Re: basic asm and memory clobbers

[David Wohlferd <dw at LimeGreenSocks dot com>]

On 11/20/2015 2:17 AM, Andrew Haley wrote:
> 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.

The intent for 24414 is to change basic asm such that it will become 
(quoting jeff) "an opaque blob that read/write/clobber any register or 
memory location."  Such being the case, "memory" is not sufficient:

#define CLOBBERALL "eax", "ebx", "ecx", "edx", "r8", "r9", "r10", "r11", 
"r12", "r13", "r14", "r15", "edi", "esi", "ebp", "cc", "memory"

int main()
{
   for (int x=0; x < 1000; x++)
      asm("#":::CLOBBERALL);
}

dw