Re: [PATCH 2/2] x86/asm: Fix inline asm call constraints for clang

[Josh Poimboeuf]

On Wed, Sep 20, 2017 at 10:32:43AM -0700, H. Peter Anvin wrote:
> On 09/19/17 11:45, Josh Poimboeuf wrote:
> > For inline asm statements which have a CALL instruction, we list the
> > stack pointer as a constraint to convince GCC to ensure the frame
> > pointer is set up first:
> > 
> >   static inline void foo()
> >   {
> >   	register void *__sp asm(_ASM_SP);
> >   	asm("call bar" : "+r" (__sp))
> >   }
> > 
> > Unfortunately, that pattern causes clang to corrupt the stack pointer.
> > 
> > There's actually an easier way to achieve the same goal in GCC, without
> > causing trouble for clang.  If we declare the stack pointer register
> > variable as a global variable, and remove the constraint altogether,
> > that convinces GCC to always set up the frame pointer before inserting
> > *any* inline asm.
> > 
> > It basically acts as if *every* inline asm statement has a CALL
> > instruction.  It's a bit overkill, but the performance impact should be
> > negligible.
> > 
> 
> Again, probably negligible, but why do we need a frame pointer just
> because we have a call assembly instruction?

It's frame pointer convention.  Without it, if dumping the stack from
the called function, a function will get skipped in the stack trace.

-- 
Josh