AW: basic asm and memory clobbers - Proposed solution

[David Wohlferd]

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