[PATCH i386] PR47698 no CMOV for volatile mem

[Richard Guenther]

On Fri, 28 Oct 2011, Sergey Ostanevich wrote:

> On Fri, Oct 28, 2011 at 12:16 PM, Richard Guenther <rguenther@suse.de> wrote:
> > On Thu, 27 Oct 2011, Uros Bizjak wrote:
> >
> >> Hello!
> >>
> >> > Here's a patch for PR47698, which is about CMOV should not be
> >> > generated for memory address marked as volatile.
> >> > Successfully bootstrapped and passed make check on x86_64-unknown-linux-gnu.
> >>
> >>
> >> Â  Â  Â  PR rtl-optimization/47698
> >> Â  Â  Â  * config/i386/i386.c (ix86_expand_int_movcc) prevent CMOV generation
> >> Â  Â  Â  for volatile mem
> >>
> >> Â  Â  Â  PR rtl-optimization/47698
> >> Â  Â  Â  * gcc.target/i386/47698.c: New test
> >>
> >> Please use punctuation marks and correct capitalization in ChangeLog entries.
> >>
> >> OTOH, do we want to fix this per-target, or in the middle-end?
> >
> > The middle-end pattern documentation does not say operands 2 and 3
> > are not evaluated if they do not end up being stored, so a middle-end
> > fix is more appropriate.
> >
> > Richard.
> >
> 
> I have two observations:
> 
> - the code for CMOV is under #ifdef in the mddle-end, which is
> explicitly marked as "have to be removed" (ifcvt.c:1446)
> - I have no clear evidence all platforms that support conditional move
> have the same semantics that lead to the PR
> 
> I think the best way to address both concerns is to implement code
> that relies on а new hookup "volatile-safe CMOV" that is false by
> default.

I suppose it's never safe for all architectures that support
memory operands in the source operand.

Richard.