Export of internal Abseil changes

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3d018c03a34bf273a4b24b3584ed77f0a6d21686 by Abseil Team <absl-team@google.com>:

Fix a spelling typo (s/boundries/boundaries).

PiperOrigin-RevId: 442041877
Change-Id: I608020697d37b85316bb9a0838e4b457659c926c

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518b8119e51db24ce7fb0fd2fe537ec43825c3e6 by Dino Radakovic <dinor@google.com>:

absl/types/internal/variant: Make include guard uppercase

https://google.github.io/styleguide/cppguide.html#The__define_Guard

PiperOrigin-RevId: 441911692
Change-Id: I9837dd07f20204d8253f20627b0917a34dc21825

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b91696c38310a7cae8c1ea9e2d479495f5dc3f69 by Greg Falcon <gfalcon@google.com>:

Add an internal-only API to wrap __builtin_prefetch() if available.

This private API is intended for future use by the Abseil implementation.  Like any internal-namespaced function, it may be changed or removed at any time.

PiperOrigin-RevId: 441894616
Change-Id: Iaa48bd4680b373f4a0d5afab0cb35e2a1908595f

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0f01e8b0551a662e02dff60840c54320f987315f by Derek Mauro <dmauro@google.com>:

C++20: Use the standard `constinit` keyword for `ABSL_CONST_INIT` when available

PiperOrigin-RevId: 441778874
Change-Id: I70c616469752ff23b326b1c615437599f42cc6aa
GitOrigin-RevId: 3d018c03a34bf273a4b24b3584ed77f0a6d21686

2 files changed, 152 insertions, 0 deletions

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