Messages in this thread Patch in this message | | | Subject | [RFC patch] cmpxchg_double: remove local variables to get better performance | From | Alex Shi <> | Date | Fri, 02 Mar 2012 16:31:03 +0800 |
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There are some local variables in cmpxchg_double macro, seems these are used to for force casting on input variables to transfer them into '*p1' type. May there are some reason I don't know. But I just saw 2 problems here:
1, user may mis-use the macro, like give a 'long' type o1, but just use a 'int*' or 'char*' p1. If we remove the force cast here, gcc will check the mis-using in compiling. and user can get the error report in compiling for such issues.
2, local variable increased the data section, and bring extra memory bus accesses, that hurt performance in this critical macro. I did a little experiment on my nhm i7 desktop, to run the macro with a fixed times, here is the data: using local vars no local variable with lock prefix, 267700578ns 232079696ns without lock prefix, 34715666ns 34687566ns
So, we may need rethink about the local variable usage here.
Signed-off-by: Alex Shi <alex.shi@intel.com> --- diff --git a/arch/x86/include/asm/cmpxchg.h b/arch/x86/include/asm/cmpxchg.h index b3b7332..8bf9127 100644 --- a/arch/x86/include/asm/cmpxchg.h +++ b/arch/x86/include/asm/cmpxchg.h @@ -210,17 +210,15 @@ extern void __add_wrong_size(void) #define __cmpxchg_double(pfx, p1, p2, o1, o2, n1, n2) \ ({ \ bool __ret; \ - __typeof__(*(p1)) __old1 = (o1), __new1 = (n1); \ - __typeof__(*(p2)) __old2 = (o2), __new2 = (n2); \ BUILD_BUG_ON(sizeof(*(p1)) != sizeof(long)); \ BUILD_BUG_ON(sizeof(*(p2)) != sizeof(long)); \ VM_BUG_ON((unsigned long)(p1) % (2 * sizeof(long))); \ VM_BUG_ON((unsigned long)((p1) + 1) != (unsigned long)(p2)); \ asm volatile(pfx "cmpxchg%c4b %2; sete %0" \ - : "=a" (__ret), "+d" (__old2), \ + : "=a" (__ret), "+d" (o2), \ "+m" (*(p1)), "+m" (*(p2)) \ - : "i" (2 * sizeof(long)), "a" (__old1), \ - "b" (__new1), "c" (__new2)); \ + : "i" (2 * sizeof(long)), "a" (o1), \ + "b" (n1), "c" (n2)); \ __ret; \ })
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