我的编译器在做什么？(优化memcpy)

Question

我正在使用VC++ 2010中的以下设置编译一些代码:/ O2/Ob2/Oi/Ot

但是我在理解生成的程序集的某些部分时遇到了一些问题,我在代码中提出了一些问题作为注释.

此外,现代cpu通常建议使用什么预取距离？我可以在我自己的cpu上进行测试,但我希望能够在更广泛的cpu上运行良好的价值.也许可以使用动态预取距离？

< - 编辑:

我很惊讶的另一件事是编译器不以某种形式交错movdqa和movntdq指令？由于这些指令在某种意义上与我的理解不同步.

此代码在预取时也假设32字节高速缓存行,但高端cpus似乎有64字节高速缓存行,因此可能会删除2个预取.

- >

void memcpy_aligned_x86(void* dest, const void* source, size_t size)
{ 
0052AC20  push        ebp  
0052AC21  mov         ebp,esp  
 const __m128i* source_128 = reinterpret_cast<const __m128i*>(source);

 for(size_t n = 0; n < size/16; n += 8) 
0052AC23  mov         edx,dword ptr [size]  
0052AC26  mov         ecx,dword ptr [dest]  
0052AC29  mov         eax,dword ptr [source]  
0052AC2C  shr         edx,4  
0052AC2F  test        edx,edx  
0052AC31  je          copy+9Eh (52ACBEh)  
 __m128i xmm0 = _mm_setzero_si128();
 __m128i xmm1 = _mm_setzero_si128();
 __m128i xmm2 = _mm_setzero_si128();
 __m128i xmm3 = _mm_setzero_si128();
 __m128i xmm4 = _mm_setzero_si128();
 __m128i xmm5 = _mm_setzero_si128();
 __m128i xmm6 = _mm_setzero_si128();
 __m128i xmm7 = _mm_setzero_si128();

 __m128i* dest_128 = reinterpret_cast<__m128i*>(dest);
0052AC37  push        esi  
0052AC38  push        edi  
0052AC39  lea         edi,[edx-1]  
0052AC3C  shr         edi,3  
0052AC3F  inc         edi  
 {
  _mm_prefetch(reinterpret_cast<const char*>(source_128+8), _MM_HINT_NTA);
  _mm_prefetch(reinterpret_cast<const char*>(source_128+10), _MM_HINT_NTA);
  _mm_prefetch(reinterpret_cast<const char*>(source_128+12), _MM_HINT_NTA);
  _mm_prefetch(reinterpret_cast<const char*>(source_128+14), _MM_HINT_NTA);

  xmm0 = _mm_load_si128(source_128++);
  xmm1 = _mm_load_si128(source_128++);
  xmm2 = _mm_load_si128(source_128++);
  xmm3 = _mm_load_si128(source_128++);
  xmm4 = _mm_load_si128(source_128++);
  xmm5 = _mm_load_si128(source_128++);
  xmm6 = _mm_load_si128(source_128++);
  xmm7 = _mm_load_si128(source_128++);
0052AC40  movdqa      xmm6,xmmword ptr [eax+70h]  // 1. Why is this moved before the pretecthes?
0052AC45  prefetchnta [eax+80h]  
0052AC4C  prefetchnta [eax+0A0h]  
0052AC53  prefetchnta [eax+0C0h]  
0052AC5A  prefetchnta [eax+0E0h]  
0052AC61  movdqa      xmm0,xmmword ptr [eax+10h]  
0052AC66  movdqa      xmm1,xmmword ptr [eax+20h]  
0052AC6B  movdqa      xmm2,xmmword ptr [eax+30h]  
0052AC70  movdqa      xmm3,xmmword ptr [eax+40h]  
0052AC75  movdqa      xmm4,xmmword ptr [eax+50h]  
0052AC7A  movdqa      xmm5,xmmword ptr [eax+60h]  
0052AC7F  lea         esi,[eax+70h]  // 2. What is happening in these 2 lines?
0052AC82  mov         edx,eax        //
0052AC84  movdqa      xmm7,xmmword ptr [edx]  // 3. Why edx? and not simply eax?

