[PATCH] x86/div64: add a shortcut if base is power of two

[Sebastian Andrzej Siewior]

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@xxxxxxxxxxxxx>
---
 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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