> On 24 Jul 2026, at 20:15, Jeffrey Law <[email protected]> wrote:
>
>
>
> On 7/22/2026 9:24 AM, [email protected] wrote:
>> From: Kyrylo Tkachov <[email protected]>
>>
>> init_noce_multiple_sets_info records earlier SET destinations that are
>> mentioned by a later SET source. When a pseudo is set more than once, only
>> its most recent prior definition reaches that source.
>>
>> Recording every definition is unsafe in the second
>> noce_convert_multiple_sets_1 attempt. The newest definition can be emitted
>> directly into its target pseudo, making its replacement a no-op. A later
>> replacement using an older definition then substitutes a stale value.
>>
>> PR126184 contains the following dependency chain:
>>
>> c = x + 1;
>> x = c * y;
>> c = z + 3;
>> y = c * x;
>>
>> The unfixed conversion computes the last multiply with the temporary that
>> holds `x + 1`, rather than the reaching `z + 3` value.
>>
>> The unfixed final sequence forms `x + 1` in x0 and later uses x0 as the
>> multiply operand:
>>
>> add x0, x1, 1
>> csel x1, x1, x3, eq
>> mul x0, x1, x0
>>
>> With the fix, x3 retains z until `z + 3` is formed and used by the multiply:
>>
>> add x3, x3, 3
>> csel x0, x0, x1, ne
>> mul x3, x0, x3
>>
>> Walk definitions from newest to oldest and record only the first match for
>> each pseudo. Add an AArch64 execution test for the reported dependency.
>>
>> Bootstrapped and tested on aarch64-none-linux-gnu and
>> x86_64-pc-linux-gnu.
>>
>> gcc/ChangeLog:
>>
>> PR rtl-optimization/126184
>> * ifcvt.cc (init_noce_multiple_sets_info): Record only the most
>> recent prior definition of each pseudo register.
>>
>> gcc/testsuite/ChangeLog:
>>
>> PR rtl-optimization/126184
>> * gcc.target/aarch64/pr126184.c: New test.
> OK
Thanks, I committed the two patches over the weekend.
Richard: is this and the prerequisite patch at:
https://gcc.gnu.org/cgit/gcc/commit/?id=7b4d5b42a78859b30c9b48fa0a426effbbd71ff7
Ok to backport for GCC 16.2?
They should be fairly localized wrong-code fixes for ifcvt.
Thanks,
Kyrill
> jeff
