Re: XCode 2.2.1 / gcc 4.0 Peephole Bug
Re: XCode 2.2.1 / gcc 4.0 Peephole Bug
- Subject: Re: XCode 2.2.1 / gcc 4.0 Peephole Bug
- From: Ben Weiss <email@hidden>
- Date: Wed, 19 Apr 2006 01:46:30 -0700
All right, so I tried it in assembly, and ran into another bug (I
think):
Given the function:
static int duplicatelabelbug(register vector unsigned short a,
register vector unsigned short b) {
register vector unsigned short vmask;
asm { vcmpgtuh. vmask, a, b; beq cr6, mylabel; };
asm { vcmpgtuh. vmask, b, a; beq cr6, mylabel; }; // <-- duplicate
label??
return 0;
mylabel: return 1; // <-- label only defined once, here!
}
XCode 2.2.1 / gcc 4.0 generates: "error: duplicate label 'LASM
$mylabel' " even though the label "mylabel" is only defined once.
(CodeWarrior handles this code just fine.) Is this a gcc bug?
Ben
On Apr 19, 2006, at 1:09 AM, Ben Weiss wrote:
Given the Altivec function:
vector unsigned short peepholebug(vector unsigned short a, vector
unsigned short b) {
vector unsigned short mask = (vector unsigned short)vec_cmplt(a, b);
if (vec_all_ge(a, b)) return a;
return mask;
}
XCode 2.2.1 / gcc 4.0 generates ( with optimizer set to -os):
mfspr r0,256
stw r0,-8(r1)
oris r0,r0,0x8000
mtspr 256,r0
vcmpgtuh. v0,v3,v2
vcmpgtuh v0,v3,v2
beq cr6,L99
vor v2,v0,v0
lwz r12,-8(r1)
mtspr 256,r12
blr
Note the second "vcmpgtuh" instruction, which is completely
superfluous. The peephole optimizer should recognize this situation
and remove the instruction. (I've filed a bug with Apple;
#4519214.) Anyone know if more recent versions of gcc are able to
do this? I have some bottleneck code that could seriously benefit
from this, and I'd rather avoid assembly if I can...
Thanks,
Ben
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