github.com/moontrade/nogc@v0.1.7/sync/Likely.h (about)

     1  /*
     2   * Copyright (c) Facebook, Inc. and its affiliates.
     3   *
     4   * Licensed under the Apache License, Version 2.0 (the "License");
     5   * you may not use this file except in compliance with the License.
     6   * You may obtain a copy of the License at
     7   *
     8   *     http://www.apache.org/licenses/LICENSE-2.0
     9   *
    10   * Unless required by applicable law or agreed to in writing, software
    11   * distributed under the License is distributed on an "AS IS" BASIS,
    12   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    13   * See the License for the specific language governing permissions and
    14   * limitations under the License.
    15   */
    16  
    17  #pragma once
    18  
    19  #if __GNUC__
    20  #define FOLLY_DETAIL_BUILTIN_EXPECT(b, t) (__builtin_expect(b, t))
    21  #else
    22  #define FOLLY_DETAIL_BUILTIN_EXPECT(b, t) b
    23  #endif
    24  
    25  //  Likeliness annotations
    26  //
    27  //  Useful when the author has better knowledge than the compiler of whether
    28  //  the branch condition is overwhelmingly likely to take a specific value.
    29  //
    30  //  Useful when the author has better knowledge than the compiler of which code
    31  //  paths are designed as the fast path and which are designed as the slow path,
    32  //  and to force the compiler to optimize for the fast path, even when it is not
    33  //  overwhelmingly likely.
    34  //
    35  //  Notes:
    36  //  * All supported compilers treat unconditionally-noreturn blocks as unlikely.
    37  //    This is true for blocks which unconditionally throw exceptions and for
    38  //    blocks which unconditionally call [[noreturn]]-annotated functions. Such
    39  //    cases do not require likeliness annotations.
    40  
    41  #define FOLLY_LIKELY(...) FOLLY_DETAIL_BUILTIN_EXPECT((__VA_ARGS__), 1)
    42  #define FOLLY_UNLIKELY(...) FOLLY_DETAIL_BUILTIN_EXPECT((__VA_ARGS__), 0)
    43  
    44  //  Un-namespaced annotations
    45  
    46  #undef LIKELY
    47  #undef UNLIKELY
    48  
    49  #if defined(__GNUC__)
    50  #define LIKELY(x) (__builtin_expect((x), 1))
    51  #define UNLIKELY(x) (__builtin_expect((x), 0))
    52  #else
    53  #define LIKELY(x) (x)
    54  #define UNLIKELY(x) (x)
    55  #endif