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Re: AW: basic asm and memory clobbers - Proposed solution


On 12/2/2015 3:34 AM, Bernd Edlinger wrote:
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.

If the goal is to order things wrt x, why wouldn't you just reference x?

   x = 1;
   asm volatile("nop":"+m"(x));
   x = 0;

If you have a dependency, stating it explicitly seems a much better approach than hoping that the implied semantics of a memory clobber might get you what you want. Not only is it crystal clear for the optimizers what the ordering needs to be, people maintaining the code can better understand your intent as well.

In summary: Yes inline asm can float. Clobbers might help. But I don't believe this is related to the "remove basic asm in functions" work. If a warning here is merited (and I'm not yet convinced), a separate case should be made for it.

However it does seem like a good fit for a section in a "How to convert basic asm to extended asm" doc. The extended docs don't mention this need or how extended can be used to address it. It's a good reason for basic asm users to switch.

dw


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