
+
+cc_test(
     name = "cord_ring_test",
     size = "medium",
     srcs = ["cord_ring_test.cc"],
diff --git a/absl/strings/CMakeLists.txt b/absl/strings/CMakeLists.txt
index 0246dc38..d79ef712 100644
--- a/absl/strings/CMakeLists.txt
+++ b/absl/strings/CMakeLists.txt
@@ -555,11 +555,13 @@ absl_cc_library(
     cord_internal
   HDRS
     "internal/cord_internal.h"
+    "internal/cord_rep_consume.h"
     "internal/cord_rep_flat.h"
     "internal/cord_rep_ring.h"
     "internal/cord_rep_ring_reader.h"
   SRCS
     "internal/cord_internal.cc"
+    "internal/cord_rep_consume.cc"
     "internal/cord_rep_ring.cc"
   COPTS
     ${ABSL_DEFAULT_COPTS}
@@ -904,6 +906,24 @@ absl_cc_test(
 
 absl_cc_test(
   NAME
+    cord_rep_consume_test
+  SRCS
+    "internal/cord_rep_consume_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::base
+    absl::config
+    absl::cord_internal
+    absl::core_headers
+    absl::function_ref
+    absl::raw_logging_internal
+    absl::strings
+    GTest::gmock_main
+)
+
+absl_cc_test(
+  NAME
     cord_ring_test
   SRCS
     "cord_ring_test.cc"
diff --git a/absl/strings/internal/cord_internal.h b/absl/strings/internal/cord_internal.h
index 1e2436ba..bddc9815 100644
--- a/absl/strings/internal/cord_internal.h
+++ b/absl/strings/internal/cord_internal.h
@@ -150,22 +150,24 @@ struct CordRepExternal;
 struct CordRepFlat;
 struct CordRepSubstring;
 class CordRepRing;
+class CordRepBtree;
 
 // Various representations that we allow
 enum CordRepKind {
   CONCAT = 0,
   SUBSTRING = 1,
-  RING = 2,
-  EXTERNAL = 3,
+  BTREE = 2,
+  RING = 3,
+  EXTERNAL = 4,
 
   // We have different tags for different sized flat arrays,
-  // starting with FLAT, and limited to MAX_FLAT_TAG. The 224 value is based on
+  // starting with FLAT, and limited to MAX_FLAT_TAG. The 225 value is based on
   // the current 'size to tag' encoding of 8 / 32 bytes. If a new tag is needed
   // in the future, then 'FLAT' and 'MAX_FLAT_TAG' should be adjusted as well
   // as the Tag <---> Size logic so that FLAT stil represents the minimum flat
   // allocation size. (32 bytes as of now).
-  FLAT = 4,
-  MAX_FLAT_TAG = 224
+  FLAT = 5,
+  MAX_FLAT_TAG = 225
 };
 
 // There are various locations where we want to check if some rep is a 'plain'
@@ -175,8 +177,9 @@ enum CordRepKind {
 // so likewise align RING to EXTERNAL.
 // Note that we can leave this optimization to the compiler. The compiler will
 // DTRT when it sees a condition like `tag == EXTERNAL || tag >= FLAT`.
-static_assert(EXTERNAL == RING + 1, "RING and EXTERNAL values not consecutive");
-static_assert(FLAT == EXTERNAL + 1, "EXTERNAL and FLAT values not consecutive");
+static_assert(RING == BTREE + 1, "BTREE and RING not consecutive");
+static_assert(EXTERNAL == RING + 1, "BTREE and EXTERNAL not consecutive");
+static_assert(FLAT == EXTERNAL + 1, "EXTERNAL and FLAT not consecutive");
 
 struct CordRep {
   CordRep() = default;
@@ -203,6 +206,9 @@ struct CordRep {
   inline CordRepFlat* flat();
   inline const CordRepFlat* flat() const;
 
