On 20/11/15 01:23, David Wohlferd wrote:
> I tried to picture the most basic case I can think of that uses
> something clobber-able:
>
> for (int x=0; x < 1000; x++)
> asm("#stuff");
>
> This generates very simple and highly performant code:
>
> movl $1000, %eax
> .L2:
> #stuff
> subl $1, %eax
> jne .L2
>
> Using extended asm to simulate the clobberall gives:
>
> movl $1000, 44(%rsp)
> .L2:
> #stuff
> subl $1, 44(%rsp)
> jne .L2
>
> It allocates an extra 4 bytes, and changed everything to memory accesses
> instead of using a register.
Can you show us your code? I get
xx:
movl $1000, %eax
.L2:
#stuff
subl $1, %eax
jne .L2
rep; ret
for
void xx() {
for (int x=0; x < 1000; x++)
asm volatile("#stuff" : : : "memory");
}
What you're describing looks like a bug: x doesn't have its address
taken.
Andrew.
