如何生成sse4.2 popcnt机器指令 [英] How to generate a sse4.2 popcnt machine instruction

问题描述

使用c程序:

int main(int argc , char** argv)
{

  return  __builtin_popcountll(0xf0f0f0f0f0f0f0f0);

}

和编译器行(gcc 4.4-Intel Xeon L3426):

and the compiler line (gcc 4.4 - Intel Xeon L3426):

gcc -msse4.2 poptest.c -o poptest

我没有得到内置的popcnt指令，而是编译器生成了一个查找表并以此方式计算了popcount.生成的二进制文件超过8000个字节. (Y！)

I do NOT get the builtin popcnt insruction rather the compiler generates a lookup table and computes the popcount that way. The resulting binary is over 8000 bytes. (Yuk!)

非常感谢您的帮助.

推荐答案

您必须告诉GCC为支持以下架构的代码生成代码 popcnt指令:

You have to tell GCC to generate code for an architecture that supports the popcnt instruction:

gcc -march=corei7 popcnt.c

或者仅启用对popcnt的支持:

Or just enable support for popcnt:

gcc -mpopcnt popcnt.c

在您的示例程序中，__builtin_popcountll的参数是一个 常量，因此编译器可能会在编译时进行计算 时间，并且永远不会发出popcnt指令.即使没有，GCC也会这样做 要求优化程序.

In your example program the parameter to __builtin_popcountll is a constant so the compiler will probably do the calculation at compile time and never emit the popcnt instruction. GCC does this even if not asked to optimize the program.

因此，请尝试在编译时传递一些它不知道的内容:

So try passing it something that it can't know at compile time:

int main (int argc, char** argv)
{
    return  __builtin_popcountll ((long long) argv);
}

$ gcc -march=corei7 -O popcnt.c && objdump -d a.out | grep '<main>' -A 2
0000000000400454 <main>:
  400454:       f3 48 0f b8 c6          popcnt %rsi,%rax
  400459:       c3                      retq

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