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authorSebastian Andrzej Siewior <bigeasy@linutronix.de>2011-11-30 14:43:34 +0400
committerIngo Molnar <mingo@elte.hu>2011-12-05 21:16:11 +0400
commit668b44846606185a9bed5a5357bc2cb132e00dd7 (patch)
tree60b2c21a71a97cc4be249e985443769e5fdbd3ca /arch/x86/include/asm/div64.h
parentf6b2bc847641ea38e2655c8424fef5d2d19f35f9 (diff)
downloadlinux-668b44846606185a9bed5a5357bc2cb132e00dd7.tar.xz
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>
Diffstat (limited to 'arch/x86/include/asm/div64.h')
-rw-r--r--arch/x86/include/asm/div64.h22
1 files changed, 14 insertions, 8 deletions
diff --git a/arch/x86/include/asm/div64.h b/arch/x86/include/asm/div64.h
index 9a2d644c08ef..ced283ac79df 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; \
})