便携式分支prediction提示 [英] Portable branch prediction hints

问题描述

是否有这样做的分支prediction暗示任何可移植的方法？请看下面的例子：

Is there any portable way of doing branch prediction hints? Consider the following example:

  if (unlikely_condition) {
    /* ..A.. */
  } else {
    /* ..B.. */
  }

这是不是做任何的不同：

Is this any different than doing:

  if (!unlikely_condition) {
    /* ..B.. */
  } else {
    /* ..A.. */
  }

或者是使用编译器的具体线索的唯一途径？ （例如__builtin_expect在GCC）

Or is the only way to use compiler specific hints? (e.g. __builtin_expect on GCC)

将编译器治疗如果情况的任何不同的基于条件的顺序？

Will compilers treat the if conditions any differently based on the ordering of the conditions?

推荐答案

的规范的方式做静态分支prediction是如果是predicted不-branched（即执行每如果子句，而不是其他），以及循环和落后 - 转到 s的拍摄。所以，如果你希望静态prediction是显著不要放在通常情况下别人。四处逛逛未获认购的循环是不容易;我从来没有尝试过但我想将它的一个其他条款应该工作pretty可移植性。

The canonical way to do static branch prediction is that if is predicted not-branched (i.e. every if clause is executed, not else), and loops and backward-gotos are taken. So, don't put the common case in else if you expect static prediction to be significant. Getting around an untaken loop isn't as easy; I've never tried but I suppose putting it an an else clause should work pretty portably.

许多编译器支持某种形式的的#pragma解开，但它仍需要以某种＃如果来保护其他的编译器。

Many compilers support some form of #pragma unroll, but it will still be necessary to guard it with some kind of #if to protect other compilers.

分公司prediction提示理论上可前preSS的如何将一个程序的流量控制图，并安排可执行的内存基本块......所以有能恩$ P $各种事物的完整描述PSS，大多数不会非常便于携带。

Branch prediction hints can theoretically express a complete description of how to transform a program's flow-control graph and arrange the basic blocks in executable memory… so there are a variety of things to express, and most won't be very portable.

由于GNU建议文档 __ builtin_expect 中，档案导引优化优于提示，更省力。

As GNU recommends in the documentation for __builtin_expect, profile-guided optimization is superior to hints, and with less effort.

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