Re: Memory outputs in inline asm

[Andrew Haley <aph at redhat dot com>]

On 02/26/2014 10:51 AM, Richard Biener wrote:
> But yes, technically you write p[0] here but as "m" merely builds
> an address to the memory I'd say that we have to treat any "m"
> operand as possibly reading from / writing to / clobbering the
> whole object that can be refered to using that address.

I think so too.

> Anything else would need extra syntax to specify a memory
> range that is accessed.
> 
> And yes, I probably should implement at least basic disambiguation
> against memory accessing asm()s ;)
> 
> Summary: I consider your asm ok without changes.  Does documentation
> tell you otherwise?  Did you run into optimization issues?

Yes.  Here is my original test case that zeroes a block of memory:

int main() {

  int arr[20];
  int *p = (int*)arr;

  int len = 20;
  __asm__("\t0:\n"
	  "\tstr wzr, [%2, #4]!\n"
	  "\tsub %1, %1, #1\n"
	  "\tcbnz %1, 0b\n"
	  : "=m"(*p), "+r"(len) : "r"(p));

  return p[5];
}

and here is the code GCC emits for main:

main:
	sub	sp, sp, #80
	ldr	w0, [sp,20]
	add	sp, sp, 80
	ret

If I change "=m"(*p) to "=m"(p[5]) I get this:

main:
	sub	sp, sp, #80
	mov	x1, sp
	mov	w0, 20
#APP
// 7 "t.c" 1
		0:
	str wzr, [x1, #4]!
	sub x0, x0, #1
	cbnz x0, 0b

// 0 "" 2
#NO_APP
	ldr	w0, [sp,20]
	add	sp, sp, 80
	ret

This has all come up because of questions on gcc-help and because I've been
looking at improving the documentation of inline asm.

If my kludge involving a struct that contains a zero-length array is
supposed to work, we can document it.

Andrew.