-O2/-O3 Optimization bug?

[Johannes Pfau]

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?