gcc 不会矢量化简单循环

Question

我正在尝试从 gcc auto-vectorize documentation 向量化示例 4 的简化版本。对于我的生活，我不知道该怎么做；

typedef int aint __attribute__ ((__aligned__(16)));
void foo1 (int n, aint * restrict px, aint *restrict qx) {

  /* feature: support for (aligned) pointer accesses.  */
  int *__restrict p = __builtin_assume_aligned (px, 16);
  int *__restrict q = __builtin_assume_aligned (qx, 16);

  while (n--){
    //*p++ += *q++; <- this is vectorized                                                                                                                                                                   
    p[n] += q[n]; // This isn't!                                                                                                                                                                            
  }
}

我正在运行 gcc 4.7.2 gcc -o 应用程序/craft_dbsplit.o -c -Wall -g -ggdb -O3 -msse2 -funsafe-math-optimizations -ffast-math -ftree-vectorize -ftree-vectorizer-verbose=5 -funsafe-loop-optimizations -std =c99

它会回复：

Analyzing loop at apps/craft_dbsplit.c:388

388: dependence distance  = 0.
388: dependence distance == 0 between *D.9363_14 and *D.9363_14
388: dependence distance  = 0.
388: accesses have the same alignment.
388: dependence distance modulo vf == 0 between *D.9363_14 and *D.9363_14
388: vect_model_load_cost: unaligned supported by hardware.
388: vect_get_data_access_cost: inside_cost = 2, outside_cost = 0.
388: vect_model_store_cost: unaligned supported by hardware.
388: vect_get_data_access_cost: inside_cost = 2, outside_cost = 0.
388: Alignment of access forced using peeling.
388: Vectorizing an unaligned access.
388: vect_model_load_cost: aligned.
388: vect_model_load_cost: inside_cost = 1, outside_cost = 0 .
388: vect_model_load_cost: unaligned supported by hardware.
388: vect_model_load_cost: inside_cost = 2, outside_cost = 0 .
388: vect_model_simple_cost: inside_cost = 1, outside_cost = 0 .
388: not vectorized: relevant stmt not supported: *D.9363_14 = D.9367_20;

apps/craft_dbsplit.c:382: note: vectorized 0 loops in function.

Answer 1

循环从高地址运行到低地址。您的 gcc 将向量操作视为从低地址运行到高地址，因此没有意识到它可以向量化。您的“优化”使循环成为while (n--)，实际上是在阻止更相关的优化。试试

#include <stddef.h>

void foo1 (size_t n, int *restrict px, int const *restrict qx)
{
  int *restrict p = __builtin_assume_aligned(px, 16);
  int const *restrict q = __builtin_assume_aligned(qx, 16);
  size_t i = 0;
  while (i < n)
    {
      p[i] += q[i];
      i++;
    }
}