AW: basic asm and memory clobbers - Proposed solution

[Bernd Edlinger]

Hi,

> Surely in code like that, you would make "x" volatile?  Memory clobbers
> are not a substitute for correct use of volatile accesses.

No,

It is as I wrote, a memory clobber is the only way to guarantee that
the asm statement is not move somewhere else.

I changed the example to use volatile and compiled it
with gcc-Version 4.6.3 (Ubuntu/Linaro 4.6.3-1ubuntu5) 

volatile int x;
void
test()
{
   x = 1;
   asm volatile("nop");
   x = 0;
}

gcc -S -O2 test.c gives:


test:
.LFB0:
	.cfi_startproc
	movl	$1, x(%rip)
	movl	$0, x(%rip)
#APP
# 6 "test.c" 1
	nop
# 0 "" 2
#NO_APP
	ret
	.cfi_endproc


While it works with asm volatile("nop" ::: "memory").
Likewise for "cli" and "sti" if you try to implement critical sections.
Although, these instructions do not touch any memory, we
need the memory clobber to prevent code motion.



Bernd.