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| author | H.J. Lu <hjl.tools@gmail.com> | 2017-10-30 10:02:16 -0700 |
|---|---|---|
| committer | H.J. Lu <hjl.tools@gmail.com> | 2017-10-30 10:02:30 -0700 |
| commit | c5cc45148c89cc5c57d1946348dd242d4db5c5f5 (patch) | |
| tree | ceee1e4fcc30daa0d3716353d5ffd6ac587508e1 /sysdeps/i386/i586/strcpy.S | |
| parent | ce12269fac8cb873df1a8785e4a6cde870855590 (diff) | |
| download | glibc-c5cc45148c89cc5c57d1946348dd242d4db5c5f5.zip glibc-c5cc45148c89cc5c57d1946348dd242d4db5c5f5.tar.gz glibc-c5cc45148c89cc5c57d1946348dd242d4db5c5f5.tar.bz2 | |
i586: Use conditional branches in strcpy.S [BZ #22353]
i586 strcpy.S used a clever trick with LEA to implement jump table:
/* ECX has the last 2 bits of the address of source - 1. */
andl $3, %ecx
call 2f
2: popl %edx
/* 0xb is the distance between 2: and 1:. */
leal 0xb(%edx,%ecx,8), %ecx
jmp *%ecx
.align 8
1: /* ECX == 0 */
orb (%esi), %al
jz L(end)
stosb
xorl %eax, %eax
incl %esi
/* ECX == 1 */
orb (%esi), %al
jz L(end)
stosb
xorl %eax, %eax
incl %esi
/* ECX == 2 */
orb (%esi), %al
jz L(end)
stosb
xorl %eax, %eax
incl %esi
/* ECX == 3 */
L(1): movl (%esi), %ecx
leal 4(%esi),%esi
This fails if there are instruction length changes before L(1):. This
patch replaces it with conditional branches:
cmpb $2, %cl
je L(Src2)
ja L(Src3)
cmpb $1, %cl
je L(Src1)
L(Src0):
which have similar performance and work with any instruction lengths.
Tested on i586 and i686 with and without --disable-multi-arch.
[BZ #22353]
* sysdeps/i386/i586/strcpy.S (STRCPY): Use conditional branches.
(1): Renamed to ...
(L(Src0)): This.
(L(Src1)): New.
(L(Src2)): Likewise.
(L(1)): Renamed to ...
(L(Src3)): This.
Diffstat (limited to 'sysdeps/i386/i586/strcpy.S')
| -rw-r--r-- | sysdeps/i386/i586/strcpy.S | 28 |
1 files changed, 11 insertions, 17 deletions
diff --git a/sysdeps/i386/i586/strcpy.S b/sysdeps/i386/i586/strcpy.S index a444604..bb73ca4 100644 --- a/sysdeps/i386/i586/strcpy.S +++ b/sysdeps/i386/i586/strcpy.S @@ -53,41 +53,35 @@ ENTRY (STRCPY) cfi_rel_offset (ebx, 0) andl $3, %ecx -#ifdef PIC - call 2f - cfi_adjust_cfa_offset (4) -2: popl %edx - cfi_adjust_cfa_offset (-4) - /* 0xb is the distance between 2: and 1: but we avoid writing - 1f-2b because the assembler generates worse code. */ - leal 0xb(%edx,%ecx,8), %ecx -#else - leal 1f(,%ecx,8), %ecx -#endif - - jmp *%ecx + cmpb $2, %cl + je L(Src2) + ja L(Src3) + cmpb $1, %cl + je L(Src1) - .align 8 -1: +L(Src0): orb (%esi), %al jz L(end) stosb xorl %eax, %eax incl %esi +L(Src1): orb (%esi), %al jz L(end) stosb xorl %eax, %eax incl %esi +L(Src2): orb (%esi), %al jz L(end) stosb xorl %eax, %eax incl %esi -L(1): movl (%esi), %ecx +L(Src3): + movl (%esi), %ecx leal 4(%esi),%esi subl %ecx, %eax @@ -107,7 +101,7 @@ L(1): movl (%esi), %ecx movl %edx, (%edi) leal 4(%edi),%edi - jmp L(1) + jmp L(Src3) L(3): movl %ecx, %edx |