http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60131
--- Comment #4 from Jeffrey A. Law <law at redhat dot com> ---
The problem here is we first have the following candidate and defining insn:
gdb) p debug_rtx (cand->insn)
(insn 24 23 25 6 (set (reg:DI 0 ax [orig:100 D.2269 ] [100])
(zero_extend:DI (reg/v:SI 0 ax [orig:91 v ] [91]))) j.c:22 133
{*zero_extendsidi2}
(nil))
$13 = void
(gdb) p debug_rtx (curr_insn)
(insn 33 32 35 7 (set (reg/v:SI 0 ax [orig:91 v ] [91])
(sign_extend:SI (reg:HI 0 ax [orig:88 D.2271 ] [88]))) j.c:19 146
{extendhisi2}
(nil))
We eliminate insn 24 by changing insn 33 into:
(insn 33 32 35 7 (set (reg:DI 0 ax)
(zero_extend:DI (sign_extend:SI (reg:HI 0 ax [orig:88 D.2271 ] [88]))))
j.c:19 -1
(nil))
Later we call combine_reaching_defs to see if we can eliminate insn 33. It now
has an unexpected form and we trigger the checking fault.
