linux-kernel - [PATCH] i386: less assembly in strlen()

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Date:	Sun, 11 Dec 2011 21:13:19 +0300
From:	Alexey Dobriyan <adobriyan@...il.com>
To:	tglx@...utronix.de, mingo@...hat.com, hpa@...or.com
Cc:	x86@...nel.org, linux-kernel@...r.kernel.org
Subject: [PATCH] i386: less assembly in strlen()

Current i386 strlen() hardcodes NOT/DEC sequence. DEC is mentioned
to be suboptimal on Core2. So, put only REPNE SCASB sequence in assembly,
compiler can do the rest.

The difference in generated code is like below (MCORE2=y):

	<strlen>:
		push   %edi
		mov    $0xffffffff,%ecx
		mov    %eax,%edi
		xor    %eax,%eax
		repnz scas %es:(%edi),%al
		not    %ecx

	-	dec    %ecx
	-	mov    %ecx,%eax
	+	lea    -0x1(%ecx),%eax

		pop    %edi
		ret

Signed-off-by: Alexey Dobriyan <adobriyan@...il.com>
---

 arch/x86/lib/string_32.c |    8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

--- a/arch/x86/lib/string_32.c
+++ b/arch/x86/lib/string_32.c
@@ -164,15 +164,13 @@ EXPORT_SYMBOL(strchr);
 size_t strlen(const char *s)
 {
 	int d0;
-	int res;
+	size_t res;
 	asm volatile("repne\n\t"
-		"scasb\n\t"
-		"notl %0\n\t"
-		"decl %0"
+		"scasb"
 		: "=c" (res), "=&D" (d0)
 		: "1" (s), "a" (0), "0" (0xffffffffu)
 		: "memory");
-	return res;
+	return ~res - 1;
 }
 EXPORT_SYMBOL(strlen);
 #endif
--
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