+  inline CordRepBtree* btree();
+  inline const CordRepBtree* btree() const;
+
   // --------------------------------------------------------------------
   // Memory management
 
diff --git a/absl/strings/internal/cord_rep_consume.cc b/absl/strings/internal/cord_rep_consume.cc
new file mode 100644
index 00000000..81514543
--- /dev/null
+++ b/absl/strings/internal/cord_rep_consume.cc
@@ -0,0 +1,129 @@
+// Copyright 2021 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/strings/internal/cord_rep_consume.h"
+
+#include <array>
+#include <utility>
+
+#include "absl/container/inlined_vector.h"
+#include "absl/functional/function_ref.h"
+#include "absl/strings/internal/cord_internal.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace cord_internal {
+
+namespace {
+
+// Unrefs the provided `substring`, and returns `substring->child`
+// Adds or assumes a reference on `substring->child`
+CordRep* ClipSubstring(CordRepSubstring* substring) {
+  CordRep* child = substring->child;
+  if (substring->refcount.IsOne()) {
+    delete substring;
+  } else {
+    CordRep::Ref(child);
+    CordRep::Unref(substring);
+  }
+  return child;
+}
+
+// Unrefs the provided `concat`, and returns `{concat->left, concat->right}`
+// Adds or assumes a reference on `concat->left` and `concat->right`.
+// Returns an array of 2 elements containing the left and right nodes.
+std::array<CordRep*, 2> ClipConcat(CordRepConcat* concat) {
+  std::array<CordRep*, 2> result{concat->left, concat->right};
+  if (concat->refcount.IsOne()) {
+    delete concat;
+  } else {
+    CordRep::Ref(result[0]);
+    CordRep::Ref(result[1]);
+    CordRep::Unref(concat);
+  }
+  return result;
+}
+
+void Consume(bool forward, CordRep* rep, ConsumeFn consume_fn) {
+  size_t offset = 0;
+  size_t length = rep->length;
+  struct Entry {
+    CordRep* rep;
+    size_t offset;
+    size_t length;
+  };
+  absl::InlinedVector<Entry, 40> stack;
+
+  for (;;) {
+    if (rep->tag == CONCAT) {
+      std::array<CordRep*, 2> res = ClipConcat(rep->concat());
+      CordRep* left = res[0];
+      CordRep* right = res[1];
+
+      if (left->length <= offset) {
+        // Don't need left node
+        offset -= left->length;
+        CordRep::Unref(left);
+        rep = right;
+        continue;
+      }
+
+      size_t length_left = left->length - offset;
+      if (length_left >= length) {
+        // Don't need right node
+        CordRep::Unref(right);
+        rep = left;
+        continue;
+      }
+
+      // Need both nodes
+      size_t length_right = length - length_left;
+      if (forward) {
+        stack.push_back({right, 0, length_right});
+        rep = left;
+        length = length_left;
+      } else {
+        stack.push_back({left, offset, length_left});
+        rep = right;
+        offset = 0;
+        length = length_right;
+      }
+    } else if (rep->tag == SUBSTRING) {
+      offset += rep->substring()->start;
+      rep = ClipSubstring(rep->substring());
+    } else {
+      consume_fn(rep, offset, length);
+      if (stack.empty()) return;
+
+      rep = stack.back().rep;
+      offset = stack.back().offset;
+      length = stack.back().length;
+      stack.pop_back();
+    }
+  }
+}
+
+}  // namespace
+
+void Consume(CordRep* rep, ConsumeFn consume_fn) {
+  return Consume(true, rep, std::move(consume_fn));
+}
+
+void ReverseConsume(CordRep* rep, ConsumeFn consume_fn) {
+  return Consume(false, rep, std::move(consume_fn));
+}
+
+}  // namespace cord_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/strings/internal/cord_rep_consume.h b/absl/strings/internal/cord_rep_consume.h
new file mode 100644
index 00000000..d46fca2b
--- /dev/null
+++ b/absl/strings/internal/cord_rep_consume.h
