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

> On 6 Aug 2026, at 06:58, Jeffrey Law <[email protected]> wrote:
> 
> 
> 
> 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 :-)

I asked an AI agent to mine SPEC2026 and a few other workloads that are of interest to me to find missing folds.
It turned out to be a quite productive exercise. I can recommend it as a way to constructively use these agents for things other than writing GCC code. I don’t think this particular fold is on a hot path or anything, but it is obviously a beneficial one anyway (it reduces the complexity of the IR, as match.pd folds intend) 

> 
> 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.

Yes, will do.
Thanks,
Kyrill

> 
> Thanks,
> 
> jeff