https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126184
--- Comment #8 from GCC Commits <cvs-commit at gcc dot gnu.org> --- The releases/gcc-16 branch has been updated by Kyrylo Tkachov <[email protected]>: https://gcc.gnu.org/g:642e52b049a24b28a0495f4d2c08f97ad2cb6666 commit r16-9423-g642e52b049a24b28a0495f4d2c08f97ad2cb6666 Author: Kyrylo Tkachov <[email protected]> Date: Wed Jul 22 16:45:51 2026 +0200 ifcvt: Keep only reaching definitions when rewiring sets [PR126184] 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. Signed-off-by: Kyrylo Tkachov <[email protected]> (cherry picked from commit f6b00aefc25a25ade7524649605fa83fba80c119)
