15 files changed, 245 insertions, 24 deletions

diff --git a/CMake/AbseilDll.cmake b/CMake/AbseilDll.cmake
index 4f7a287c..0b5f0a54 100644
--- a/CMake/AbseilDll.cmake
+++ b/CMake/AbseilDll.cmake
@@ -26,6 +26,7 @@ set(ABSL_INTERNAL_DLL_FILES
   "base/internal/low_level_alloc.h"
   "base/internal/low_level_scheduling.h"
   "base/internal/per_thread_tls.h"
+  "base/internal/prefetch.h"
   "base/internal/pretty_function.h"
   "base/internal/raw_logging.cc"
   "base/internal/raw_logging.h"
diff --git a/absl/base/BUILD.bazel b/absl/base/BUILD.bazel
index 6f2f09b9..49e3a95f 100644
--- a/absl/base/BUILD.bazel
+++ b/absl/base/BUILD.bazel
@@ -704,6 +704,31 @@ cc_test(
     ],
 )
 
+cc_library(
+    name = "prefetch",
+    hdrs = ["internal/prefetch.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        "//absl:__subpackages__",
+    ],
+    deps = [
+        ":config",
+    ],
+)
+
+cc_test(
+    name = "prefetch_test",
+    size = "small",
+    srcs = ["internal/prefetch_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":prefetch",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
+
 cc_test(
     name = "unique_small_name_test",
     size = "small",
diff --git a/absl/base/CMakeLists.txt b/absl/base/CMakeLists.txt
index 7e600f0b..0e1e0413 100644
--- a/absl/base/CMakeLists.txt
+++ b/absl/base/CMakeLists.txt
@@ -642,6 +642,32 @@ absl_cc_test(
     GTest::gtest_main
 )
 
+# Internal-only target, do not depend on directly.
+absl_cc_library(
+  NAME
+    prefetch
+  HDRS
+    "internal/prefetch.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::config
+)
+
+absl_cc_test(
+  NAME
+    prefetch_test
+  SRCS
+    "internal/prefetch_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::prefetch
+    GTest::gtest_main
+)
+
 absl_cc_test(
   NAME
     optimization_test
diff --git a/absl/base/attributes.h b/absl/base/attributes.h
index c71038c7..e4e7a3d8 100644
--- a/absl/base/attributes.h
+++ b/absl/base/attributes.h
@@ -682,9 +682,18 @@
 // not compile (on supported platforms) unless the variable has a constant
 // initializer. This is useful for variables with static and thread storage
 // duration, because it guarantees that they will not suffer from the so-called
-// "static init order fiasco".  Prefer to put this attribute on the most visible
-// declaration of the variable, if there's more than one, because code that
-// accesses the variable can then use the attribute for optimization.
+// "static init order fiasco".
+//
+// This attribute must be placed on the initializing declaration of the
+// variable. Some compilers will give a -Wmissing-constinit warning when this
+// attribute is placed on some other declaration but missing from the
+// initializing declaration.
+//
+// In some cases (notably with thread_local variables), `ABSL_CONST_INIT` can
+// also be used in a non-initializing declaration to tell the compiler that a
+// variable is already initialized, reducing overhead that would otherwise be
+// incurred by a hidden guard variable. Thus annotating all declarations with
+// this attribute is recommended to potentially enhance optimization.
 //
 // Example:
 //
@@ -693,14 +702,19 @@
 //     ABSL_CONST_INIT static MyType my_var;
 //   };
 //
-//   MyType MyClass::my_var = MakeMyType(...);
+//   ABSL_CONST_INIT MyType MyClass::my_var = MakeMyType(...);
+//
+// For code or headers that are assured to only build with C++20 and up, prefer
+// just using the standard `constinit` keyword directly over this macro.
 //
 // Note that this attribute is redundant if the variable is declared constexpr.
-#if ABSL_HAVE_CPP_ATTRIBUTE(clang::require_constant_initialization)
+#if defined(__cpp_constinit) && __cpp_constinit >= 201907L
+#define ABSL_CONST_INIT constinit
+#elif ABSL_HAVE_CPP_ATTRIBUTE(clang::require_constant_initialization)
 #define ABSL_CONST_INIT [[clang::require_constant_initialization]]
 #else
 #define ABSL_CONST_INIT
-#endif  // ABSL_HAVE_CPP_ATTRIBUTE(clang::require_constant_initialization)
+#endif
 
