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SubjectRe: [RFC PATCH 3/4] x86/asm: Make alternative macro interfaces more clear and consistent
On Sat, Sep 02, 2017 at 12:32:21PM +0200, Ingo Molnar wrote:
>
> * Josh Poimboeuf <jpoimboe@redhat.com> wrote:
>
> > On Thu, Aug 31, 2017 at 12:25:42PM -0500, Josh Poimboeuf wrote:
> > > 2) Put "sp" in the clobbers list instead of as an i/o constraint. This
> > > mostly works for GCC, and doesn't break clang. However, it causes
> > > GCC to insert a "lea -0x10(%rbp),%rsp" in the epilogue of every
> > > affected function.
> >
> > And maybe this extra instruction is negligible for performance and not a
> > big deal? I might look at this one after the holiday too.
>
> Please do statistics of how many functions are affected, on a defconfig-ish
> kernel.

As it turns out, the real problem with this option is that it imposes a
penalty for CONFIG_FRAME_POINTER=n: even with frame pointers disabled,
it forces the frame pointer to be saved for each function which uses the
inline asm "call" statements. Our current solution doesn't do that.

- On a defconfig-based kernel, this adds +6k of .text (+0.06%).

- On a Fedora distro-based config, it adds +27k of .text (+0.3%).
(I think the difference from defconfig is mostly caused by
CONFIG_PARAVIRT.)

I'll try a few more experiments, but I'll probably end up engaging the
compiler people as Linus suggested.

--
Josh

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