Re: [PATCH] match.pd: Use ranges to optimize some x * y / y to x [PR97997]

[Jakub Jelinek <jakub@redhat.com>]

On Thu, Nov 26, 2020 at 09:24:30AM +0000, Richard Biener wrote:
> > For signed integers with undefined overflow we already optimize x * y / y
> > into x, but for signed integers with -fwrapv or unsigned integers we don't.
> > The following patch allows optimizing that into just x if value ranges
> > prove that x * y will never overflow.
> > It uses the global SSA_NAME_RANGE_INFO only, because like mentioned
> > in another PR we don't currently have a way to tell the ranger from match.pd
> > the use stmt (and we'd need in that case to tell ranger to only follow
> > SSA_NAME_DEF_STMTs + SSA_NAME_RANGE_INFO and never go in the other
> > direction, as following immediate uses seems forbidden in match.pd).
> > Another possibility would be to optimize this during vrp, but on the
> > other side the optimization itself is match.pd-ish.
> > 
> > Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
> 
> Hmm, can't you match
> 
> >    (div (mult@3:c @0 @1) @1)
> 
> and then look at the range of @3 directly?

No.  I need to check whether the multiplication will never overflow,
i.e. I need @3 range in the infinite precision (for multiplication
twice as large precision as the multiplication (when the arguments and
result have the same precision, which is the case for IL MULT_EXPR)),
while the @3 precision is that after wrapping it into the @3's precision.
So, e.g. the multiplication could always overflow, yet the range
wouldn't be VARYING, consider
@0 in [0x4000000, 0x40000ff] and @1 64, then the infinite precision
range would be [0x100000000, 0x100003fc0], but @3 range is
[0, 0x3fc0], etc.  What I need to check is essentially that
__builtin_mul_overflow_p (@0, @1, (typeof (@0)) 0) folds to constant 0.

	Jakub