 // ABSL_ATTRIBUTE_PURE_FUNCTION
 //
diff --git a/absl/base/internal/prefetch.h b/absl/base/internal/prefetch.h
new file mode 100644
index 00000000..a71b3897
--- /dev/null
+++ b/absl/base/internal/prefetch.h
@@ -0,0 +1,109 @@
+// Copyright 2022 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_BASE_INTERNAL_PREFETCH_H_
+#define ABSL_BASE_INTERNAL_PREFETCH_H_
+
+#include "absl/base/config.h"
+
+// Compatibility wrappers around __builtin_prefetch, to prefetch data
+// for read if supported by the toolchain.
+
+// Move data into the cache before it is read, or "prefetch" it.
+//
+// The value of `addr` is the address of the memory to prefetch. If
+// the target and compiler support it, data prefetch instructions are
+// generated. If the prefetch is done some time before the memory is
+// read, it may be in the cache by the time the read occurs.
+//
+// The function names specify the temporal locality heuristic applied,
+// using the names of Intel prefetch instructions:
+//
+//   T0 - high degree of temporal locality; data should be left in as
+//        many levels of the cache possible
+//   T1 - moderate degree of temporal locality
+//   T2 - low degree of temporal locality
+//   Nta - no temporal locality, data need not be left in the cache
+//         after the read
+//
+// Incorrect or gratuitous use of these functions can degrade
+// performance, so use them only when representative benchmarks show
+// an improvement.
+//
+// Example usage:
+//
+//   absl::base_internal::PrefetchT0(addr);
+//
+// Currently, the different prefetch calls behave on some Intel
+// architectures as follows:
+//
+//                 SNB..SKL   SKX
+// PrefetchT0()   L1/L2/L3  L1/L2
+// PrefetchT1()      L2/L3     L2
+// PrefetchT2()      L2/L3     L2
+// PrefetchNta()  L1/--/L3  L1*
+//
+// * On SKX PrefetchNta() will bring the line into L1 but will evict
+//   from L3 cache. This might result in surprising behavior.
+//
+// SNB = Sandy Bridge, SKL = Skylake, SKX = Skylake Xeon.
+//
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace base_internal {
+
+void PrefetchT0(const void* addr);
+void PrefetchT1(const void* addr);
+void PrefetchT2(const void* addr);
+void PrefetchNta(const void* addr);
+
+// Implementation details follow.
+
+#if ABSL_HAVE_BUILTIN(__builtin_prefetch) || defined(__GNUC__)
+
+// See __builtin_prefetch:
+// https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html.
+//
+// These functions speculatively load for read only. This is
+// safe for all currently supported platforms. However, prefetch for
+// store may have problems depending on the target platform.
+//
+inline void PrefetchT0(const void* addr) {
+  // Note: this uses prefetcht0 on Intel.
+  __builtin_prefetch(addr, 0, 3);
+}
+inline void PrefetchT1(const void* addr) {
+  // Note: this uses prefetcht1 on Intel.
+  __builtin_prefetch(addr, 0, 2);
+}
+inline void PrefetchT2(const void* addr) {
+  // Note: this uses prefetcht2 on Intel.
+  __builtin_prefetch(addr, 0, 1);
+}
+inline void PrefetchNta(const void* addr) {
+  // Note: this uses prefetchtnta on Intel.
+  __builtin_prefetch(addr, 0, 0);
+}
+#else
+inline void PrefetchT0(const void*) {}
+inline void PrefetchT1(const void*) {}
+inline void PrefetchT2(const void*) {}
+inline void PrefetchNta(const void*) {}
+#endif
+
+}  // namespace base_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_BASE_INTERNAL_PREFETCH_H_
diff --git a/absl/base/internal/prefetch_test.cc b/absl/base/internal/prefetch_test.cc
new file mode 100644
index 00000000..7c1dae46
