https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87871
--- Comment #12 from Segher Boessenkool <segher at gcc dot gnu.org> ---
(In reply to Segher Boessenkool from comment #11)
> (In reply to Wilco from comment #8)
> > mov r4, r0
> > cmp r4, #0
>
> Why does it copy r0 to r4 and then compare r4? On more modern machines it
> is faster to compare r0 itself, and it would allow shrink-wrapping to work
> fine here
We get this in combine:
Trying 2 -> 7:
2: r112:SI=r116:SI
REG_DEAD r116:SI
7: cc:CC=cmp(r112:SI,0)
Successfully matched this instruction:
(parallel [
(set (reg:CC 100 cc)
(compare:CC (reg:SI 116)
(const_int 0 [0])))
(set (reg/v:SI 112 [ a ])
(reg:SI 116))
])
(that's *movsi_compare0).
This is preceded by
(insn 50 3 7 2 (set (reg:SI 116)
(reg:SI 0 r0 [ a ])) "ira-shrinkwrap-prep-1.c":14:1 179
{*arm_movsi_insn}
(nil))
And it stays that way until IRA, which does
Disposition:
0:r111 l0 0 3:r112 l0 4 1:r113 l0 2 2:r114 l0 3
5:r116 l0 4 4:r117 l0 0
If r116 had been allocated hard reg 0 all would be fine (and we know r116
dies in insn 7 already, there is a REG_DEAD note on it).
