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SubjectRe: [RFC PATCH 3/4] x86/asm: Make alternative macro interfaces more clear and consistent
On Fri, Sep 15, 2017 at 07:53:46PM +0300, Andrey Ryabinin wrote:
>
>
> On 08/31/2017 08:25 PM, Josh Poimboeuf wrote:
> >
> > There have been a few other ideas which have *almost* worked:
> >
> > 1) Make the 'register void *__sp asm(_ASM_SP)' a global variable instead
> > of a local one. This works for GCC and doesn't break clang. However
> > it resulted in a lot of changed code on the GCC side. It looked like
> > some optimizations had been disabled, even in functions which
> > shouldn't have been affected.
> >
>
> (For those who wasn't following previous discussion there is the patch - http://lkml.kernel.org/r/<75850bb7-a60e-057d-d88b-afa0c79e94a0@gmail.com>
> But, Josh discovered that the patch causes regression in .text size with gcc > 7.x
> http://lkml.kernel.org/r/<20170721132452.ihpws67e3e7ym3al@treble> )
>
> I'm not so sure that this is disabled optimization. I assume that global rsp makes
> changes something in gcc's register allocation logic, or something like that leading
> to subtle changes in generated code.
>
> But what I recently find out, is that this "regression" sometimes is actually improvement in .text size.
> It all depends on .config, e.g:
>
>
> allnoconfig:
> text data bss dec hex filename
> patched 938094 207700 1215752 2361546 2408ca allnoconfig_p/vmlinux
> unpatched 938768 211796 1215752 2366316 241b6c allnoconfig/vmlinux
>
>
> defconfig:
> text data bss dec hex filename
> patched 10691469 4882856 876544 16450869 fb0535 defconfig_p/vmlinux
> unpatched 10691035 4882856 876544 16450435 fb0383 defconfig/vmlinux
>
> allyesconfig:
> text data bss dec hex filename
> patched 159216239 153294411 49692672 362203322 1596c8ba allyesconfig_p/vmlinux
> unpatched 159224951 153294387 49692672 362212010 1596eaaa allyesconfig/vmlinux
>
>
> So there is regression with Josh's config and defconfig, but allnoconfig and allyesconfig shows improvement.
> Note that sometimes there is the difference in .data size too, don't ask me why.
>
> Given that, perhaps the best thing to do here would be asking gcc devs why is generated code changed.
> And if such changes are harmless (as I suspect) we can just fix the problem with this simple patch http://lkml.kernel.org/r/<75850bb7-a60e-057d-d88b-afa0c79e94a0@gmail.com> ?
> Despite the increased .text on some configs.

Can you change all the other code sites in a similar way, and then run
the numbers again?

$ git grep __sp |grep register |grep void
arch/x86/include/asm/alternative.h: register void *__sp asm(_ASM_SP); \
arch/x86/include/asm/mshyperv.h: register void *__sp asm(_ASM_SP);
arch/x86/include/asm/mshyperv.h: register void *__sp asm(_ASM_SP);
arch/x86/include/asm/paravirt_types.h: register void *__sp asm("esp")
arch/x86/include/asm/paravirt_types.h: register void *__sp asm("rsp")
arch/x86/include/asm/preempt.h: register void *__sp asm(_ASM_SP); \
arch/x86/include/asm/preempt.h: register void *__sp asm(_ASM_SP); \
arch/x86/include/asm/processor.h: register void *__sp asm(_ASM_SP);
arch/x86/include/asm/rwsem.h: register void *__sp asm(_ASM_SP); \
arch/x86/include/asm/uaccess.h: register void *__sp asm(_ASM_SP); \
arch/x86/include/asm/xen/hypercall.h: register void *__sp asm(_ASM_SP);
arch/x86/kvm/emulate.c: register void *__sp asm(_ASM_SP);
arch/x86/kvm/vmx.c: register void *__sp asm(_ASM_SP);
arch/x86/mm/fault.c: register void *__sp asm("rsp");

--
Josh

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