--- /dev/null
+++ b/absl/base/internal/prefetch_test.cc
@@ -0,0 +1,43 @@
+// Copyright 2022 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/base/internal/prefetch.h"
+
+#include "gtest/gtest.h"
+
+namespace {
+
+int number = 42;
+
+TEST(Prefetch, TemporalLocalityNone) {
+  absl::base_internal::PrefetchNta(&number);
+  EXPECT_EQ(number, 42);
+}
+
+TEST(Prefetch, TemporalLocalityLow) {
+  absl::base_internal::PrefetchT2(&number);
+  EXPECT_EQ(number, 42);
+}
+
+TEST(Prefetch, TemporalLocalityMedium) {
+  absl::base_internal::PrefetchT1(&number);
+  EXPECT_EQ(number, 42);
+}
+
+TEST(Prefetch, TemporalLocalityHigh) {
+  absl::base_internal::PrefetchT0(&number);
+  EXPECT_EQ(number, 42);
+}
+
+}  // namespace
diff --git a/absl/container/flat_hash_map.h b/absl/container/flat_hash_map.h
index 13779eda..cbb24693 100644
--- a/absl/container/flat_hash_map.h
+++ b/absl/container/flat_hash_map.h
@@ -76,7 +76,7 @@ struct FlatHashMapPolicy;
 // absl/hash/hash.h for information on extending Abseil hashing to user-defined
 // types.
 //
-// Using `absl::flat_hash_map` at interface boundries in dynamically loaded
+// Using `absl::flat_hash_map` at interface boundaries in dynamically loaded
 // libraries (e.g. .dll, .so) is unsupported due to way `absl::Hash` values may
 // be randomized across dynamically loaded libraries.
 //
diff --git a/absl/container/flat_hash_set.h b/absl/container/flat_hash_set.h
index 304c2abb..4938c703 100644
--- a/absl/container/flat_hash_set.h
+++ b/absl/container/flat_hash_set.h
@@ -72,7 +72,7 @@ struct FlatHashSetPolicy;
 // absl/hash/hash.h for information on extending Abseil hashing to user-defined
 // types.
 //
-// Using `absl::flat_hash_set` at interface boundries in dynamically loaded
+// Using `absl::flat_hash_set` at interface boundaries in dynamically loaded
 // libraries (e.g. .dll, .so) is unsupported due to way `absl::Hash` values may
 // be randomized across dynamically loaded libraries.
 //
diff --git a/absl/container/node_hash_map.h b/absl/container/node_hash_map.h
index f2175d16..c91cae69 100644
--- a/absl/container/node_hash_map.h
+++ b/absl/container/node_hash_map.h
@@ -78,7 +78,7 @@ class NodeHashMapPolicy;
 // absl/hash/hash.h for information on extending Abseil hashing to user-defined
 // types.
 //
-// Using `absl::node_hash_map` at interface boundries in dynamically loaded
+// Using `absl::node_hash_map` at interface boundaries in dynamically loaded
 // libraries (e.g. .dll, .so) is unsupported due to way `absl::Hash` values may
 // be randomized across dynamically loaded libraries.
 //
diff --git a/absl/container/node_hash_set.h b/absl/container/node_hash_set.h
index a9ff7e1b..f2cc70c3 100644
--- a/absl/container/node_hash_set.h
+++ b/absl/container/node_hash_set.h
@@ -74,7 +74,7 @@ struct NodeHashSetPolicy;
 // absl/hash/hash.h for information on extending Abseil hashing to user-defined
 // types.
 //
-// Using `absl::node_hash_set` at interface boundries in dynamically loaded
+// Using `absl::node_hash_set` at interface boundaries in dynamically loaded
 // libraries (e.g. .dll, .so) is unsupported due to way `absl::Hash` values may
 // be randomized across dynamically loaded libraries.
 //
diff --git a/absl/debugging/internal/vdso_support.cc b/absl/debugging/internal/vdso_support.cc
index e63ac4a3..40eb055f 100644
--- a/absl/debugging/internal/vdso_support.cc
+++ b/absl/debugging/internal/vdso_support.cc
@@ -69,7 +69,9 @@ ABSL_CONST_INIT
 std::atomic<const void *> VDSOSupport::vdso_base_(
     debugging_internal::ElfMemImage::kInvalidBase);
 
