Re: [PATCH] match: fold a remainder compared with its dividend

On 8/4/2026 4:06 AM, [email protected] wrote:
> From: Kyrylo Tkachov <[email protected]>
>
> A remainder of non-negative operands equals its dividend exactly when the
> dividend is smaller than the divisor.  Comparing the two therefore does
> not need the division at all.  This sits next to the (X / Y) == 0 rule,
> which has the same shape and the same non-negativity requirement.
>
>    int f (unsigned x, unsigned y) { return x % y == x; }
>
> aarch64 -O2:
>
>    before                          after
>      udiv  w2, w0, w1                cmp   w0, w1
>      msub  w2, w2, w1, w0            cset  w0, cc
>      cmp   w2, w0
>      cset  w0, eq
>
> The :s marker cannot reject this flat replacement.  Remove the inactive
> marker.
>
> A zero divisor can raise a non-call exception.  Keep the division or
> remainder when the divisor might be zero and non-call exceptions are
> enabled.  Also keep an explicit zero divisor for diagnostics.
>
> Bootstrapped and tested on aarch64-none-linux-gnu.
> Ok for trunk?
> Thanks,
> Kyrill
>
> gcc/ChangeLog:
>
> 	* match.pd ((X / Y) ==/!= 0): Preserve a possible zero-divisor
> 	exception.
> 	((X % Y) ==/!= X): New simplification.  Preserve a possible
> 	zero-divisor exception.
>
> gcc/testsuite/ChangeLog:
>
> 	* gcc.dg/tree-ssa/modcmp-1.c: New test.
> 	* gcc.dg/tree-ssa/modcmp-noncall-1.c: Likewise.
Like most of these, I'm curious where this showed up :-)

It looks correct to me.  OK for the trunk.  Note that your cover 
indicates you removed a :s tag, but I don't see that in the patch. 
  Probably best to resolve that one way or another before committing.

Thanks,

jeff