@@ -0,0 +1,50 @@
+// Copyright 2021 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_STRINGS_INTERNAL_CORD_REP_CONSUME_H_
+#define ABSL_STRINGS_INTERNAL_CORD_REP_CONSUME_H_
+
+#include <functional>
+
+#include "absl/functional/function_ref.h"
+#include "absl/strings/internal/cord_internal.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace cord_internal {
+
+// Functor for the Consume() and ReverseConsume() functions:
+//   void ConsumeFunc(CordRep* rep, size_t offset, size_t length);
+// See the Consume() and ReverseConsume() function comments for documentation.
+using ConsumeFn = FunctionRef<void(CordRep*, size_t, size_t)>;
+
+// Consume() and ReverseConsume() consume CONCAT based trees and invoke the
+// provided functor with the contained nodes in the proper forward or reverse
+// order, which is used to convert CONCAT trees into other tree or cord data.
+// All CONCAT and SUBSTRING nodes are processed internally. The 'offset`
+// parameter of the functor is non-zero for any nodes below SUBSTRING nodes.
+// It's up to the caller to form these back into SUBSTRING nodes or otherwise
+// store offset / prefix information. These functions are intended to be used
+// only for migration / transitional code where due to factors such as ODR
+// violations, we can not 100% guarantee that all code respects 'new format'
+// settings and flags, so we need to be able to parse old data on the fly until
+// all old code is deprecated / no longer the default format.
+void Consume(CordRep* rep, ConsumeFn consume_fn);
+void ReverseConsume(CordRep* rep, ConsumeFn consume_fn);
+
+}  // namespace cord_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_STRINGS_INTERNAL_CORD_REP_CONSUME_H_
diff --git a/absl/strings/internal/cord_rep_consume_test.cc b/absl/strings/internal/cord_rep_consume_test.cc
new file mode 100644
index 00000000..e507824b
--- /dev/null
+++ b/absl/strings/internal/cord_rep_consume_test.cc
@@ -0,0 +1,173 @@
+// Copyright 2021 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/strings/internal/cord_rep_consume.h"
+
+#include <functional>
+#include <utility>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/strings/internal/cord_internal.h"
+#include "absl/strings/internal/cord_rep_flat.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace cord_internal {
+namespace {
+
+using testing::InSequence;
+using testing::MockFunction;
+
+// Returns the depth of a node
+int Depth(const CordRep* rep) {
+  return (rep->tag == CONCAT) ? rep->concat()->depth() : 0;
+}
+
+// Creates a concatenation of the specified nodes.
+CordRepConcat* CreateConcat(CordRep* left, CordRep* right) {
+  auto* concat = new CordRepConcat();
+  concat->tag = CONCAT;
+  concat->left = left;
+  concat->right = right;
+  concat->length = left->length + right->length;
+  concat->set_depth(1 + (std::max)(Depth(left), Depth(right)));
+  return concat;
+}
+
+// Creates a flat with the length set to `length`
+CordRepFlat* CreateFlatWithLength(size_t length) {
+  auto* flat = CordRepFlat::New(length);
+  flat->length = length;
+  return flat;
+}
+
+// Creates a substring node on the specified child.
+CordRepSubstring* CreateSubstring(CordRep* child, size_t start, size_t length) {
+  auto* rep = new CordRepSubstring();
+  rep->length = length;
+  rep->tag = SUBSTRING;
+  rep->start = start;
+  rep->child = child;
+  return rep;
+}
+
+// Flats we use in the tests
+CordRep* flat[6];
+
+// Creates a test tree
+CordRep* CreateTestTree() {
+  flat[0] = CreateFlatWithLength(1);
+  flat[1] = CreateFlatWithLength(7);
+  CordRepConcat* left = CreateConcat(flat[0], CreateSubstring(flat[1], 2, 4));
+
+  flat[2] = CreateFlatWithLength(9);
+  flat[3] = CreateFlatWithLength(13);
