https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118739
--- Comment #18 from GCC Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Uros Bizjak <u...@gcc.gnu.org>: https://gcc.gnu.org/g:a92dc3fe31c95d56019b2fb95a58414bca06241f commit r15-7793-ga92dc3fe31c95d56019b2fb95a58414bca06241f Author: Uros Bizjak <ubiz...@gmail.com> Date: Wed Feb 12 11:19:57 2025 +0100 combine: Discard REG_UNUSED note in i2 when register is also referenced in i3 [PR118739] The combine pass is trying to combine: Trying 16, 22, 21 -> 23: 16: r104:QI=flags:CCNO>0 22: {r120:QI=r104:QI^0x1;clobber flags:CC;} REG_UNUSED flags:CC 21: r119:QI=flags:CCNO<=0 REG_DEAD flags:CCNO 23: {r110:QI=r119:QI|r120:QI;clobber flags:CC;} REG_DEAD r120:QI REG_DEAD r119:QI REG_UNUSED flags:CC and creates the following two insn sequence: modifying insn i2 22: r104:QI=flags:CCNO>0 REG_DEAD flags:CC deferring rescan insn with uid = 22. modifying insn i3 23: r110:QI=flags:CCNO<=0 REG_DEAD flags:CC deferring rescan insn with uid = 23. where the REG_DEAD note in i2 is not correct, because the flags register is still referenced in i3. In try_combine() megafunction, we have this part: --cut here-- /* Distribute all the LOG_LINKS and REG_NOTES from I1, I2, and I3. */ if (i3notes) distribute_notes (i3notes, i3, i3, newi2pat ? i2 : NULL, elim_i2, elim_i1, elim_i0); if (i2notes) distribute_notes (i2notes, i2, i3, newi2pat ? i2 : NULL, elim_i2, elim_i1, elim_i0); if (i1notes) distribute_notes (i1notes, i1, i3, newi2pat ? i2 : NULL, elim_i2, local_elim_i1, local_elim_i0); if (i0notes) distribute_notes (i0notes, i0, i3, newi2pat ? i2 : NULL, elim_i2, elim_i1, local_elim_i0); if (midnotes) distribute_notes (midnotes, NULL, i3, newi2pat ? i2 : NULL, elim_i2, elim_i1, elim_i0); --cut here-- where the compiler distributes REG_UNUSED note from i2: 22: {r120:QI=r104:QI^0x1;clobber flags:CC;} REG_UNUSED flags:CC via distribute_notes() using the following: --cut here-- /* Otherwise, if this register is used by I3, then this register now dies here, so we must put a REG_DEAD note here unless there is one already. */ else if (reg_referenced_p (XEXP (note, 0), PATTERN (i3)) && ! (REG_P (XEXP (note, 0)) ? find_regno_note (i3, REG_DEAD, REGNO (XEXP (note, 0))) : find_reg_note (i3, REG_DEAD, XEXP (note, 0)))) { PUT_REG_NOTE_KIND (note, REG_DEAD); place = i3; } --cut here-- Flags register is used in I3, but there already is a REG_DEAD note in I3. The above condition doesn't trigger and continues in the "else" part where REG_DEAD note is put to I2. The proposed solution corrects the above logic to trigger every time the register is referenced in I3, avoiding the "else" part. PR rtl-optimization/118739 gcc/ChangeLog: * combine.cc (distribute_notes) <case REG_UNUSED>: Correct the logic when the register is used by I3. gcc/testsuite/ChangeLog: * gcc.target/i386/pr118739.c: New test.
