| Date | Sun, 19 Aug 2012 17:18:25 +0200 | From | Andi Kleen <> | Subject | Re: [PATCH 48/74] x86, lto: Use inline assembler instead of global register variable to get sp |
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On Sun, Aug 19, 2012 at 09:37:27AM +0100, Jan Beulich wrote: > >>> Andi Kleen <andi@firstfloor.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@linux.intel.com -- Speaking for myself only.
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