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x86/div64: Add a micro-optimization shortcut if base is power of two 

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>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/1322649814-544-1-git-send-email-bigeasy@linutronix.de
Signed-off-by: Ingo Molnar <mingo@elte.hu>
This commit is contained in:
Sebastian Andrzej Siewior 2011-11-30 11:43:34 +01:00 committed by Ingo Molnar
parent f6b2bc8476
commit 668b448466
1 changed files with 14 additions and 8 deletions
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22
arch/x86/include/asm/div64.h
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@ -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; \
})