编译器是否自动优化对数学函数的重复调用? [英] Do compilers automatically optimise repeated calls to mathematical functions?

问题描述

说我有以下代码段:

#include <cmath>

// ...

float f = rand();
std::cout << sin(f) << " " << sin(f);

由于sin(f)是定义明确的函数，因此很容易优化:

As sin(f) is a well defined function there is an easy optimisation:

float f = rand();
float sin_f = sin(f);
std::cout << sin_f << " " << sin_f;

期望现代C ++编译器自己执行此优化是合理的吗?还是编译器无法确定sin(f)应该始终为f的相等值返回相同的值?

Is this an optimisation that it's reasonable to expect a modern C++ compiler to do by itself? Or is there no way for the compiler to determine that sin(f) should always return the same value for an equal value of f?

推荐答案

使用通过默认优化标记构建的g ++:

Using g++ built with default optimization flags:

float f = rand();
40117e: e8 75 01 00 00          call   4012f8 <_rand>
401183: 89 44 24 1c             mov    %eax,0x1c(%esp)
401187: db 44 24 1c             fildl  0x1c(%esp)
40118b: d9 5c 24 2c             fstps  0x2c(%esp)
std::cout << sin(f) << " " << sin(f);
40118f: d9 44 24 2c             flds   0x2c(%esp)
401193: dd 1c 24                fstpl  (%esp)
401196: e8 65 01 00 00          call   401300 <_sin>  <----- 1st call
40119b: dd 5c 24 10             fstpl  0x10(%esp)
40119f: d9 44 24 2c             flds   0x2c(%esp)
4011a3: dd 1c 24                fstpl  (%esp)
4011a6: e8 55 01 00 00          call   401300 <_sin>  <----- 2nd call
4011ab: dd 5c 24 04             fstpl  0x4(%esp)
4011af: c7 04 24 e8 60 40 00    movl   $0x4060e8,(%esp)

内置-O2:

float f = rand();
4011af: e8 24 01 00 00          call   4012d8 <_rand>
4011b4: 89 44 24 1c             mov    %eax,0x1c(%esp)
4011b8: db 44 24 1c             fildl  0x1c(%esp)
std::cout << sin(f) << " " << sin(f);
4011bc: dd 1c 24                fstpl  (%esp)
4011bf: e8 1c 01 00 00          call   4012e0 <_sin>  <----- 1 call

由此我们可以看到，没有优化，编译器只使用2个调用，而只有1个带有优化，我凭经验可以说编译器确实优化了调用.

From this we can see that without optimizations the compiler uses 2 calls and just 1 with optimizations, empirically I guess, we can say the compiler does optimize the call.

这篇关于编译器是否自动优化对数学函数的重复调用?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！