linux-kernel - [PATCH] Reduce the number of expensive division instructions done by _parse

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Date:	Thu, 09 Feb 2012 15:48:20 +0000
From:	David Howells <dhowells@...hat.com>
To:	adobriyan@...il.com
Cc:	torvalds@...ux-foundation.org, dhowells@...hat.com,
	linux-kernel@...r.kernel.org
Subject: [PATCH] Reduce the number of expensive division instructions done by
 _parse_integer()

_parse_integer() does one or two division instructions (which are slow) per
digit parsed to perform the overflow check.

Furthermore, these are particularly expensive examples of division instruction
as the number of clock cycles required to complete them may go up with the
position of the most significant set bit in the dividend:

	if (*res > div_u64(ULLONG_MAX - val, base))

which is as maximal as possible.

Worse, on 32-bit arches, more than one of these division instructions may be
required per digit.

So, assuming we don't support a base of more than 16, skip the check if the
top nibble of the result is not set at this point.

Signed-off-by: David Howells <dhowells@...hat.com>
---

 lib/kstrtox.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/lib/kstrtox.c b/lib/kstrtox.c
index 7a94c8f..f80c896 100644
--- a/lib/kstrtox.c
+++ b/lib/kstrtox.c
@@ -64,7 +64,7 @@ unsigned int _parse_integer(const char *s, unsigned int base, unsigned long long
 
 		if (val >= base)
 			break;
-		if (*res > div_u64(ULLONG_MAX - val, base))
+		if (unlikely(*res >> 60) && *res > div_u64(ULLONG_MAX - val, base))
 			overflow = 1;
 		*res = *res * base + val;
 		rv++;

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