245 lines
9.0 KiB
C++
245 lines
9.0 KiB
C++
//
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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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// https://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 <assert.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. This macro is
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// useful when you wish to preserve the existing function order within a stack
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// 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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#include <intrin.h>
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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: callers should replace uses of this macro with `alignas()` using
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// `std::hardware_constructive_interference_size` and/or
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// `std::hardware_destructive_interference_size` when C++17 becomes available to
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// them.
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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 an `__attribute__`
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// or `__declspec` attribute. For compilers where this is not known to work,
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// 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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// NOTE: Some compilers are picky about the locations of annotations such as
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// this attribute, so prefer to put it at the beginning of your declaration.
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// For example,
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//
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// ABSL_CACHELINE_ALIGNED static Foo* foo = ...
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//
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// class ABSL_CACHELINE_ALIGNED Bar { ...
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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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#elif defined(_MSC_VER)
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#define ABSL_CACHELINE_SIZE 64
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#define ABSL_CACHELINE_ALIGNED __declspec(align(ABSL_CACHELINE_SIZE))
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#else
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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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//
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// Recommendation: Modern CPUs dynamically predict branch execution paths,
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// typically with accuracy greater than 97%. As a result, annotating every
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// branch in a codebase is likely counterproductive; however, annotating
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// specific branches that are both hot and consistently mispredicted is likely
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// to yield performance improvements.
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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(false || (x), false))
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#define ABSL_PREDICT_TRUE(x) (__builtin_expect(false || (x), true))
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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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// ABSL_INTERNAL_ASSUME(cond)
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// Informs the compiler that a condition is always true and that it can assume
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// it to be true for optimization purposes. The call has undefined behavior if
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// the condition is false.
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// In !NDEBUG mode, the condition is checked with an assert().
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// NOTE: The expression must not have side effects, as it will only be evaluated
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// in some compilation modes and not others.
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//
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// Example:
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//
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// int x = ...;
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// ABSL_INTERNAL_ASSUME(x >= 0);
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// // The compiler can optimize the division to a simple right shift using the
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// // assumption specified above.
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// int y = x / 16;
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//
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#if !defined(NDEBUG)
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#define ABSL_INTERNAL_ASSUME(cond) assert(cond)
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#elif ABSL_HAVE_BUILTIN(__builtin_assume)
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#define ABSL_INTERNAL_ASSUME(cond) __builtin_assume(cond)
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#elif defined(__GNUC__) || ABSL_HAVE_BUILTIN(__builtin_unreachable)
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#define ABSL_INTERNAL_ASSUME(cond) \
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do { \
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if (!(cond)) __builtin_unreachable(); \
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} while (0)
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#elif defined(_MSC_VER)
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#define ABSL_INTERNAL_ASSUME(cond) __assume(cond)
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#else
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#define ABSL_INTERNAL_ASSUME(cond) \
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do { \
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static_cast<void>(false && (cond)); \
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} while (0)
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#endif
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// ABSL_INTERNAL_UNIQUE_SMALL_NAME(cond)
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// This macro forces small unique name on a static file level symbols like
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// static local variables or static functions. This is intended to be used in
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// macro definitions to optimize the cost of generated code. Do NOT use it on
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// symbols exported from translation unit since it may cause a link time
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// conflict.
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//
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// Example:
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//
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// #define MY_MACRO(txt)
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// namespace {
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// char VeryVeryLongVarName[] ABSL_INTERNAL_UNIQUE_SMALL_NAME() = txt;
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// const char* VeryVeryLongFuncName() ABSL_INTERNAL_UNIQUE_SMALL_NAME();
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// const char* VeryVeryLongFuncName() { return txt; }
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// }
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//
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#if defined(__GNUC__)
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#define ABSL_INTERNAL_UNIQUE_SMALL_NAME2(x) #x
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#define ABSL_INTERNAL_UNIQUE_SMALL_NAME1(x) ABSL_INTERNAL_UNIQUE_SMALL_NAME2(x)
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#define ABSL_INTERNAL_UNIQUE_SMALL_NAME() \
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asm(ABSL_INTERNAL_UNIQUE_SMALL_NAME1(.absl.__COUNTER__))
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#else
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#define ABSL_INTERNAL_UNIQUE_SMALL_NAME()
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#endif
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#endif // ABSL_BASE_OPTIMIZATION_H_
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