linux-kernel - hex_to

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Date:	Thu, 27 May 2010 12:07:29 +0200
From:	Fredrik Gustafsson <iveqy@...qy.com>
To:	linux-kernel@...r.kernel.org
Subject: hex_to_bin speedup

Hi,
I looked a bit at the newly added hex_to_bin function in lib/hexdump.c.
I do believe this is a speedup (at least according to my benchmark[1]).

However in the comments to commit 
903788892ea0fc7fcaf7e8e5fac9a77379fc215b you can read
"[akpm@...ux-foundation.org: use tolower(), saving 3 bytes, test the more common case first - it's quicker]"

I don't know the change that akpm has done, so I'm unsure if there's any
problems that I miss with my patch.

--
iveqy

[1] Benchmark

#include <sys/time.h>
#include <stdio.h>

int main()
{
	struct timeval start, end;
	int i,itr;
	itr = 100000;
	char c = 'A';
	time_t diff;
	gettimeofday(&start,NULL);
	for(i = 0; i < itr; i++) {
		if((c >= 'A') && (c <= 'F')) {}
	}
	gettimeofday(&end,NULL);
	diff = end.tv_usec - start.tv_usec;
	printf("if-statement: %d ms\n",diff);
	gettimeofday(&start,NULL);
	for(i = 0; i < itr; i++) {
		c = tolower(c);
	}
	gettimeofday(&end,NULL);
	diff = end.tv_usec - start.tv_usec;
	printf("tolower(): %d ms\n",diff);
}

[2] Patch

This is faster (at least on i686).
---
 lib/hexdump.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)

diff --git a/lib/hexdump.c b/lib/hexdump.c
index 5d7a480..f01d11c 100644
--- a/lib/hexdump.c
+++ b/lib/hexdump.c
@@ -26,9 +26,10 @@ int hex_to_bin(char ch)
 {
 	if ((ch >= '0') && (ch <= '9'))
 		return ch - '0';
-	ch = tolower(ch);
 	if ((ch >= 'a') && (ch <= 'f'))
 		return ch - 'a' + 10;
+	if ((ch >= 'A') && (ch <= 'F'))
+		return ch - 'A' + 10;
 	return -1;
 }
 EXPORT_SYMBOL(hex_to_bin);
--
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