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SubjectRe: [PATCH 2/2] x86/asm: Fix inline asm call constraints for clang
On Tue, Sep 19, 2017 at 08:18:23PM -0500, Josh Poimboeuf wrote:
> On Tue, Sep 19, 2017 at 01:45:28PM -0500, 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.
> >
> > Here are the vmlinux .text size differences with the following configs
> > on GCC:
> >
> > - defconfig
> > - defconfig without frame pointers
> > - Fedora distro config
> > - Fedora distro config without frame pointers
> >
> > defconfig defconfig-nofp distro distro-nofp
> > before 9796300 9466764 9076191 8789745
> > after 9796941 9462859 9076381 8785325
> >
> > With frame pointers, the text size increases slightly. Without frame
> > pointers, the text size decreases, and a little more significantly.
> >
> > Reported-by: Matthias Kaehlcke <mka@chromium.org>
> > Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
>
> NAK - kbuild bot is reporting some cases where this patch doesn't force
> the frame pointer setup.

So it turns out that for GCC 7, it works as described above: the global
register variable results in the frame pointer getting set up before
*all* inline asm.

But for GCC 6, it doesn't work that way. The global register variable
has no such effect.

So we need both the global register variable *and* the output constraint
after all. Will post another patch after some more testing.

--
Josh

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