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Subject[tip:x86/asm] x86/asm/64: Micro-optimize __clear_user() - Use immediate constants
Commit-ID:  1153933703d927b3d4874c0bc801de32b1b58be9
Gitweb: https://git.kernel.org/tip/1153933703d927b3d4874c0bc801de32b1b58be9
Author: Alexey Dobriyan <adobriyan@gmail.com>
AuthorDate: Tue, 8 May 2018 00:39:37 +0300
Committer: Ingo Molnar <mingo@kernel.org>
CommitDate: Mon, 14 May 2018 11:38:51 +0200

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@avx2
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
arch/x86/lib/usercopy_64.c | 9 ++++-----
1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/arch/x86/lib/usercopy_64.c b/arch/x86/lib/usercopy_64.c
index 75d3776123cc..a624dcc4de10 100644
--- a/arch/x86/lib/usercopy_64.c
+++ b/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;
}
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