-std::atomic<VDSOSupport::GetCpuFn> VDSOSupport::getcpu_fn_(&InitAndGetCPU);
+ABSL_CONST_INIT std::atomic<VDSOSupport::GetCpuFn> VDSOSupport::getcpu_fn_(
+    &InitAndGetCPU);
+
 VDSOSupport::VDSOSupport()
     // If vdso_base_ is still set to kInvalidBase, we got here
     // before VDSOSupport::Init has been called. Call it now.
diff --git a/absl/strings/internal/cord_internal.h b/absl/strings/internal/cord_internal.h
index 5ca5e589..6ae418e7 100644
--- a/absl/strings/internal/cord_internal.h
+++ b/absl/strings/internal/cord_internal.h
@@ -411,7 +411,8 @@ struct ConstInitExternalStorage {
 };
 
 template <typename Str>
-CordRepExternal ConstInitExternalStorage<Str>::value(Str::value);
+ABSL_CONST_INIT CordRepExternal
+    ConstInitExternalStorage<Str>::value(Str::value);
 
 enum {
   kMaxInline = 15,
diff --git a/absl/strings/internal/escaping.cc b/absl/strings/internal/escaping.cc
index 7f87e124..cfea0961 100644
--- a/absl/strings/internal/escaping.cc
+++ b/absl/strings/internal/escaping.cc
@@ -21,7 +21,7 @@ namespace absl {
 ABSL_NAMESPACE_BEGIN
 namespace strings_internal {
 
-const char kBase64Chars[] =
+ABSL_CONST_INIT const char kBase64Chars[] =
     "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
 
 size_t CalculateBase64EscapedLenInternal(size_t input_len, bool do_padding) {
diff --git a/absl/strings/numbers.cc b/absl/strings/numbers.cc
index cbd84c91..e798fc69 100644
--- a/absl/strings/numbers.cc
+++ b/absl/strings/numbers.cc
@@ -757,8 +757,8 @@ struct LookupTables {
 //
 // uint128& operator/=(uint128) is not constexpr, so hardcode the resulting
 // array to avoid a static initializer.
-template<>
-const uint128 LookupTables<uint128>::kVmaxOverBase[] = {
+template <>
+ABSL_CONST_INIT const uint128 LookupTables<uint128>::kVmaxOverBase[] = {
     0,
     0,
     MakeUint128(9223372036854775807u, 18446744073709551615u),
@@ -809,8 +809,8 @@ const uint128 LookupTables<uint128>::kVmaxOverBase[] = {
 //
 // int128& operator/=(int128) is not constexpr, so hardcode the resulting array
 // to avoid a static initializer.
-template<>
-const int128 LookupTables<int128>::kVmaxOverBase[] = {
+template <>
+ABSL_CONST_INIT const int128 LookupTables<int128>::kVmaxOverBase[] = {
     0,
     0,
     MakeInt128(4611686018427387903, 18446744073709551615u),
@@ -862,8 +862,8 @@ const int128 LookupTables<int128>::kVmaxOverBase[] = {
 //
 // int128& operator/=(int128) is not constexpr, so hardcode the resulting array
 // to avoid a static initializer.
-template<>
-const int128 LookupTables<int128>::kVminOverBase[] = {
+template <>
+ABSL_CONST_INIT const int128 LookupTables<int128>::kVminOverBase[] = {
     0,
     0,
     MakeInt128(-4611686018427387904, 0u),
@@ -904,11 +904,11 @@ const int128 LookupTables<int128>::kVminOverBase[] = {
 };
 
 template <typename IntType>
-const IntType LookupTables<IntType>::kVmaxOverBase[] =
+ABSL_CONST_INIT const IntType LookupTables<IntType>::kVmaxOverBase[] =
     X_OVER_BASE_INITIALIZER(std::numeric_limits<IntType>::max());
 
 template <typename IntType>
-const IntType LookupTables<IntType>::kVminOverBase[] =
+ABSL_CONST_INIT const IntType LookupTables<IntType>::kVminOverBase[] =
     X_OVER_BASE_INITIALIZER(std::numeric_limits<IntType>::min());
 
 #undef X_OVER_BASE_INITIALIZER
diff --git a/absl/types/internal/variant.h b/absl/types/internal/variant.h
index 772008c7..7c402ece 100644
--- a/absl/types/internal/variant.h
+++ b/absl/types/internal/variant.h
@@ -16,8 +16,8 @@
 // separate file to avoid cluttering the top of the API header with
 // implementation details.
 
-#ifndef ABSL_TYPES_variant_internal_H_
-#define ABSL_TYPES_variant_internal_H_
+#ifndef ABSL_TYPES_VARIANT_INTERNAL_H_
+#define ABSL_TYPES_VARIANT_INTERNAL_H_
 
 #include <cassert>
 #include <cstddef>
@@ -1643,4 +1643,4 @@ ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // !defined(ABSL_USES_STD_VARIANT)
-#endif  // ABSL_TYPES_variant_internal_H_
+#endif  // ABSL_TYPES_VARIANT_INTERNAL_H_