Commit 890890cb authored Dec 11, 2011 by Alexey Dobriyan Committed by Ingo Molnar Dec 12, 2011

x86/i386: Use less assembly in strlen(), speed things up a bit



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@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Jan Beulich <JBeulich@suse.com>
Link: http://lkml.kernel.org/r/20111211181319.GA17097@p183.telecom.by


Signed-off-by: Ingo Molnar <mingo@elte.hu>

parent 79f1ddd0

arch/x86/lib/string_32.c

+3 −5

Original line number	Diff line number	Diff line
		@@ -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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