On Mon, 17 Aug 2026 at 13:57, Richard Biener <[email protected]> wrote:
> On Thu, 13 Aug 2026, Dominic P wrote: > > > A product t * u is an exact multiple of u, so t * u % u is zero under > > every rounding convention whenever the multiplication does not wrap. > > For signed types the overflow is undefined so this always holds; for > > others use value ranges to prove the multiply is overflow-free, exactly > > as the neighbouring (t * u) / u -> t already does. This is the > > remainder counterpart of that fold, for all four modulo codes: the > > C family only produces TRUNC_MOD_EXPR, but Fortran's MODULO and Ada's > > mod produce the floor and ceiling forms, which fold equally. > > > > Assisted-by: Claude Opus 4.8 (Anthropic) > > Looks OK, but how did you test this patch? > Bootstrapped and regtested on x86_64-pc-linux-gnu, no regressions. > > Richard. > > > gcc/ChangeLog: > > > > * match.pd ((t * u) % u -> 0): New simplification. > > > > gcc/testsuite/ChangeLog: > > > > * gcc.dg/fold-mod-mult-1.c: New test. > > > > Signed-off-by: Dominic P <[email protected]> > > --- > > gcc/match.pd | 19 +++++++++++++++++++ > > gcc/testsuite/gcc.dg/fold-mod-mult-1.c | 25 +++++++++++++++++++++++++ > > 2 files changed, 44 insertions(+) > > create mode 100644 gcc/testsuite/gcc.dg/fold-mod-mult-1.c > > > > diff --git a/gcc/match.pd b/gcc/match.pd > > index 3f476cc4baa..90fb2acede4 100644 > > --- a/gcc/match.pd > > +++ b/gcc/match.pd > > @@ -1144,6 +1144,25 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT) > > #endif > > )))) > > > > +/* Simplify (t * u) % u -> 0. The product is an exact multiple of u, so > > + the remainder is zero under every rounding convention as long as the > > + multiplication does not overflow. Mirrors (t * u) / u -> t above. > */ > > +(for mod (trunc_mod floor_mod ceil_mod round_mod) > > + (simplify > > + (mod (mult:c@2 @0 @1) @1) > > + (if (ANY_INTEGRAL_TYPE_P (type)) > > + (if (TYPE_OVERFLOW_UNDEFINED (type) && !TYPE_OVERFLOW_SANITIZED > (type)) > > + { build_zero_cst (type); } > > +#if GIMPLE > > + (with {int_range_max vr0, vr1;} > > + (if (INTEGRAL_TYPE_P (type) > > + && gimple_match_range_of_expr (vr0, @0, @2) > > + && gimple_match_range_of_expr (vr1, @1, @2) > > + && range_op_handler (MULT_EXPR).overflow_free_p (vr0, vr1)) > > + { build_zero_cst (type); })) > > +#endif > > + )))) > > + > > #if GIMPLE > > (for div (trunc_div exact_div) > > /* Simplify (X + M*N) / N -> X / N + M. */ > > diff --git a/gcc/testsuite/gcc.dg/fold-mod-mult-1.c > b/gcc/testsuite/gcc.dg/fold-mod-mult-1.c > > new file mode 100644 > > index 00000000000..9315e94822c > > --- /dev/null > > +++ b/gcc/testsuite/gcc.dg/fold-mod-mult-1.c > > @@ -0,0 +1,25 @@ > > +/* (t * u) % u is zero whenever the product does not overflow: for > signed > > + types the overflow is undefined so it always folds; for unsigned > types it > > + folds when value ranges prove the multiply cannot wrap. */ > > +/* { dg-do compile } */ > > +/* { dg-require-effective-target int32plus } */ > > +/* { dg-options "-O2 -fdump-tree-optimized" } */ > > + > > +int > > +f_signed (int a, int b) > > +{ > > + return (a * b) % b; > > +} > > + > > +unsigned > > +f_ranged (unsigned a, unsigned b) > > +{ > > + a &= 0xffff; > > + b &= 0xffff; > > + if (b == 0) > > + return 7; > > + return (a * b) % b; /* a*b <= 0xfffe0001, cannot wrap > */ > > +} > > + > > +/* Both remainders fold away; no modulo survives. */ > > +/* { dg-final { scan-tree-dump-not " % " "optimized" } } */ > > > > -- > Richard Biener <[email protected]> > SUSE Software Solutions Germany GmbH, > Frankenstrasse 146, 90461 Nuernberg, Germany; > GF: Jochen Jaser, Andrew McDonald, Abhinav Puri; (HRB 36809, AG Nuernberg) > >
