https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107172
--- Comment #28 from Segher Boessenkool <segher at gcc dot gnu.org> ---
> So the issue is with the consumer:
>
> (insn 50 49 51 2 (parallel [
> (set (reg:SI 93)
> (neg:SI (ltu:SI (reg:CCC 17 flags)
> (const_int 0 [0]))))
> (clobber (reg:CC 17 flags))
> ]) "107172.c":4:10 1258 {*x86_movsicc_0_m1_neg}
> (expr_list:REG_DEAD (reg:CCC 17 flags)
> (expr_list:REG_UNUSED (reg:CC 17 flags)
> (nil))))
>
> There are many similar patterns in different backends. They work as long as
> the flags register isn't a known constant since simplify-rtx.cc leaves them
> alone. They become a problem only when the flags register is a known
> constant.
Such patterns are fine. The problem is that this consumer of MODE_CC does not
fit together with the producer of that reg 17: it only has meaning together,
that
is how this stuff works; and it has no meaning at all like this.
