https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117095
--- Comment #2 from Stefan Schulze Frielinghaus <stefansf at gcc dot gnu.org>
---
Prior cse2 we have
(jump_insn 217 78 216 10 (parallel [
(set (pc)
(if_then_else (ne (reg:SI 165)
(const_int 1 [0x1]))
(label_ref 216)
(pc)))
(set (reg:SI 165)
(plus:SI (reg:SI 165)
(const_int -1 [0xffffffffffffffff])))
(clobber (scratch:SI))
(clobber (reg:CC 33 %cc))
]) "t.c":14:17 discrim 1 2192 {doloop_si64}
(int_list:REG_BR_PROB 955630228 (nil))
-> 216)
...
(insn 99 98 100 12 (set (reg:SI 138)
(const_int 1 [0x1])) "t.c":9:31 1507 {*movsi_zarch}
(nil))
(insn 100 99 103 12 (parallel [
(set (reg:SI 137)
(minus:SI (reg:SI 138)
(subreg:SI (reg:HI 135 [ a ]) 0)))
(clobber (reg:CC 33 %cc))
]) "t.c":9:31 1904 {*subsi3}
(expr_list:REG_DEAD (reg:SI 138)
(expr_list:REG_DEAD (reg:HI 135 [ a ])
(expr_list:REG_UNUSED (reg:CC 33 %cc)
(nil)))))
where insn 100 computes 1-a. After cse2 insn 99 is removed and pseudo 138 is
replaced with the branch-on-count pseudo 165 from jump_insn 217:
(insn 100 98 103 12 (parallel [
(set (reg:SI 137)
(minus:SI (reg:SI 165)
(subreg:SI (reg:HI 135 [ a ]) 0)))
(clobber (reg:CC 33 %cc))
]) "t.c":9:31 1904 {*subsi3}
(expr_list:REG_DEAD (reg:SI 138)
(expr_list:REG_DEAD (reg:HI 135 [ a ])
(expr_list:REG_UNUSED (reg:CC 33 %cc)
(nil)))))
However, pseudo 165 is always 0 which means the result of the subtraction is
also always 0 since variable `a` is always 0. Looks like cse2 missed that in
the parallel pseudo 165 is decreased.
