x86-64: Modify memcpy()/memset() alternatives mechanism
In order to avoid unnecessary chains of branches, rather than
implementing memcpy()/memset()'s access to their alternative
implementations via a jump, patch the (larger) original function
directly.
The memcpy() part of this is slightly subtle: while alternative
instruction patching does itself use memcpy(), with the
replacement block being less than 64-bytes in size the main loop
of the original function doesn't get used for copying memcpy_c()
over memcpy(), and hence we can safely write over its beginning.
Also note that the CFI annotations are fine for both variants of
each of the functions.
Signed-off-by: Jan Beulich <jbeulich@novell.com>
Cc: Nick Piggin <npiggin@suse.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
LKML-Reference: <4B2BB8D30200007800026AF2@vpn.id2.novell.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
diff --git a/arch/x86/lib/memset_64.S b/arch/x86/lib/memset_64.S
index 2c59481..e88d3b8 100644
--- a/arch/x86/lib/memset_64.S
+++ b/arch/x86/lib/memset_64.S
@@ -12,9 +12,8 @@
*
* rax original destination
*/
- ALIGN
-memset_c:
- CFI_STARTPROC
+ .section .altinstr_replacement, "ax", @progbits
+.Lmemset_c:
movq %rdi,%r9
movl %edx,%r8d
andl $7,%r8d
@@ -29,8 +28,8 @@
rep stosb
movq %r9,%rax
ret
- CFI_ENDPROC
-ENDPROC(memset_c)
+.Lmemset_e:
+ .previous
ENTRY(memset)
ENTRY(__memset)
@@ -118,16 +117,11 @@
#include <asm/cpufeature.h>
- .section .altinstr_replacement,"ax"
-1: .byte 0xeb /* jmp <disp8> */
- .byte (memset_c - memset) - (2f - 1b) /* offset */
-2:
- .previous
.section .altinstructions,"a"
.align 8
.quad memset
- .quad 1b
+ .quad .Lmemset_c
.byte X86_FEATURE_REP_GOOD
.byte .Lfinal - memset
- .byte 2b - 1b
+ .byte .Lmemset_e - .Lmemset_c
.previous