+  CordRepConcat* right1 = CreateConcat(flat[2], flat[3]);
+
+  flat[4] = CreateFlatWithLength(15);
+  flat[5] = CreateFlatWithLength(19);
+  CordRepConcat* right2 = CreateConcat(flat[4], flat[5]);
+
+  CordRepConcat* right = CreateConcat(right1, CreateSubstring(right2, 5, 17));
+  return CreateConcat(left, right);
+}
+
+TEST(CordRepConsumeTest, Consume) {
+  InSequence in_sequence;
+  CordRep* tree = CreateTestTree();
+  MockFunction<void(CordRep*, size_t, size_t)> consume;
+  EXPECT_CALL(consume, Call(flat[0], 0, 1));
+  EXPECT_CALL(consume, Call(flat[1], 2, 4));
+  EXPECT_CALL(consume, Call(flat[2], 0, 9));
+  EXPECT_CALL(consume, Call(flat[3], 0, 13));
+  EXPECT_CALL(consume, Call(flat[4], 5, 10));
+  EXPECT_CALL(consume, Call(flat[5], 0, 7));
+  Consume(tree, consume.AsStdFunction());
+  for (CordRep* rep : flat) {
+    EXPECT_TRUE(rep->refcount.IsOne());
+    CordRep::Unref(rep);
+  }
+}
+
+TEST(CordRepConsumeTest, ConsumeShared) {
+  InSequence in_sequence;
+  CordRep* tree = CreateTestTree();
+  MockFunction<void(CordRep*, size_t, size_t)> consume;
+  EXPECT_CALL(consume, Call(flat[0], 0, 1));
+  EXPECT_CALL(consume, Call(flat[1], 2, 4));
+  EXPECT_CALL(consume, Call(flat[2], 0, 9));
+  EXPECT_CALL(consume, Call(flat[3], 0, 13));
+  EXPECT_CALL(consume, Call(flat[4], 5, 10));
+  EXPECT_CALL(consume, Call(flat[5], 0, 7));
+  Consume(CordRep::Ref(tree), consume.AsStdFunction());
+  for (CordRep* rep : flat) {
+    EXPECT_FALSE(rep->refcount.IsOne());
+    CordRep::Unref(rep);
+  }
+  CordRep::Unref(tree);
+}
+
+TEST(CordRepConsumeTest, Reverse) {
+  InSequence in_sequence;
+  CordRep* tree = CreateTestTree();
+  MockFunction<void(CordRep*, size_t, size_t)> consume;
+  EXPECT_CALL(consume, Call(flat[5], 0, 7));
+  EXPECT_CALL(consume, Call(flat[4], 5, 10));
+  EXPECT_CALL(consume, Call(flat[3], 0, 13));
+  EXPECT_CALL(consume, Call(flat[2], 0, 9));
+  EXPECT_CALL(consume, Call(flat[1], 2, 4));
+  EXPECT_CALL(consume, Call(flat[0], 0, 1));
+  ReverseConsume(tree, consume.AsStdFunction());
+  for (CordRep* rep : flat) {
+    EXPECT_TRUE(rep->refcount.IsOne());
+    CordRep::Unref(rep);
+  }
+}
+
+TEST(CordRepConsumeTest, ReverseShared) {
+  InSequence in_sequence;
+  CordRep* tree = CreateTestTree();
+  MockFunction<void(CordRep*, size_t, size_t)> consume;
+  EXPECT_CALL(consume, Call(flat[5], 0, 7));
+  EXPECT_CALL(consume, Call(flat[4], 5, 10));
+  EXPECT_CALL(consume, Call(flat[3], 0, 13));
+  EXPECT_CALL(consume, Call(flat[2], 0, 9));
+  EXPECT_CALL(consume, Call(flat[1], 2, 4));
+  EXPECT_CALL(consume, Call(flat[0], 0, 1));
+  ReverseConsume(CordRep::Ref(tree), consume.AsStdFunction());
+  for (CordRep* rep : flat) {
+    EXPECT_FALSE(rep->refcount.IsOne());
+    CordRep::Unref(rep);
+  }
+  CordRep::Unref(tree);
+}
+
+TEST(CordRepConsumeTest, UnreachableFlat) {
+  InSequence in_sequence;
+  CordRepFlat* flat1 = CreateFlatWithLength(10);
+  CordRepFlat* flat2 = CreateFlatWithLength(20);
+  CordRepConcat* concat = CreateConcat(flat1, flat2);
+  CordRepSubstring* tree = CreateSubstring(concat, 15, 10);
+  MockFunction<void(CordRep*, size_t, size_t)> consume;
+  EXPECT_CALL(consume, Call(flat2, 5, 10));
+  Consume(tree, consume.AsStdFunction());
+  EXPECT_TRUE(flat2->refcount.IsOne());
+  CordRep::Unref(flat2);
+}
+
+}  // namespace
+}  // namespace cord_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/strings/internal/cord_rep_flat.h b/absl/strings/internal/cord_rep_flat.h
index a98aa9df..cddb282f 100644
--- a/absl/strings/internal/cord_rep_flat.h
+++ b/absl/strings/internal/cord_rep_flat.h
@@ -44,11 +44,11 @@ static constexpr size_t kMaxFlatLength = kMaxFlatSize - kFlatOverhead;
 static constexpr size_t kMinFlatLength = kMinFlatSize - kFlatOverhead;
 
