Re: [PATCH] RISC-V: Don't let popretz combine across a call [PR target/126454]

On 8/5/2026 8:31 AM, Kito Cheng wrote:
> The popretz pass scans backwards from cm.popret for the "li a0, 0" that
> feeds the (use a0), but nothing stopped the scan at a call: the argument
> registers of a call live in CALL_INSN_FUNCTION_USAGE, which
> reg_referenced_p does not look at, and the a0 set by a call_value hides
> in a PARALLEL, which the bare SET test did not match.  A "li a0, 0" that
> sets up the first argument of a call was therefore deleted and cm.popret
> became cm.popretz, so the callee got garbage in a0 and the caller
> returned 0 instead of the result of the callee.
>
> Use insn level helpers instead, find_reg_fusage for the uses and
> reg_set_p for the definitions, and ask them about the word_mode a0
> rather than about the a0 of the (use a0), whose mode covers a0 and a1
> for a DImode return value on rv32.
>
> gcc/ChangeLog:
>
> 	PR target/126454
> 	* config/riscv/riscv-opt-popretz.cc (pass_combine_popretz::execute):
> 	Stop the backward scan at any use or definition of a0, including
> 	those hidden in CALL_INSN_FUNCTION_USAGE or in a PARALLEL.
>
> gcc/testsuite/ChangeLog:
>
> 	PR target/126454
> 	* gcc.target/riscv/pr126454.c: New test.
Looks sensible, but you know this code far better than I. Presumably 
it's run after register allocation, so you don't have to worry about 
something like (subreg (reg A0)) as that will have been simplified as we 
leave register allocation.

OK by me.
jeff