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absl/base/optimization.h
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absl/base/optimization.h
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//
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// Copyright 2017 The Abseil Authors.
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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//
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// -----------------------------------------------------------------------------
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// File: optimization.h
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// -----------------------------------------------------------------------------
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//
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// This header file defines portable macros for performance optimization.
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#ifndef ABSL_BASE_OPTIMIZATION_H_
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#define ABSL_BASE_OPTIMIZATION_H_
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#include "absl/base/config.h"
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// ABSL_BLOCK_TAIL_CALL_OPTIMIZATION
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//
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// Instructs the compiler to avoid optimizing tail-call recursion. Use of this
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// macro is useful when you wish to preserve the existing function order within
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// a stack trace for logging, debugging, or profiling purposes.
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//
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// Example:
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//
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// int f() {
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// int result = g();
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// ABSL_BLOCK_TAIL_CALL_OPTIMIZATION();
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// return result;
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// }
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#if defined(__pnacl__)
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#define ABSL_BLOCK_TAIL_CALL_OPTIMIZATION() if (volatile int x = 0) { (void)x; }
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#elif defined(__clang__)
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// Clang will not tail call given inline volatile assembly.
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#define ABSL_BLOCK_TAIL_CALL_OPTIMIZATION() __asm__ __volatile__("")
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#elif defined(__GNUC__)
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// GCC will not tail call given inline volatile assembly.
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#define ABSL_BLOCK_TAIL_CALL_OPTIMIZATION() __asm__ __volatile__("")
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#elif defined(_MSC_VER)
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// The __nop() intrinsic blocks the optimisation.
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#define ABSL_BLOCK_TAIL_CALL_OPTIMIZATION() __nop()
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#else
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#define ABSL_BLOCK_TAIL_CALL_OPTIMIZATION() if (volatile int x = 0) { (void)x; }
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#endif
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// ABSL_CACHELINE_SIZE
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//
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// Explicitly defines the size of the L1 cache for purposes of alignment.
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// Setting the cacheline size allows you to specify that certain objects be
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// aligned on a cacheline boundary with `ABSL_CACHELINE_ALIGNED` declarations.
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// (See below.)
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//
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// NOTE: this macro should be replaced with the following C++17 features, when
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// those are generally available:
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//
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// * `std::hardware_constructive_interference_size`
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// * `std::hardware_destructive_interference_size`
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//
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// See http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0154r1.html
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// for more information.
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#if defined(__GNUC__)
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// Cache line alignment
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#if defined(__i386__) || defined(__x86_64__)
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#define ABSL_CACHELINE_SIZE 64
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#elif defined(__powerpc64__)
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#define ABSL_CACHELINE_SIZE 128
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#elif defined(__aarch64__)
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// We would need to read special register ctr_el0 to find out L1 dcache size.
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// This value is a good estimate based on a real aarch64 machine.
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#define ABSL_CACHELINE_SIZE 64
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#elif defined(__arm__)
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// Cache line sizes for ARM: These values are not strictly correct since
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// cache line sizes depend on implementations, not architectures. There
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// are even implementations with cache line sizes configurable at boot
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// time.
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#if defined(__ARM_ARCH_5T__)
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#define ABSL_CACHELINE_SIZE 32
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#elif defined(__ARM_ARCH_7A__)
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#define ABSL_CACHELINE_SIZE 64
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#endif
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#endif
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#ifndef ABSL_CACHELINE_SIZE
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// A reasonable default guess. Note that overestimates tend to waste more
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// space, while underestimates tend to waste more time.
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#define ABSL_CACHELINE_SIZE 64
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#endif
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// ABSL_CACHELINE_ALIGNED
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//
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// Indicates that the declared object be cache aligned using
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// `ABSL_CACHELINE_SIZE` (see above). Cacheline aligning objects allows you to
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// load a set of related objects in the L1 cache for performance improvements.
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// Cacheline aligning objects properly allows constructive memory sharing and
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// prevents destructive (or "false") memory sharing.
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//
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// NOTE: this macro should be replaced with usage of `alignas()` using
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// `std::hardware_constructive_interference_size` and/or
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// `std::hardware_destructive_interference_size` when available within C++17.
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//
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// See http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0154r1.html
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// for more information.
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//
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// On some compilers, `ABSL_CACHELINE_ALIGNED` expands to
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// `__attribute__((aligned(ABSL_CACHELINE_SIZE)))`. For compilers where this is
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// not known to work, the macro expands to nothing.
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//
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// No further guarantees are made here. The result of applying the macro
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// to variables and types is always implementation-defined.
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//
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// WARNING: It is easy to use this attribute incorrectly, even to the point
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// of causing bugs that are difficult to diagnose, crash, etc. It does not
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// of itself guarantee that objects are aligned to a cache line.
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//
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// Recommendations:
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//
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// 1) Consult compiler documentation; this comment is not kept in sync as
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// toolchains evolve.
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// 2) Verify your use has the intended effect. This often requires inspecting
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// the generated machine code.
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// 3) Prefer applying this attribute to individual variables. Avoid
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// applying it to types. This tends to localize the effect.
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#define ABSL_CACHELINE_ALIGNED __attribute__((aligned(ABSL_CACHELINE_SIZE)))
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#else // not GCC
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#define ABSL_CACHELINE_SIZE 64
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#define ABSL_CACHELINE_ALIGNED
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#endif
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// ABSL_PREDICT_TRUE, ABSL_PREDICT_FALSE
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//
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// Enables the compiler to prioritize compilation using static analysis for
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// likely paths within a boolean branch.
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//
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// Example:
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//
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// if (ABSL_PREDICT_TRUE(expression)) {
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// return result; // Faster if more likely
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// } else {
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// return 0;
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// }
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//
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// Compilers can use the information that a certain branch is not likely to be
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// taken (for instance, a CHECK failure) to optimize for the common case in
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// the absence of better information (ie. compiling gcc with `-fprofile-arcs`).
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#if ABSL_HAVE_BUILTIN(__builtin_expect) || \
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(defined(__GNUC__) && !defined(__clang__))
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#define ABSL_PREDICT_FALSE(x) (__builtin_expect(x, 0))
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#define ABSL_PREDICT_TRUE(x) (__builtin_expect(!!(x), 1))
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#else
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#define ABSL_PREDICT_FALSE(x) x
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#define ABSL_PREDICT_TRUE(x) x
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#endif
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#endif // ABSL_BASE_OPTIMIZATION_H_
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