linux-kernel - Re: [PATCH 48/74] x86, lto: Use inline assembler instead of global register variable to get sp

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Date:	Sun, 19 Aug 2012 17:18:25 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Jan Beulich <jbeulich@...e.com>
Cc:	andi@...stfloor.org, ak@...ux.intel.com,
	linux-kernel@...r.kernel.org, x86@...nel.org,
	akpm@...ux-foundation.org, mmarek@...e.cz,
	linux-kbuild@...r.kernel.org
Subject: Re: [PATCH 48/74] x86, lto: Use inline assembler instead of global register variable to get sp

On Sun, Aug 19, 2012 at 09:37:27AM +0100, Jan Beulich wrote:
> >>> Andi Kleen <andi@...stfloor.org> 08/19/12 4:59 AM >>>
> >I verified this generates the same binary (on 64bit) as the original
> >register variable.
> 
> This isn't very surprising given that the modified code is inside a
> CONFIG_X86_32 conditional (as ought to be obvious from the code using
> %%esp). Given that it's being used as operand to a binary &, the resulting
> code - if the compiler handles this only half way sensibly - can hardly be
> expected to be identical.

Doh! Thanks. I'll double check.

You're right it'll likely change code. But it shouldn't be common.

> 
> >-register unsigned long current_stack_pointer asm("esp") __used;
> >+#define current_stack_pointer ({         \
> >+    unsigned long sp;            \
> >+    asm("mov %%esp,%0" : "=r" (sp));    \
> >+    sp;                    \
> >+})
>  
> It would get closer to the original if you used "=g" (I noticed in a few
> earlier patches already that you like to use "=r" in places where a register
> is not strictly required, thus reducing the flexibility the compiler has).

My fingers have =r hardcoded. Will fix.

> 
> Also, given that this is more a workaround for a compiler deficiency,
> shouldn't this be conditional upon use of LTO?

I think it's cleaner than the global reg var, so unconditional should 
be fine. It wouldn't surprise me if global reg causes trouble even
without LTO, i probably just triggered some latent bug.

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only.
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