https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83304
--- Comment #9 from Segher Boessenkool <segher at gcc dot gnu.org> ---
At some point during combine we have
insn_cost 4 for 72: r127:SI=0xffffffffffffff81
insn_cost 4 for 9: r130:SI=ctz(r125:SI)
insn_cost 4 for 69: cc:CC=cmp(r125:SI,0)
insn_cost 4 for 70: r131:SI={(cc:CC!=0)?r130:SI:0xffffffffffffffff}
REG_DEAD r130:SI
insn_cost 12 for 16: r133:SI=cc:CC==0
REG_DEAD cc:CC
19: {r134:SI=r131:SI!=0xffffffffffffffff;clobber cc:CC;}
REG_UNUSED cc:CC
REG_DEAD r131:SI
and then it does
Trying 70 -> 19:
70: r131:SI={(cc:CC!=0)?r130:SI:0xffffffffffffffff}
REG_DEAD r130:SI
19: {r134:SI=r131:SI!=0xffffffffffffffff;clobber cc:CC;}
REG_UNUSED cc:CC
REG_DEAD r131:SI
Failed to match this instruction:
(parallel [
(set (reg:SI 134)
(ne:SI (reg:CC 100 cc)
(const_int 0 [0])))
(clobber (reg:CC 100 cc))
])
Successfully matched this instruction:
(set (reg:SI 134)
(ne:SI (reg:CC 100 cc)
(const_int 0 [0])))
allowing combination of insns 70 and 19
original costs 4 + 16 = 20
replacement cost 12
deferring deletion of insn with uid = 70.
deferring deletion of insn with uid = 9.
modifying insn i3 19: r134:SI=cc:CC!=0
REG_DEAD cc:CC
deferring rescan insn with uid = 19.
but the REG_DEAD note on insn 16 is left, although now 19 uses it too.
