x86/asm/64: Micro-optimize __clear_user() - Use immediate constants

Save two registers. Adding constants should be just as fast as adding registers.
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20180507213937.GB32406@avx2Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/lib/usercopy_64.c

@@ -23,13 +23,13 @@ unsigned long __clear_user(void __user *addr, unsigned long size)
 	asm volatile(
 		"	testq  %[size8],%[size8]\n"
 		"	jz     4f\n"
-		"0:	movq %[zero],(%[dst])\n"
-		"	addq   %[eight],%[dst]\n"
+		"0:	movq $0,(%[dst])\n"
+		"	addq   $8,%[dst]\n"
 		"	decl %%ecx ; jnz   0b\n"
 		"4:	movq  %[size1],%%rcx\n"
 		"	testl %%ecx,%%ecx\n"
 		"	jz     2f\n"
-		"1:	movb   %b[zero],(%[dst])\n"
+		"1:	movb   $0,(%[dst])\n"
 		"	incq   %[dst]\n"
 		"	decl %%ecx ; jnz  1b\n"
 		"2:\n"
@@ -40,8 +40,7 @@ unsigned long __clear_user(void __user *addr, unsigned long size)
 		_ASM_EXTABLE(0b,3b)
 		_ASM_EXTABLE(1b,2b)
 		: [size8] "=&c"(size), [dst] "=&D" (__d0)
-		: [size1] "r"(size & 7), "[size8]" (size / 8), "[dst]"(addr),
-		  [zero] "r" (0UL), [eight] "r" (8UL));
+		: [size1] "r"(size & 7), "[size8]" (size / 8), "[dst]"(addr));
 	clac();
 	return size;
 }