diff options
Diffstat (limited to 'absl/base')
-rw-r--r-- | absl/base/BUILD.bazel | 25 | ||||
-rw-r--r-- | absl/base/CMakeLists.txt | 26 | ||||
-rw-r--r-- | absl/base/attributes.h | 26 | ||||
-rw-r--r-- | absl/base/internal/prefetch.h | 109 | ||||
-rw-r--r-- | absl/base/internal/prefetch_test.cc | 43 |
5 files changed, 223 insertions, 6 deletions
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 |