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 <[e-mail]>
Cc: Linus Torvalds <[e-mail]>
Cc: Jan Beulich <[e-mail]>
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Ingo Molnar <[e-mail]>

arch/x86/lib/string_32.c

···
 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