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.

diff --git a/ChangeLog b/ChangeLog
index 5961238..5ea3d85 100644 (file)
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,14 @@
+2017-10-30  H.J. Lu  <hongjiu.lu@intel.com>
+
+       [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.
+
 2017-10-30  Joseph Myers  <joseph@codesourcery.com>
 
        * math/math.h [__HAVE_FLOAT16 && __USE_GNU] (M_Ef16): New macro.

diff --git a/sysdeps/i386/i586/strcpy.S b/sysdeps/i386/i586/strcpy.S
index a444604..bb73ca4 100644 (file)
--- 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