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Diffstat (limited to 'third_party/abseil_cpp/absl/base/optimization.h')
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1 files changed, 241 insertions, 0 deletions
diff --git a/third_party/abseil_cpp/absl/base/optimization.h b/third_party/abseil_cpp/absl/base/optimization.h new file mode 100644 index 000000000000..92bf9cd38840 --- /dev/null +++ b/third_party/abseil_cpp/absl/base/optimization.h @@ -0,0 +1,241 @@ +// +// Copyright 2017 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. +// +// ----------------------------------------------------------------------------- +// File: optimization.h +// ----------------------------------------------------------------------------- +// +// This header file defines portable macros for performance optimization. + +#ifndef ABSL_BASE_OPTIMIZATION_H_ +#define ABSL_BASE_OPTIMIZATION_H_ + +#include "absl/base/config.h" + +// ABSL_BLOCK_TAIL_CALL_OPTIMIZATION +// +// Instructs the compiler to avoid optimizing tail-call recursion. Use of this +// macro is useful when you wish to preserve the existing function order within +// a stack trace for logging, debugging, or profiling purposes. +// +// Example: +// +// int f() { +// int result = g(); +// ABSL_BLOCK_TAIL_CALL_OPTIMIZATION(); +// return result; +// } +#if defined(__pnacl__) +#define ABSL_BLOCK_TAIL_CALL_OPTIMIZATION() if (volatile int x = 0) { (void)x; } +#elif defined(__clang__) +// Clang will not tail call given inline volatile assembly. +#define ABSL_BLOCK_TAIL_CALL_OPTIMIZATION() __asm__ __volatile__("") +#elif defined(__GNUC__) +// GCC will not tail call given inline volatile assembly. +#define ABSL_BLOCK_TAIL_CALL_OPTIMIZATION() __asm__ __volatile__("") +#elif defined(_MSC_VER) +#include <intrin.h> +// The __nop() intrinsic blocks the optimisation. +#define ABSL_BLOCK_TAIL_CALL_OPTIMIZATION() __nop() +#else +#define ABSL_BLOCK_TAIL_CALL_OPTIMIZATION() if (volatile int x = 0) { (void)x; } +#endif + +// ABSL_CACHELINE_SIZE +// +// Explicitly defines the size of the L1 cache for purposes of alignment. +// Setting the cacheline size allows you to specify that certain objects be +// aligned on a cacheline boundary with `ABSL_CACHELINE_ALIGNED` declarations. +// (See below.) +// +// NOTE: this macro should be replaced with the following C++17 features, when +// those are generally available: +// +// * `std::hardware_constructive_interference_size` +// * `std::hardware_destructive_interference_size` +// +// See http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0154r1.html +// for more information. +#if defined(__GNUC__) +// Cache line alignment +#if defined(__i386__) || defined(__x86_64__) +#define ABSL_CACHELINE_SIZE 64 +#elif defined(__powerpc64__) +#define ABSL_CACHELINE_SIZE 128 +#elif defined(__aarch64__) +// We would need to read special register ctr_el0 to find out L1 dcache size. +// This value is a good estimate based on a real aarch64 machine. +#define ABSL_CACHELINE_SIZE 64 +#elif defined(__arm__) +// Cache line sizes for ARM: These values are not strictly correct since +// cache line sizes depend on implementations, not architectures. There +// are even implementations with cache line sizes configurable at boot +// time. +#if defined(__ARM_ARCH_5T__) +#define ABSL_CACHELINE_SIZE 32 +#elif defined(__ARM_ARCH_7A__) +#define ABSL_CACHELINE_SIZE 64 +#endif +#endif + +#ifndef ABSL_CACHELINE_SIZE +// A reasonable default guess. Note that overestimates tend to waste more +// space, while underestimates tend to waste more time. +#define ABSL_CACHELINE_SIZE 64 +#endif + +// ABSL_CACHELINE_ALIGNED +// +// Indicates that the declared object be cache aligned using +// `ABSL_CACHELINE_SIZE` (see above). Cacheline aligning objects allows you to +// load a set of related objects in the L1 cache for performance improvements. +// Cacheline aligning objects properly allows constructive memory sharing and +// prevents destructive (or "false") memory sharing. +// +// NOTE: this macro should be replaced with usage of `alignas()` using +// `std::hardware_constructive_interference_size` and/or +// `std::hardware_destructive_interference_size` when available within C++17. +// +// See http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0154r1.html +// for more information. +// +// On some compilers, `ABSL_CACHELINE_ALIGNED` expands to an `__attribute__` +// or `__declspec` attribute. For compilers where this is not known to work, +// the macro expands to nothing. +// +// No further guarantees are made here. The result of applying the macro +// to variables and types is always implementation-defined. +// +// WARNING: It is easy to use this attribute incorrectly, even to the point +// of causing bugs that are difficult to diagnose, crash, etc. It does not +// of itself guarantee that objects are aligned to a cache line. +// +// NOTE: Some compilers are picky about the locations of annotations such as +// this attribute, so prefer to put it at the beginning of your declaration. +// For example, +// +// ABSL_CACHELINE_ALIGNED static Foo* foo = ... +// +// class ABSL_CACHELINE_ALIGNED Bar { ... +// +// Recommendations: +// +// 1) Consult compiler documentation; this comment is not kept in sync as +// toolchains evolve. +// 2) Verify your use has the intended effect. This often requires inspecting +// the generated machine code. +// 3) Prefer applying this attribute to individual variables. Avoid +// applying it to types. This tends to localize the effect. +#define ABSL_CACHELINE_ALIGNED __attribute__((aligned(ABSL_CACHELINE_SIZE))) +#elif defined(_MSC_VER) +#define ABSL_CACHELINE_SIZE 64 +#define ABSL_CACHELINE_ALIGNED __declspec(align(ABSL_CACHELINE_SIZE)) +#else +#define ABSL_CACHELINE_SIZE 64 +#define ABSL_CACHELINE_ALIGNED +#endif + +// ABSL_PREDICT_TRUE, ABSL_PREDICT_FALSE +// +// Enables the compiler to prioritize compilation using static analysis for +// likely paths within a boolean branch. +// +// Example: +// +// if (ABSL_PREDICT_TRUE(expression)) { +// return result; // Faster if more likely +// } else { +// return 0; +// } +// +// Compilers can use the information that a certain branch is not likely to be +// taken (for instance, a CHECK failure) to optimize for the common case in +// the absence of better information (ie. compiling gcc with `-fprofile-arcs`). +// +// Recommendation: Modern CPUs dynamically predict branch execution paths, +// typically with accuracy greater than 97%. As a result, annotating every +// branch in a codebase is likely counterproductive; however, annotating +// specific branches that are both hot and consistently mispredicted is likely +// to yield performance improvements. +#if ABSL_HAVE_BUILTIN(__builtin_expect) || \ + (defined(__GNUC__) && !defined(__clang__)) +#define ABSL_PREDICT_FALSE(x) (__builtin_expect(x, 0)) +#define ABSL_PREDICT_TRUE(x) (__builtin_expect(false || (x), true)) +#else +#define ABSL_PREDICT_FALSE(x) (x) +#define ABSL_PREDICT_TRUE(x) (x) +#endif + +// ABSL_INTERNAL_ASSUME(cond) +// Informs the compiler than a condition is always true and that it can assume +// it to be true for optimization purposes. The call has undefined behavior if +// the condition is false. +// In !NDEBUG mode, the condition is checked with an assert(). +// NOTE: The expression must not have side effects, as it will only be evaluated +// in some compilation modes and not others. +// +// Example: +// +// int x = ...; +// ABSL_INTERNAL_ASSUME(x >= 0); +// // The compiler can optimize the division to a simple right shift using the +// // assumption specified above. +// int y = x / 16; +// +#if !defined(NDEBUG) +#define ABSL_INTERNAL_ASSUME(cond) assert(cond) +#elif ABSL_HAVE_BUILTIN(__builtin_assume) +#define ABSL_INTERNAL_ASSUME(cond) __builtin_assume(cond) +#elif defined(__GNUC__) || ABSL_HAVE_BUILTIN(__builtin_unreachable) +#define ABSL_INTERNAL_ASSUME(cond) \ + do { \ + if (!(cond)) __builtin_unreachable(); \ + } while (0) +#elif defined(_MSC_VER) +#define ABSL_INTERNAL_ASSUME(cond) __assume(cond) +#else +#define ABSL_INTERNAL_ASSUME(cond) \ + do { \ + static_cast<void>(false && (cond)); \ + } while (0) +#endif + +// ABSL_INTERNAL_UNIQUE_SMALL_NAME(cond) +// This macro forces small unique name on a static file level symbols like +// static local variables or static functions. This is intended to be used in +// macro definitions to optimize the cost of generated code. Do NOT use it on +// symbols exported from translation unit since it may casue a link time +// conflict. +// +// Example: +// +// #define MY_MACRO(txt) +// namespace { +// char VeryVeryLongVarName[] ABSL_INTERNAL_UNIQUE_SMALL_NAME() = txt; +// const char* VeryVeryLongFuncName() ABSL_INTERNAL_UNIQUE_SMALL_NAME(); +// const char* VeryVeryLongFuncName() { return txt; } +// } +// + +#if defined(__GNUC__) +#define ABSL_INTERNAL_UNIQUE_SMALL_NAME2(x) #x +#define ABSL_INTERNAL_UNIQUE_SMALL_NAME1(x) ABSL_INTERNAL_UNIQUE_SMALL_NAME2(x) +#define ABSL_INTERNAL_UNIQUE_SMALL_NAME() \ + asm(ABSL_INTERNAL_UNIQUE_SMALL_NAME1(.absl.__COUNTER__)) +#else +#define ABSL_INTERNAL_UNIQUE_SMALL_NAME() +#endif + +#endif // ABSL_BASE_OPTIMIZATION_H_ |