  _mm_stream_si128(dest_128++, xmm0);
0052AC88  mov         esi,ecx  // 4. Is esi never used?
0052AC8A  movntdq     xmmword ptr [esi],xmm7  
  _mm_stream_si128(dest_128++, xmm1);
0052AC8E  movntdq     xmmword ptr [ecx+10h],xmm0  
  _mm_stream_si128(dest_128++, xmm2);
0052AC93  movntdq     xmmword ptr [ecx+20h],xmm1  
  _mm_stream_si128(dest_128++, xmm3);
0052AC98  movntdq     xmmword ptr [ecx+30h],xmm2  
  _mm_stream_si128(dest_128++, xmm4);
0052AC9D  movntdq     xmmword ptr [ecx+40h],xmm3  
  _mm_stream_si128(dest_128++, xmm5);
0052ACA2  movntdq     xmmword ptr [ecx+50h],xmm4  
  _mm_stream_si128(dest_128++, xmm6);
0052ACA7  movntdq     xmmword ptr [ecx+60h],xmm5  
  _mm_stream_si128(dest_128++, xmm7);
0052ACAC  lea         edx,[ecx+70h]  
0052ACAF  sub         eax,0FFFFFF80h  
0052ACB2  sub         ecx,0FFFFFF80h  
0052ACB5  dec         edi  
0052ACB6  movntdq     xmmword ptr [edx],xmm6  // 5. Why not simply ecx?
0052ACBA  jne         copy+20h (52AC40h)  
0052ACBC  pop         edi  
0052ACBD  pop         esi  
 }
}

原始代码:

void memcpy_aligned_x86(void* dest, const void* source, size_t size)
{ 
 assert(dest != nullptr);
 assert(source != nullptr);
 assert(source != dest);
 assert(size % 128 == 0);

 __m128i xmm0 = _mm_setzero_si128();
 __m128i xmm1 = _mm_setzero_si128();
 __m128i xmm2 = _mm_setzero_si128();
 __m128i xmm3 = _mm_setzero_si128();
 __m128i xmm4 = _mm_setzero_si128();
 __m128i xmm5 = _mm_setzero_si128();
 __m128i xmm6 = _mm_setzero_si128();
 __m128i xmm7 = _mm_setzero_si128();

 __m128i* dest_128 = reinterpret_cast<__m128i*>(dest);
 const __m128i* source_128 = reinterpret_cast<const __m128i*>(source);

 for(size_t n = 0; n < size/16; n += 8) 
 {
  _mm_prefetch(reinterpret_cast<const char*>(source_128+8), _MM_HINT_NTA);
  _mm_prefetch(reinterpret_cast<const char*>(source_128+10), _MM_HINT_NTA);
  _mm_prefetch(reinterpret_cast<const char*>(source_128+12), _MM_HINT_NTA);
  _mm_prefetch(reinterpret_cast<const char*>(source_128+14), _MM_HINT_NTA);

  xmm0 = _mm_load_si128(source_128++);
  xmm1 = _mm_load_si128(source_128++);
  xmm2 = _mm_load_si128(source_128++);
  xmm3 = _mm_load_si128(source_128++);
  xmm4 = _mm_load_si128(source_128++);
  xmm5 = _mm_load_si128(source_128++);
  xmm6 = _mm_load_si128(source_128++);
  xmm7 = _mm_load_si128(source_128++);

  _mm_stream_si128(dest_128++, xmm0);
  _mm_stream_si128(dest_128++, xmm1);
  _mm_stream_si128(dest_128++, xmm2);
  _mm_stream_si128(dest_128++, xmm3);
  _mm_stream_si128(dest_128++, xmm4);
  _mm_stream_si128(dest_128++, xmm5);
  _mm_stream_si128(dest_128++, xmm6);
  _mm_stream_si128(dest_128++, xmm7);
 }
}

Answer 1

eax+70h 读取会向上移动，因为 eax+70h 与 eax 位于不同的缓存行中，并且编译器可能希望硬件预取器尽快忙于获取该行。

它不进行交错，要么是因为它希望通过避免加载到存储的依赖关系来最大化性能（即使 AMD 优化指南明确指出要交错），要么只是因为它不确定存储不会覆盖加载。如果将 __restrict 关键字添加到源和目标，它会改变行为吗？

其余部分的目的也让我无法理解。对于 AMD 或 Intel 来说，可能是一些模糊的指令解码或硬件预取器考虑因素，但我找不到任何理由。我想知道当你删除这些指令时代码会变得更快还是更慢？

建议的预取距离取决于循环大小。需要足够远，以便数据在需要时有时间从内存到达。我认为你通常需要给它至少 100 个时钟周期。