On 07/05/2013 01:35 AM, dw wrote:
>
>>> So, you are getting the duplicate zeroing of eax, but not
>>> the duplicate loading of rdi.
>>>
>>> I'm using 4.8.0. You?
>> Ah, I'm on 4.7. So this might be a regression: we should check.
>
> Compiling with 4.7.2 and 4.7.3, I also get no duplicate loads on rdi.
> But both 4.8.0 and 4.8.1 do. All 4 duplicate the zeroing of eax.
>
> What now?
First, I'd correct the asm:
asm volatile (
"rep stosb"
: "+D" (Dest), "+c" (Count)
: "a" (Data)
: "memory");
then see if there was a real regression.
As far as I can see current GCC is (nearly) optimal:
main:
.LFB1:
leaq -32(%rsp), %rdi
movl $32, %ecx
xorl %eax, %eax
#APP
# 2 "z.c" 1
rep stosb
# 0 "" 2
#NO_APP
movl $32, %ecx
leaq -32(%rsp), %rdi
#APP
# 2 "z.c" 1
rep stosb
# 0 "" 2
#NO_APP
xorl %eax, %eax
ret
Andrew.
