Re: [PATCH 48/74] x86, lto: Use inline assembler instead ofglobal register variable to get sp

[Jan Beulich]

>>> Andi Kleen <andi@xxxxxxxxxxxxxx> 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.

>-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).

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

Jan

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