c++ - 使用#pragma 优化的代码的可移植性如何？

Question

使用 #pragma optimize 的代码的可移植性如何？大多数编译器都支持它吗？对此 #pragma 的支持有多完整？

Answer 1

#pragma 是编译器添加非认可和不可移植语言扩展的认可和可移植方式^*.

基本上，您永远无法确定，并且至少有一个主要的 C++ 编译器 (g++) 不按原样支持此 pragma。

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^*:

来自 C++ 标准 (N3242):

16.6 Pragma directive [cpp.pragma]

A preprocessing directive of the form

# pragma pp-tokens_opt new-line

causes the implementation to behave in an implementation-defined manner. The behavior might cause translation to fail or cause the translator or the resulting program to behave in a non-conforming manner. Any pragma that is not recognized by the implementation is ignored.

来自 C 标准(委员会草案 — 2011 年 4 月 12 日):

6.10.6 Pragma directive

Semantics

A preprocessing directive of the form

# pragma pp-tokens_opt new-line

where the preprocessing token STDC does not immediately follow pragma in the directive (prior to any macro replacement)¹⁷⁴⁾ causes the implementation to behave in an implementation-defined manner. The behavior might cause translation to fail or cause the translator or the resulting program to behave in a non-conforming manner. Any such pragma that is not recognized by the implementation is ignored.

下面是一个例子:

int main () {
    #pragma omp parallel for
    for (int i=0; i<16; ++i) {}
}

C 和 C++ OpenMP API 的很大一部分是作为 #pragma 实现的。