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>

1 files changed, 6 insertions, 12 deletions

diff --git a/arch/x86/lib/memset_64.S b/arch/x86/lib/memset_64.S
index 2c5948116bd2..e88d3b81644a 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 @@ memset_c:
         rep stosb
         movq %r9,%rax
         ret
-        CFI_ENDPROC
-ENDPROC(memset_c)
+.Lmemset_e:
+        .previous
 
 ENTRY(memset)
 ENTRY(__memset)
@@ -118,16 +117,11 @@ ENDPROC(__memset)
 
 #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