Re: [PATCH v6] forwprop: add simplify_phi_result_movdiv() [PR101179]

On 8/10/2026 10:32 AM, Daniel Barboza wrote:
> This new forwprop  step is my attempt to implement Richi's suggestions from
> v1 of this work [1] where he suggested to push things out of match.pd.
>
> The idea is to simplify DIV/MOD into RSHIFT/BIT_AND ops in which the
> divisor are pow2 integers in a PHI.  E.g.:
>
> phi_var = PHI <16,4>
> _x = _y % phi_var
>
> Can be turned into:
>
> phi_var = PHI <15,3>
> _x = _y & phi_var
>
> As long as we know that _y is a positive number or '_x' is used just in
> zero equality comparisons.
>
> Most of 101179 use cases are solved by this change.  PHI with 2+ args
> are supported as long as every phi_arg meets the criteria.
>
> Boostrapped and regression tested with x86_64, aarch64 and riscv64.
>
> [1] https://gcc.gnu.org/pipermail/gcc-patches/2026-May/716303.html
>
> 	PR tree-optimization/101179
>
> gcc/ChangeLog:
>
> 	* tree-ssa-forwprop.cc (simplify_phi_result_movdiv): New
> 	forwprop step where MOD/DIV ops with pow2 divisors can be
> 	simplified to BIT_AND/RSHIFT.
> 	(pass_forwprop::execute): Call simplify_phi_result_movdiv.
>
> gcc/testsuite/ChangeLog:
>
> 	* gcc.dg/tree-ssa/pr101179.c: New test.
Can we really optimize the signed case when the result is used in a 
comparison against zero?  Consider something like:

y / (x ? 4 : 16) == 0

Where "y" is a small negative number, say -2.  In the original form that 
expression would be true.  In the right shifted form it would be false.

Did I miss something here?

Jeff