 constexpr uint8_t AllocatedSizeToTagUnchecked(size_t size) {
-  return static_cast<uint8_t>((size <= 1024) ? size / 8
-                                             : 128 + size / 32 - 1024 / 32);
+  return static_cast<uint8_t>((size <= 1024) ? size / 8 + 1
+                                             : 129 + size / 32 - 1024 / 32);
 }
 
-static_assert(kMinFlatSize / 8 >= FLAT, "");
+static_assert(kMinFlatSize / 8 + 1 >= FLAT, "");
 static_assert(AllocatedSizeToTagUnchecked(kMaxFlatSize) <= MAX_FLAT_TAG, "");
 
 // Helper functions for rounded div, and rounding to exact sizes.
@@ -73,7 +73,7 @@ inline uint8_t AllocatedSizeToTag(size_t size) {
 
 // Converts the provided tag to the corresponding allocated size
 constexpr size_t TagToAllocatedSize(uint8_t tag) {
-  return (tag <= 128) ? (tag * 8) : (1024 + (tag - 128) * 32);
+  return (tag <= 129) ? ((tag - 1) * 8) : (1024 + (tag - 129) * 32);
 }
 
 // Converts the provided tag to the corresponding available data length
@@ -82,7 +82,7 @@ constexpr size_t TagToLength(uint8_t tag) {
 }
 
 // Enforce that kMaxFlatSize maps to a well-known exact tag value.
-static_assert(TagToAllocatedSize(224) == kMaxFlatSize, "Bad tag logic");
+static_assert(TagToAllocatedSize(225) == kMaxFlatSize, "Bad tag logic");
 
 struct CordRepFlat : public CordRep {
   // Creates a new flat node.
diff --git a/absl/strings/internal/cord_rep_ring.cc b/absl/strings/internal/cord_rep_ring.cc
index f78c94e1..20a6fc2b 100644
--- a/absl/strings/internal/cord_rep_ring.cc
+++ b/absl/strings/internal/cord_rep_ring.cc
@@ -26,6 +26,7 @@
 #include "absl/base/macros.h"
 #include "absl/container/inlined_vector.h"
 #include "absl/strings/internal/cord_internal.h"
+#include "absl/strings/internal/cord_rep_consume.h"
 #include "absl/strings/internal/cord_rep_flat.h"
 
 namespace absl {
@@ -49,14 +50,6 @@ inline void CheckCapacity(size_t n, size_t extra) {
   }
 }
 
-// Removes a reference from `rep` only.
-// Asserts that the refcount after decrement is not zero.
-inline bool UnrefNeverOne(CordRep* rep) {
-  bool result = rep->refcount.Decrement();
-  assert(result);
-  return result;
-}
-
 // Creates a flat from the provided string data, allocating up to `extra`
 // capacity in the returned flat depending on kMaxFlatLength limitations.
 // Requires `len` to be less or equal to `kMaxFlatLength`
@@ -68,40 +61,6 @@ CordRepFlat* CreateFlat(const char* s, size_t n, size_t extra = 0) {  // NOLINT
   return rep;
 }
 
-// Unrefs the provided `substring`, and returns `substring->child`
-// Adds or assumes a reference on `substring->child`
-CordRep* ClipSubstring(CordRepSubstring* substring) {
-  CordRep* child = substring->child;
-  if (substring->refcount.IsOne()) {
-    delete substring;
-  } else {
-    CordRep::Ref(child);
-    if (ABSL_PREDICT_FALSE(!substring->refcount.Decrement())) {
-      UnrefNeverOne(child);
-      delete substring;
-    }
-  }
-  return child;
-}
-
-// Unrefs the provided `concat`, and returns `{concat->left, concat->right}`
-// Adds or assumes a reference on `concat->left` and `concat->right`.
-std::pair<CordRep*, CordRep*> ClipConcat(CordRepConcat* concat) {
-  auto result = std::make_pair(concat->left, concat->right);
-  if (concat->refcount.IsOne()) {
-    delete concat;
-  } else {
-    CordRep::Ref(result.first);
-    CordRep::Ref(result.second);
-    if (ABSL_PREDICT_FALSE(!concat->refcount.Decrement())) {
-      UnrefNeverOne(result.first);
-      UnrefNeverOne(result.second);
-      delete concat;
-    }
-  }
-  return result;
-}
-
 // Unrefs the entries in `[head, tail)`.
 // Requires all entries to be a FLAT or EXTERNAL node.
 void UnrefEntries(const CordRepRing* rep, index_type head, index_type tail) {
@@ -117,79 +76,6 @@ void UnrefEntries(const CordRepRing* rep, index_type head, index_type tail) {
   });
 }
 
