Commit 668b4484 authored Nov 30, 2011 by Sebastian Andrzej Siewior Committed by Ingo Molnar Dec 05, 2011

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>

parent f6b2bc84

arch/x86/include/asm/div64.h

+14 −8

Original line number	Diff line number	Diff line
		@@ -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
		@@ -21,6 +22,10 @@
		({ \
		unsigned long __upper, __low, __high, __mod, __base; \
		__base = (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) { \
		@@ -30,6 +35,7 @@
		asm("divl %2" : "=a" (__low), "=d" (__mod) \
		: "rm" (__base), "0" (__low), "1" (__upper)); \
		asm("" : "=A" (n) : "a" (__low), "d" (__high)); \
		} \
		__mod; \
		})

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