On Sat, Dec 02, 2023 at 11:04:04AM +0000, Richard Sandiford wrote:
> I still maintain that so much stuff relies on the lack of false-positive
> REG_UNUSED notes that (whatever the intention might have been) we need
> to prevent the false positive. Like Andrew says, any use of single_set
> is suspect if there's a REG_UNUSED note for something that is in fact used.
The false positive REG_UNUSED in that case comes from
(insn 15 14 35 2 (set (reg:CCZ 17 flags)
(compare:CCZ (reg:DI 0 ax [111])
(reg:DI 1 dx [112]))) "pr112760.c":11:22 12 {*cmpdi_1}
(expr_list:REG_UNUSED (reg:CCZ 17 flags)
(nil)))
(insn 35 15 36 2 (set (reg:CCZ 17 flags)
(compare:CCZ (reg:DI 0 ax [111])
(reg:DI 1 dx [112]))) "pr112760.c":11:22 12 {*cmpdi_1}
(expr_list:REG_DEAD (reg:DI 1 dx [112])
(expr_list:REG_DEAD (reg:DI 0 ax [111])
(nil))))
...
use of flags
Haven't verified what causes the redundant comparison, but postreload cse
then does:
110 if (!count && cselib_redundant_set_p (body))
111 {
112 if (check_for_inc_dec (insn))
113 delete_insn_and_edges (insn);
114 /* We're done with this insn. */
115 goto done;
116 }
So, we'd in such cases need to look up what instruction was the earlier
setter and if it has REG_UNUSED note, drop it.
Jakub