-template <typename F>
-void Consume(Direction direction, CordRep* rep, F&& fn) {
-  size_t offset = 0;
-  size_t length = rep->length;
-  struct Entry {
-    CordRep* rep;
-    size_t offset;
-    size_t length;
-  };
-  absl::InlinedVector<Entry, 40> stack;
-
-  for (;;) {
-    if (rep->tag >= FLAT || rep->tag == EXTERNAL || rep->tag == RING) {
-      fn(rep, offset, length);
-      if (stack.empty()) return;
-
-      rep = stack.back().rep;
-      offset = stack.back().offset;
-      length = stack.back().length;
-      stack.pop_back();
-    } else if (rep->tag == SUBSTRING) {
-      offset += rep->substring()->start;
-      rep = ClipSubstring(rep->substring());
-    } else if (rep->tag == CONCAT) {
-      auto res = ClipConcat(rep->concat());
-      CordRep* left = res.first;
-      CordRep* right = res.second;
-
-      if (left->length <= offset) {
-        // Don't need left node
-        offset -= left->length;
-        CordRep::Unref(left);
-        rep = right;
-        continue;
-      }
-
-      size_t length_left = left->length - offset;
-      if (length_left >= length) {
-        // Don't need right node
-        CordRep::Unref(right);
-        rep = left;
-        continue;
-      }
-
-      // Need both nodes
-      size_t length_right = length - length_left;
-      if (direction == Direction::kReversed) {
-        stack.push_back({left, offset, length_left});
-        rep = right;
-        offset = 0;
-        length = length_right;
-      } else {
-        stack.push_back({right, 0, length_right});
-        rep = left;
-        length = length_left;
-      }
-    } else {
-      assert("Valid tag" == nullptr);
-      return;
-    }
-  }
-}
-
-template <typename F>
-void Consume(CordRep* rep, F&& fn) {
-  return Consume(Direction::kForward, rep, std::forward<F>(fn));
-}
-
-template <typename F>
-void RConsume(CordRep* rep, F&& fn) {
-  return Consume(Direction::kReversed, rep, std::forward<F>(fn));
-}
-
 }  // namespace
 
 std::ostream& operator<<(std::ostream& s, const CordRepRing& rep) {
@@ -581,7 +467,7 @@ CordRepRing* CordRepRing::Append(CordRepRing* rep, CordRep* child) {
 }
 
 CordRepRing* CordRepRing::PrependSlow(CordRepRing* rep, CordRep* child) {
-  RConsume(child, [&](CordRep* child_arg, size_t offset, size_t len) {
+  ReverseConsume(child, [&](CordRep* child_arg, size_t offset, size_t len) {
     if (IsFlatOrExternal(child_arg)) {
       rep = PrependLeaf(rep, child_arg, offset, len);
     } else {
diff --git a/absl/strings/internal/str_split_internal.h b/absl/strings/internal/str_split_internal.h
index 17c1bfe8..e7664216 100644
--- a/absl/strings/internal/str_split_internal.h
+++ b/absl/strings/internal/str_split_internal.h
@@ -64,7 +64,7 @@ class ConvertibleToStringView {
   ConvertibleToStringView(const std::string& s)  // NOLINT(runtime/explicit)
       : value_(s) {}
 
-  // Matches rvalue strings and moves their data to a member.
+  // Disable conversion from rvalue strings.
   ConvertibleToStringView(std::string&& s) = delete;
   ConvertibleToStringView(const std::string&& s) = delete;