标准C ++ 11代码相当于PEXT Haswell指令（可能由编译器优化） [英] Standard C++11 code equivalent to the PEXT Haswell instruction (and likely to be optimized by compiler)

问题描述

Haswell架构提供了几个新指令。其中一个是 PEXT （ < a href =https://github.com/fmatthew5876/stdcxx-bitops =nofollow> stdcxx-bitops GitHub repo 浮动到 std-proposals mailinglist作为初步建议，为C ++添加 constexpr 位运算库。

  #ifndef HAS_CXX14_CONSTEXPR 
 #define HAS_CXX14_CONSTEXPR 0 
 #endif 
 
 #if HAS_CXX14_CONSTEXPR 
 #define constexpr14 constexpr 
 #else 
＃定义constexpr14 
 #endif 
 
 //平行位提取
 // x HGFEDCBA 
 //掩码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; 
} 
  

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):

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

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.

解决方案

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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