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

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