我正在尝试学习gcc自动向量化模块。在阅读这里的文档之后。
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,我只需运行:
$ 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.
根据文档,我本以为会看到明确的一行文字,比如说:
ex1.c:5: note: LOOP VECTORIZED.
但情况并非如此。我使用了命令行选项:
-fopt-info-vec-missed
,因为命令行选项:-ftree-vectorizer-verbose
现在是一个空操作,正如报告所述。那么我的问题是:我如何阅读上面的输出以提取循环实际上是向量化的某种方式?
如果有帮助的话:
$ gcc -dumpversion
4.9.2