Re: Can shrink-wrapping ever move prologue past an ASM statement?

[Josh Poimboeuf <jpoimboe at redhat dot com>]

On Wed, Jul 08, 2015 at 05:36:31AM -0500, Segher Boessenkool wrote:
> On Wed, Jul 08, 2015 at 11:23:09AM +0200, Martin Jambor wrote:
> > > For other archs, e.g. x86-64, you can do
> > > 
> > > 	register void *sp asm("%sp");
> > > 	asm volatile("call func" : "+r"(sp));
> 
> <snip>
> 
> > Well, I only have had a quick look at where things "go wrong" and have
> > not spent much time thinking about all the things that coud break by
> > moving an asm statement before stack frame setup so I don't know.
> 
> What goes wrong is that the "plain" asm (just the call) does not say it
> needs the prologue of this function to be before it.  But it needs some
> of its effects: at least it needs the stack pointer changes (by the
> prologue) to be done before it.
> 
> Writing the asm with a clobber of the stack pointer causes all stack
> accesses to go via the frame pointer, which causes pretty horrible
> code.

As far as I can tell, most (but not all) kernel stack accesses already
occur via the frame pointer anyway.  Is that bad?  Can you clarify what
you mean by horrible code?

> Using just an input of the stack pointer, like in
> 
> 	register void *sp asm("%sp");
> 	asm volatile("call func" : : "r"(sp));
> 
> also works, at least in my simple test case; but the version that also
> writes to sp prevents the asm's movement more.
> 
> > Nevertheless, given that this was discovered only now, it might not be
> > a common problem but of course it may become one if we improve
> > shrink-wrapping further.
> 
> GCC 5 and/or GCC 6 make shrink wrapping more aggressive, quite nice.
> 
> > Using a register variable seems like a workaround,
> 
> That is only a workaround for the fact that there is no other way to
> say "this input should be in this specific register" (in general;
> some ports have such constraints for some registers).
> 
> Having the asm take the stack pointer as input is hackish yes.
> 
> > so having a "stack" input operand is probably worth some
> > further thought.
> 
> It would be nice if you could just say "this asm has to stay behind
> the prologue", yes.  I suggested using a special clobber, not an input,
> but that is just syntax quibbles.
> 
> The question is if this usage is common enough to warrant a special
> syntax at all.

In the kernel it only affects a handful of code locations which are
mostly wrapped in macros, so I don't really see the need for a special
syntax.  IMO, the existing syntax is fine for the kernel.

Thanks!

-- 
Josh