linux-kernel - [PATCH 1/5] div64.h: optimize do_div() for power-of-two constant divisors

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Date:	Mon, 02 Nov 2015 17:33:26 -0500
From:	Nicolas Pitre <nicolas.pitre@...aro.org>
To:	Alexey Brodkin <Alexey.Brodkin@...opsys.com>,
	Måns Rullgård <mans@...sr.com>
Cc:	Arnd Bergmann <arnd@...db.de>, rmk+kernel@....linux.org.uk,
	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH 1/5] div64.h: optimize do_div() for power-of-two constant
 divisors

On 32-bit targets, gcc is able to do the right thing with a constant
divisor that happens to be a power of two i.e. it turns the division
into a right shift inline.

Signed-off-by: Nicolas Pitre <nico@...aro.org>
---
 include/asm-generic/div64.h | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/include/asm-generic/div64.h b/include/asm-generic/div64.h
index 8f4e319334..47aa1e2134 100644
--- a/include/asm-generic/div64.h
+++ b/include/asm-generic/div64.h
@@ -41,7 +41,12 @@ extern uint32_t __div64_32(uint64_t *dividend, uint32_t divisor);
 	uint32_t __base = (base);			\
 	uint32_t __rem;					\
 	(void)(((typeof((n)) *)0) == ((uint64_t *)0));	\
-	if (likely(((n) >> 32) == 0)) {			\
+	if (__builtin_constant_p(__base) &&		\
+	    (__base & (__base - 1)) == 0) {		\
+		/* constant power of 2: gcc is fine */	\
+		__rem = (n) & (__base - 1);		\
+		(n) /= __base;				\
+	} else if (likely(((n) >> 32) == 0)) {		\
 		__rem = (uint32_t)(n) % __base;		\
 		(n) = (uint32_t)(n) / __base;		\
 	} else 						\
-- 
2.4.3

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