Messages in this thread Patch in this message | | | From | Sebastian Andrzej Siewior <> | Subject | [PATCH] x86/div64: add a shortcut if base is power of two | Date | Wed, 30 Nov 2011 11:43:34 +0100 |
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In the target code I have a do_div(x, PAGE_SIZE). The x86-64 version of it was doing a shift and a mask which is clever. The 32bit version of it had a div operation in it which made me think. After digging I noticed that x86 has an optimized version of it. This patch adds this shift and mask optimization if base is constant so we don't have any runtime "checking" overhead since most users use a power of ten.
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> --- arch/x86/include/asm/div64.h | 22 ++++++++++++++-------- 1 files changed, 14 insertions(+), 8 deletions(-)
diff --git a/arch/x86/include/asm/div64.h b/arch/x86/include/asm/div64.h index 9a2d644..ced283a 100644 --- a/arch/x86/include/asm/div64.h +++ b/arch/x86/include/asm/div64.h @@ -4,6 +4,7 @@ #ifdef CONFIG_X86_32 #include <linux/types.h> +#include <linux/log2.h> /* * do_div() is NOT a C function. It wants to return @@ -21,15 +22,20 @@ ({ \ unsigned long __upper, __low, __high, __mod, __base; \ __base = (base); \ - asm("":"=a" (__low), "=d" (__high) : "A" (n)); \ - __upper = __high; \ - if (__high) { \ - __upper = __high % (__base); \ - __high = __high / (__base); \ + if (__builtin_constant_p(__base) && is_power_of_2(__base)) { \ + __mod = n & (__base - 1); \ + n >>= ilog2(__base); \ + } else { \ + asm("" : "=a" (__low), "=d" (__high) : "A" (n));\ + __upper = __high; \ + if (__high) { \ + __upper = __high % (__base); \ + __high = __high / (__base); \ + } \ + asm("divl %2" : "=a" (__low), "=d" (__mod) \ + : "rm" (__base), "0" (__low), "1" (__upper)); \ + asm("" : "=A" (n) : "a" (__low), "d" (__high)); \ } \ - asm("divl %2":"=a" (__low), "=d" (__mod) \ - : "rm" (__base), "0" (__low), "1" (__upper)); \ - asm("":"=A" (n) : "a" (__low), "d" (__high)); \ __mod; \ }) -- 1.7.7.3
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