Re: [PATCH] [RFC] PR target/52813 and target/11807

[Dimitar Dimitrov <dimitar@dinux.eu>]

On Thu, Dec 13, 2018 at 8:48:38 EET Segher Boessenkool wrote:
> On Wed, Dec 12, 2018 at 06:26:10PM +0200, Dimitar Dimitrov wrote:
> > I expect that if I mark a HW register as "clobber", compiler would save
> > its
> > contents before executing the asm statement, and after that it would
> > restore its contents. This is the GCC behaviour for all but the SP and
> > PIC registers. That is why I believe that PR52813 is a valid bug.
> 
> It won't do it for *any* fixed registers.  But you do not want to error
> or even warn for some fixed registers, for example the "flags" register
> on x86 is *always* written to by asm.

Yes, you are correct.

> 
> But you never want to warn for non-fixed registers, and e.g.
> PIC_OFFSET_TABLE_REGNUM isn't always a fixed register (when flag_pic is 0
> for example).
I  could not trace how PIC_OFFSET_TABLE_REGNUM on i386 gets marked as fixed 
register. I'll dig more through the source.

> 
> > I'm not sure how GCC could recover if SP is clobbered. If SP is clobbered
> > in such a way that GCC will not notice (e.g. thread switching), then why
> > should GCC know about it in the first place?
> 
> Up until today, GCC has always just ignored it if you claimed to clobber
> the stack pointer.

My point is that the silent ignoring is confusing to users, as shown by 
PR52813. How would you suggest me to proceed:
 - Leave patch as-is.
 - Revert patch. Update documentation to point that clobber marker for fixed 
registers is ignored by GCC. Close PR52813 as invalid.
 - Revert patch. Discuss more broadly and specify behaviour of asm clobber for 
fixed registers (and SP in particular).

Thanks,
Dimitar