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SubjectRe: [RFC PATCH 3/4] x86/asm: Make alternative macro interfaces more clear and consistent
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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.

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