Re: [PATCH] Fix PR77407

[Richard Biener <rguenther at suse dot de>]

On Tue, 1 Nov 2016, Marc Glisse wrote:

> On Mon, 31 Oct 2016, Richard Biener wrote:
> 
> > On Fri, 28 Oct 2016, Marc Glisse wrote:
> > 
> > > On Wed, 28 Sep 2016, Richard Biener wrote:
> > > 
> > > > The following patch implements patterns to catch x / abs (x)
> > > > and x / -x, taking advantage of undefinedness at x == 0 as
> > > > opposed to the PR having testcases with explicit != 0 checks.
> > > > 
> > > > Bootstrap / regtest pending on x86_64-unknown-linux-gnu.
> > > > 
> > > > Richard.
> > > > 
> > > > 2016-09-28  Richard Biener  <rguenther@suse.de>
> > > > 
> > > > 	PR middle-end/77407
> > > > 	* match.pd: Add X / abs (X) -> X < 0 ? -1 : 1 and
> > > > 	X / -X -> -1 simplifications.
> > > 
> > > I notice that we still have the following comment a few lines above:
> > > 
> > > /* Make sure to preserve divisions by zero.  This is the reason why
> > >    we don't simplify x / x to 1 or 0 / x to 0.  */
> > > 
> > > Did we give up on preserving divisions by 0? Can we now do the 2
> > > simplifications listed by the comment?
> > 
> > At some point there was at least diagnostics fallout when doing them.
> > There may be also undefined sanitizer fallout depending on when we
> > instrument for that.
> > 
> > But in general yes, we do want to do the two simplifications.  Maybe
> > we can compromise (in case of early fallout) to do them on GIMPLE
> > only.
> > 
> > We could at least add them with a proper nonzero_p predicate.
> 
>  (for div (trunc_div ceil_div floor_div round_div exact_div)
> + (simplify (div @0 @0) { build_one_cst (type); })
> + (simplify (div integer_zerop@0 @1) @0)
> 
> causes no regression on powerpc64le-unknown-linux-gnu with
> --enable-languages=all,obj-c++,go.

Good.  I probably tried last before the C++ early folding changes.

If you'd formally post a patch adding the above (and adjusting the
comment) I'll approve that.

This eventually means we can remove the if (integer_zerop ()) early out
in fold_binary_loc as well.

Richard.