On Sun, Oct 5, 2025 at 11:20 PM Richard Biener
<[email protected]> wrote:
>
> On Mon, Oct 6, 2025 at 2:15 AM Andrew Pinski
> <[email protected]> wrote:
> >
> > When I tried to fix this before I didn't realize there was already a
> > pattern for
> > `-(a ptrdiff b) -> (b ptrdiff a)`, I had added a complex pattern to match
> > `ptr0 - (ptr0 - ptr1)`.
> > But with there being a pattern for `-(a ptrdiff b)`, we just need to extend
> > the pattern
> > to support a nop conversion inbetween the negative and the ptrdiff.
> > Also the check for TYPE_OVERFLOW_UNDEFINED was wrong, in the case of `-(a -
> > b) -> (b - a)`, the check
> > is !TYPE_OVERFLOW_SANITIZED so this pattern should use the same check.
> >
> > PR tree-optimization/121921
> >
> > gcc/ChangeLog:
> >
> > * match.pd (`-(a ptrdiff b)`): Extend for a nop_convert
> > between the neg and ptrdiff.
> >
> > gcc/testsuite/ChangeLog:
> >
> > * gcc.dg/pr121921-1.c: New test.
> >
> > Signed-off-by: Andrew Pinski <[email protected]>
> > ---
> > gcc/match.pd | 6 +++---
> > gcc/testsuite/gcc.dg/pr121921-1.c | 15 +++++++++++++++
> > 2 files changed, 18 insertions(+), 3 deletions(-)
> > create mode 100644 gcc/testsuite/gcc.dg/pr121921-1.c
> >
> > diff --git a/gcc/match.pd b/gcc/match.pd
> > index 6f896aa7961..a2353be099f 100644
> > --- a/gcc/match.pd
> > +++ b/gcc/match.pd
> > @@ -2133,9 +2133,9 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
> > && !HONOR_SIGNED_ZEROS (type)))
> > (minus @1 @0)))
> > (simplify
> > - (negate (pointer_diff @0 @1))
> > - (if (TYPE_OVERFLOW_UNDEFINED (type))
> > - (pointer_diff @1 @0)))
> > + (negate (nop_convert? (pointer_diff@2 @0 @1)))
> > + (if (ANY_INTEGRAL_TYPE_P (type) && !TYPE_OVERFLOW_SANITIZED (type))
> > + (convert (pointer_diff:ssizetype @1 @0))))
>
> The patch is OK, but is this technically OK when @1 is from a
> named address-space with smaller pointers? The gimple
> verifier requires the same precision on the difference and the
> pointer, so I guess a more defensive variant would be
>
> (with { tree ptrdifft = signed_type_for (TREE_TYPE (@0)); }
> (convert (pointer_diff:ptrdifft @1 @0))))
>
> ? I realize there's at least one other place using pointer_diff:ssizetype
> but that guards against this with TYPE_PRECISION (TREE_TYPE (@0)) ==
> TYPE_PRECISION (sizetype)
Actually since we already have the ptrdifftype, I am just going to use that.
So the pattern becomes:
```
(simplify
(negate (nop_convert? (pointer_diff@2 @0 @1)))
(if (ANY_INTEGRAL_TYPE_P (type) && !TYPE_OVERFLOW_SANITIZED (type))
(with { tree ptrdifftype = TREE_TYPE (@2); }
(convert (pointer_diff:ptrdifftype @1 @0)))))
```
I actually captured the pointer_diff in the old patch as I was trying
to figure out what the best choice was for the type was and then saw
ssizetype being used below (though I had missed the check that you
mentioned). We check to make sure both @0 and @1 are the same mode in
tree-cfg and I assume for pointers we always have the same precision
as the mode's precision too; that is PSI mode if we have say 20bit
pointers.
For the other pattern that has :ssizetype, we only check if one of the
pointers is the same precision as ssizetype and NOT both so using
ptrdifftype here should be ok too.
I suspect we are not allowing different precision pointers for
pointer_diff indirectly via the mode check too.
Thanks,
Andrew
>
> Richard.
>
> >
> > /* A - B -> A + (-B) if B is easily negatable. */
> > (simplify
> > diff --git a/gcc/testsuite/gcc.dg/pr121921-1.c
> > b/gcc/testsuite/gcc.dg/pr121921-1.c
> > new file mode 100644
> > index 00000000000..828472abbd5
> > --- /dev/null
> > +++ b/gcc/testsuite/gcc.dg/pr121921-1.c
> > @@ -0,0 +1,15 @@
> > +/* { dg-do compile } */
> > +/* { dg-options "-O -fdump-tree-cddce1" } */
> > +
> > +/* PR tree-optimization/121921 */
> > +
> > +int *
> > +fx (int *b, int *e)
> > +{
> > + __SIZE_TYPE__ p = b - e;
> > + /* The first forwprop pass should optimize this to return e; */
> > + return b - p;
> > +}
> > +
> > +/* { dg-final { scan-tree-dump "return e" "cddce1" } } */
> > +
> > --
> > 2.43.0
> >
