| // |
| // 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) |
| #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 |
| // `__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_ |
