On Wed, Feb 12, 2025 at 4:16 PM Richard Sandiford
<richard.sandif...@arm.com> wrote:
>
> Uros Bizjak <ubiz...@gmail.com> writes:
> > 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
>
> It looks like something has already gone wrong in this sequence,
> in that insn 21 is using the flags after the register has been clobbered.
> If the flags result of insn 22 is useful, the insn should be setting the
> flags using a parallel of two sets.
Please note that the insn sequence before combine pass is correct:
16: r104:QI=flags:CCNO>0
21: r119:QI=flags:CCNO<=0
REG_DEAD flags:CCNO
22: {r120:QI=r104:QI^0x1;clobber flags:CC;}
REG_UNUSED flags:CC
23: {r110:QI=r119:QI|r120:QI;clobber flags:CC;}
REG_DEAD r120:QI
REG_DEAD r119:QI
REG_UNUSED flags:CC
then combine tries to make the combined insn with:
Trying 16, 22, 21 -> 23:
where:
Failed to match this instruction:
(parallel [
(set (reg:QI 110 [ d_lsm_flag.20 ])
(le:QI (reg:CCNO 17 flags)
(const_int 0 [0])))
(clobber (reg:CC 17 flags))
(set (reg:QI 104 [ _36 ])
(gt:QI (reg:CCNO 17 flags)
(const_int 0 [0])))
])
Failed to match this instruction:
(parallel [
(set (reg:QI 110 [ d_lsm_flag.20 ])
(le:QI (reg:CCNO 17 flags)
(const_int 0 [0])))
(set (reg:QI 104 [ _36 ])
(gt:QI (reg:CCNO 17 flags)
(const_int 0 [0])))
])
Successfully matched this instruction:
(set (reg:QI 104 [ _36 ])
(gt:QI (reg:CCNO 17 flags)
(const_int 0 [0])))
Successfully matched this instruction:
(set (reg:QI 110 [ d_lsm_flag.20 ])
(le:QI (reg:CCNO 17 flags)
(const_int 0 [0])))
allowing combination of insns 16, 21, 22 and 23
original costs 4 + 4 + 4 + 4 = 16
replacement costs 4 + 4 = 8
deferring deletion of insn with uid = 21.
deferring deletion of insn with uid = 16.
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.
Combined instruction is OK, but when insn is split into two, it
propagates REG_UNUSED from insn 22 (and converting them to REG_DEAD on
the fly in the referred code) to insn i2. This is clearly wrong when
flags reg is used in insn i3. I don't see anything wrong besides this.
Uros.
