标准C ++ 11 code相当于PEXT的Haswell指令（并可能被编译器优化） [英] Standard C++11 code equivalent to the PEXT Haswell instruction (and likely to be optimized by compiler)

问题描述

在Haswell的结构出现了几个新的指令。其中之一是 PEXT （并行位提取），其功能通过这一形象（来源这里）解释说：

The Haswell architectures comes up with several new instructions. One of them is PEXT (parallel bits extract) whose functionality is explained by this image (source here):

这需要一个值 R2 和掩码 R3 ，并把的提取位R2 到 R1 。

It takes a value r2 and a mask r3 and puts the extracted bits of r2 into r1.

我的问题是：这将是一个优化的模板功能的纯标准 C ++ 11，这很可能会进行优化，以通过编译器该指令相当于code在未来的。

My question is the following: what would be the equivalent code of an optimized templated function in pure standard C++11, that would be likely to be optimized to this instruction by compilers in the future.

推荐答案

下面是的马太福音Fioravante的 href=\"https://github.com/fmatthew5876/stdcxx-bitops\" rel=\"nofollow\"> stdcxx-bitops GitHub库这是浮动到 STD-建议 邮件列表为preliminary建议增加一个 constexpr 位运算库，用于C ++。

Here is some code from Matthew Fioravante's stdcxx-bitops GitHub repo that was floated to the std-proposals mailinglist as a preliminary proposal to add a constexpr bitwise operations library for C++.

#ifndef HAS_CXX14_CONSTEXPR
#define HAS_CXX14_CONSTEXPR 0
#endif

#if HAS_CXX14_CONSTEXPR
#define constexpr14 constexpr
#else
#define constexpr14
#endif

//Parallel Bits Extract
//x    HGFEDCBA
//mask 01100100
//res  00000GFC
//x86_64 BMI2: PEXT
template <typename Integral>
constexpr14 Integral extract_bits(Integral x, Integral mask) {
  Integral res = 0;
  for(Integral bb = 1; mask != 0; bb += bb) {
    if(x & mask & -mask) {
      res |= bb;
    }
    mask &= (mask - 1);
  }
  return res;
}

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