On Wed, 26 Feb 2025, Uros Bizjak wrote:
> 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.
>
> Bootstrapped and regression tested on x86_64-linux-gnu {,-m32}.
>
> OK for master and eventual backports?
OK if there's no other comments on this until Monday.
Thanks,
Richard.
