GCC missed optimization?

[Richard Guenther]

On 7/28/06, François-Xavier Coudert <fxcoudert@gmail.com> wrote:
> I've been doing some benchmarking of gfortran, and reducing the
> testcase leads to what seems a missed optimization in the following
> code:
>
> $ cat a.c
> void foo (float * restrict x, float * restrict y)
> {
>   int i;
>
>   for (i = 0; i < 10000; i++)
>     x[i] = y[i] * y[i];
> }
> $ gcc a.c -O1 -ffast-math -msse -mfpmath=sse -ftree-vectorize
> -ftree-vectorizer-verbose=5 -std=c99 -c
>
> a.c:5: note: Alignment of access forced using peeling.
> a.c:5: note: Vectorizing an unaligned access.
> a.c:5: note: not vectorized: relevant stmt not supported: D.1353_14 =
> __builtin_powf (D.1352_13, 2.0e+0)
> a.c:5: note: vectorized 0 loops in function.
>
>
> I find in fold-const.c:fold_binary, around line 9091, I found the following:
>               /* Optimize x*x as pow(x,2.0), which is expanded as x*x.  */
> Should I file it as a missed-optimization bug in bugzilla, or is there
> some clever reason for that behaviour?

It's canonicalization.  I thought about folding to __builtin_powif
instead, which
would make it easier for the vectorizer to recognize it as x*x, which
it needs to
do in any case.

There was also a patch to expand those again at the tree level, which could be
done before the vectorizer.

A bugreport is useful so we don't forget.

Richard.