https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78751
--- Comment #2 from Segher Boessenkool <segher at gcc dot gnu.org> ---
In this testcase ifcvt happens upon a branch like:
(jump_insn 28 27 65 2 (set (pc)
(if_then_else (eq (reg:CCEQ 183)
(const_int 0 [0]))
(label_ref:SI 65)
(pc))) "78751-0.c":8 796 {*rs6000.md:12174}
(expr_list:REG_DEAD (reg:CCEQ 183)
(int_list:REG_BR_PROB 5000 (nil)))
where that reg 183 is set up with
(insn 27 26 28 2 (set (reg:CCEQ 183)
(compare:CCEQ (gt:SI (reg:CC 168)
(const_int 0 [0]))
(ne:SI (reg:CC 179)
(const_int 0 [0])))) "78751-0.c":8 805 {*rs6000.md:12298}
(expr_list:REG_DEAD (reg:CC 179)
(expr_list:REG_DEAD (reg:CC 168)
(nil))))
This is a crxor on the outputs of two compares, an unnamed
define_insn_and_split (search for "creqv or crxor" in rs6000.md).
ifcvt wants to combine this with a trap in the branch block to become
a conditional branch. To do that, it makes insns like
(insn 63 26 64 2 (set (reg:SI 189)
(gt:SI (reg:CC 168)
(const_int 0 [0]))) -1
(nil))
(insn 64 63 65 2 (set (reg:SI 190)
(ne:SI (reg:CC 179)
(const_int 0 [0]))) -1
(nil))
(insn 65 64 27 2 (trap_if (ne (reg:SI 189)
(reg:SI 190))
(const_int 0 [0])) -1
(nil))
but those do not exist (there is no pattern without clobbers for NE).
