-O2/-O3 Optimization bug?

[Iain Buclaw]

On 22 January 2014 15:03, Johannes Pfau <nospam at example.com> wrote:
> Am Wed, 22 Jan 2014 00:28:32 +0000
> schrieb "Mike" <none at none.com>:
>
>>
>> "So what's the question, Mike?" I hear you say:
>> 1.  Is this just one of the consequences of using -O2/-O3, and I
>> should just suck it up and deal with it?
>> 2.  Is this potentially a bug in the GCC backend?
>> 3.  Is this potentially a bug in GDC or the DMD frontend?
>>
>> Thanks for the help,
>> Mike
>
> I can only guess, but this looks like another 'volatile' problem. You'd
> have to post the ASM of the optimized version somewhere, and probably
> the output of -fdump-tree-optimized for the optimized version.
>
> But anyway, I guess it inlines 'SendCommand' and then thinks you're not
> using the message and probably completely optimizes the call away. Then
> it sees you're never using message and removes the rest of your code.
> If SendCommand was written in D you'd have to mark the target of the
> copy volatile (or shared).
>
> But I'm not sure how this applies to the inline asm though. In C you
> have asm volatile, but I never used that. This answer seems to state
> that you have to use asm volatile:
> http://stackoverflow.com/a/5057270/471401
>
>
>
> So the questions for Iain:
>  * should we mark all inline ASM blocks as volatile?
>  * shared can't replace volatile in this case as `shared asm{...}`
>    isn't valid
>  * Should we add some GDC specific way to mark extended ASM blocks as
>    volatile? As DMD doesn't optimize ASM blocks at all there's probably
>    no need for a standard solution?

We already do (ExtAsmStatement::toIR -> ASM_VOLATILE_P (exp) = 1;)

Regards
Iain