Re: GCC missed optimization?

[Dorit Nuzman <DORIT at il dot ibm dot com>]

I think this patch by Zdenek - vectorizing function calls - is related:
http://gcc.gnu.org/ml/gcc-patches/2006-03/msg01655.html
(would need to be extended to cover this case).

dorit


> 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.