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    SubjectRe: [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@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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