optimization.h - mozsearch

//

// 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 <assert.h>

#include "absl/base/config.h"

#include "absl/base/options.h"

// ABSL_BLOCK_TAIL_CALL_OPTIMIZATION

//

// Instructs the compiler to avoid optimizing tail-call recursion. 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

#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: callers should replace uses of this macro with `alignas()` using

// `std::hardware_constructive_interference_size` and/or

// `std::hardware_destructive_interference_size` when C++17 becomes available to

// them.

//

// 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.

#if defined(__clang__) || defined(__GNUC__)

#define ABSL_CACHELINE_ALIGNED __attribute__((aligned(ABSL_CACHELINE_SIZE)))

#elif defined(_MSC_VER)

#define ABSL_CACHELINE_ALIGNED __declspec(align(ABSL_CACHELINE_SIZE))

#else

#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(false || (x), false))

#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_IMMEDIATE_ABORT_IMPL()` aborts the program in the fastest

// possible way, with no attempt at logging. One use is to implement hardening

// aborts with ABSL_OPTION_HARDENED.  Since this is an internal symbol, it

// should not be used directly outside of Abseil.

#if ABSL_HAVE_BUILTIN(__builtin_trap) || \

    (defined(__GNUC__) && !defined(__clang__))

#define ABSL_INTERNAL_IMMEDIATE_ABORT_IMPL() __builtin_trap()

#else

#define ABSL_INTERNAL_IMMEDIATE_ABORT_IMPL() abort()

#endif

// `ABSL_INTERNAL_UNREACHABLE_IMPL()` is the platform specific directive to

// indicate that a statement is unreachable, and to allow the compiler to

// optimize accordingly. Clients should use `ABSL_UNREACHABLE()`, which is

// defined below.

#if defined(__cpp_lib_unreachable) && __cpp_lib_unreachable >= 202202L

#define ABSL_INTERNAL_UNREACHABLE_IMPL() std::unreachable()

#elif defined(__GNUC__) || ABSL_HAVE_BUILTIN(__builtin_unreachable)

#define ABSL_INTERNAL_UNREACHABLE_IMPL() __builtin_unreachable()

#elif ABSL_HAVE_BUILTIN(__builtin_assume)

#define ABSL_INTERNAL_UNREACHABLE_IMPL() __builtin_assume(false)

#elif defined(_MSC_VER)

#define ABSL_INTERNAL_UNREACHABLE_IMPL() __assume(false)

#else

#define ABSL_INTERNAL_UNREACHABLE_IMPL()

#endif

// `ABSL_UNREACHABLE()` is an unreachable statement.  A program which reaches

// one has undefined behavior, and the compiler may optimize accordingly.

#if ABSL_OPTION_HARDENED == 1 && defined(NDEBUG)

// Abort in hardened mode to avoid dangerous undefined behavior.

#define ABSL_UNREACHABLE()                \

  do {                                    \

    ABSL_INTERNAL_IMMEDIATE_ABORT_IMPL(); \

    ABSL_INTERNAL_UNREACHABLE_IMPL();     \

  } while (false)

#else

// The assert only fires in debug mode to aid in debugging.

// When NDEBUG is defined, reaching ABSL_UNREACHABLE() is undefined behavior.

#define ABSL_UNREACHABLE()                       \

  do {                                           \

    /* NOLINTNEXTLINE: misc-static-assert */     \

    assert(false && "ABSL_UNREACHABLE reached"); \

    ABSL_INTERNAL_UNREACHABLE_IMPL();            \

  } while (false)

#endif

// ABSL_ASSUME(cond)

//

// Informs the compiler that a condition is always true and that it can assume

// it to be true for optimization purposes.

//

// WARNING: If the condition is false, the program can produce undefined and

// potentially dangerous behavior.

//

// In !NDEBUG mode, the condition is checked with an assert().

//

// NOTE: The expression must not have side effects, as it may only be evaluated

// in some compilation modes and not others. Some compilers may issue a warning

// if the compiler cannot prove the expression has no side effects. For example,

// the expression should not use a function call since the compiler cannot prove

// that a function call does not have side effects.

//

// Example:

//

//   int x = ...;

//   ABSL_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_ASSUME(cond) assert(cond)

#elif ABSL_HAVE_BUILTIN(__builtin_assume)

#define ABSL_ASSUME(cond) __builtin_assume(cond)

#elif defined(_MSC_VER)

#define ABSL_ASSUME(cond) __assume(cond)

#elif defined(__cpp_lib_unreachable) && __cpp_lib_unreachable >= 202202L

#define ABSL_ASSUME(cond)            \

  do {                               \

    if (!(cond)) std::unreachable(); \

  } while (false)

#elif defined(__GNUC__) || ABSL_HAVE_BUILTIN(__builtin_unreachable)

#define ABSL_ASSUME(cond)                 \

  do {                                    \

    if (!(cond)) __builtin_unreachable(); \

  } while (false)

#else

#define ABSL_ASSUME(cond)               \

  do {                                  \

    static_cast<void>(false && (cond)); \

  } while (false)

#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 cause 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_

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