了解 gcc 4.9.2 自动矢量化输出

Question

我正在尝试学习 gcc 自动矢量化模块。阅读 here.

的文档后

这是我尝试过的 (debian jessie amd64)：

$ cat ex1.c
int a[256], b[256], c[256];
foo () {
  int i;

  for (i=0; i<256; i++){
    a[i] = b[i] + c[i];
  }
}

然后，我只是运行:

$ gcc  -x c -Ofast -msse2 -c   -ftree-vectorize -fopt-info-vec-missed ex1.c
ex1.c:5:3: note: misalign = 0 bytes of ref b[i_11]
ex1.c:5:3: note: misalign = 0 bytes of ref c[i_11]
ex1.c:5:3: note: misalign = 0 bytes of ref a[i_11]
ex1.c:5:3: note: virtual phi. skip.
ex1.c:5:3: note: num. args = 4 (not unary/binary/ternary op).
ex1.c:5:3: note: not ssa-name.
ex1.c:5:3: note: use not simple.
ex1.c:5:3: note: num. args = 4 (not unary/binary/ternary op).
ex1.c:5:3: note: not ssa-name.
ex1.c:5:3: note: use not simple.
ex1.c:2:1: note: not vectorized: not enough data-refs in basic block.
ex1.c:6:13: note: not vectorized: no vectype for stmt: vect__4.5_1 = MEM[(int *)vectp_b.3_9];
 scalar_type: vector(4) int
ex1.c:6:13: note: not vectorized: not enough data-refs in basic block.
ex1.c:2:1: note: not vectorized: not enough data-refs in basic block.
ex1.c:8:1: note: not vectorized: not enough data-refs in basic block.

根据 documentation，我会假设看到一条清晰的线，上面写着这样的话：

ex1.c:5: note: LOOP VECTORIZED.

但事实并非如此。我使用了命令行选项：-fopt-info-vec-missed，因为命令行选项：-ftree-vectorizer-verbose 现在是空操作，因为 per report.

所以我的问题是：如何读取上面的输出以提取循环实际上以某种方式矢量化的信息？

如果有帮助：

$ gcc -dumpversion
4.9.2

Answer 1

实际上在 gcc 在线文档中挖掘，我终于发现我应该使用：-fopt-info-vec-optimized（或者可能是 -fopt-info-vec-all）。见 here and here:

optimized: Print information when an optimization is successfully applied. It is up to a pass to decide which information is relevant. For example, the vectorizer passes print the source location of loops which are successfully vectorized.

missed: Print information about missed optimizations. Individual passes control which information to include in the output.

note: Print verbose information about optimizations, such as certain transformations, more detailed messages about decisions etc.

all: Print detailed optimization information. This includes ‘optimized’, ‘missed’, and ‘note’.

了解 gcc 4.9.2 自动矢量化输出

Understanding gcc 4.9.2 auto-vectorization output

c

gcc

auto-vectorization