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Diffstat (limited to 'absl/base/optimization.h')
| -rw-r--r-- | absl/base/optimization.h | 164 |
1 files changed, 164 insertions, 0 deletions
diff --git a/absl/base/optimization.h b/absl/base/optimization.h new file mode 100644 index 000000000000..db8beafa3f2e --- /dev/null +++ b/absl/base/optimization.h @@ -0,0 +1,164 @@ +// +// 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 +// +// http://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) +// 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 +// `__attribute__((aligned(ABSL_CACHELINE_SIZE)))`. 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. +// +// 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))) + +#else // not GCC +#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`). +#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(!!(x), 1)) +#else +#define ABSL_PREDICT_FALSE(x) x +#define ABSL_PREDICT_TRUE(x) x +#endif + +#endif // ABSL_BASE_OPTIMIZATION_H_ |