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Re: Can shrink-wrapping ever move prologue past an ASM statement?
- From: Josh Poimboeuf <jpoimboe at redhat dot com>
- To: Segher Boessenkool <segher at kernel dot crashing dot org>
- Cc: GCC Mailing List <gcc at gcc dot gnu dot org>, Vojtech Pavlik <vojtech at suse dot cz>
- Date: Wed, 8 Jul 2015 11:22:34 -0500
- Subject: Re: Can shrink-wrapping ever move prologue past an ASM statement?
- Authentication-results: sourceware.org; auth=none
- References: <20150707175349 dot GA2325 at virgil dot suse dot cz> <20150707184415 dot GB13004 at gate dot crashing dot org> <20150708092309 dot GB2325 at virgil dot suse dot cz> <20150708103631 dot GA23975 at gate dot crashing dot org>
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));
> > 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
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.