======
Perform the following substitutions:
ip -> r8, then
r0 -> ip,
and insert 'mov ip, r0' as the first statement of the function.
At this point, we have a memset() implementation returning the proper result,
but corrupting r8 on some paths (the ones that were using ip).

Step 2
======
Make sure r8 is saved and restored when (! CALGN(1)+0) == 1:

save r8:
-       str     lr, [sp, #-4]!
+       stmfd   sp!, {r8, lr}

and restore r8 on both exit paths:
-       ldmeqfd sp!, {pc}               @ Now <64 bytes to go.
+       ldmeqfd sp!, {r8, pc}           @ Now <64 bytes to go.
(...)
        tst     r2, #16
        stmneia ip!, {r1, r3, r8, lr}
-       ldr     lr, [sp], #4
+       ldmfd   sp!, {r8, lr}

Step 3
======
Make sure r8 is saved and restored when (! CALGN(1)+0) == 0:

save r8:
-       stmfd   sp!, {r4-r7, lr}
+       stmfd   sp!, {r4-r8, lr}

and restore r8 on both exit paths:
        bgt     3b
-       ldmeqfd sp!, {r4-r7, pc}
+       ldmeqfd sp!, {r4-r8, pc}
(...)
        tst     r2, #16
        stmneia ip!, {r4-r7}
-       ldmfd   sp!, {r4-r7, lr}
+       ldmfd   sp!, {r4-r8, lr}

Step 4
======
Rewrite register list "r4-r7, r8" as "r4-r8".

Signed-off-by: Ivan Djelic <ivan.djelic@parrot.com>
Reviewed-by: Nicolas Pitre <nico@linaro.org>
---
v2 changelog:
- added patch breakdown explanation to commit text
 arch/arm/lib/memset.S |   85 +++++++++++++++++++++++++------------------------
 1 file changed, 44 insertions(+), 41 deletions(-)

diff --git a/arch/arm/lib/memset.S b/arch/arm/lib/memset.S
index 650d592..d912e73 100644
--- a/arch/arm/lib/memset.S
+++ b/arch/arm/lib/memset.S
@@ -19,9 +19,9 @@
 1:	subs	r2, r2, #4		@ 1 do we have enough
 	blt	5f			@ 1 bytes to align with?
 	cmp	r3, #2			@ 1
-	strltb	r1, [r0], #1		@ 1
-	strleb	r1, [r0], #1		@ 1
-	strb	r1, [r0], #1		@ 1
+	strltb	r1, [ip], #1		@ 1
+	strleb	r1, [ip], #1		@ 1
+	strb	r1, [ip], #1		@ 1
 	add	r2, r2, r3		@ 1 (r2 = r2 - (4 - r3))
 /*
  * The pointer is now aligned and the length is adjusted.  Try doing the
@@ -29,10 +29,14 @@
  */
 
 ENTRY(memset)
-	ands	r3, r0, #3		@ 1 unaligned?
+/*
+ * Preserve the contents of r0 for the return value.
+ */
+	mov	ip, r0
+	ands	r3, ip, #3		@ 1 unaligned?
 	bne	1b			@ 1
 /*
- * we know that the pointer in r0 is aligned to a word boundary.
+ * we know that the pointer in ip is aligned to a word boundary.
  */
 	orr	r1, r1, r1, lsl #8
 	orr	r1, r1, r1, lsl #16
@@ -43,29 +47,28 @@ ENTRY(memset)
 #if ! CALGN(1)+0
 
 /*
- * We need an extra register for this loop - save the return address and
- * use the LR
+ * We need 2 extra registers for this loop - use r8 and the LR
  */
-	str	lr, [sp, #-4]!
-	mov	ip, r1
+	stmfd	sp!, {r8, lr}
+	mov	r8, r1
 	mov	lr, r1
 
 2:	subs	r2, r2, #64
-	stmgeia	r0!, {r1, r3, ip, lr}	@ 64 bytes at a time.
-	stmgeia	r0!, {r1, r3, ip, lr}
-	stmgeia	r0!, {r1, r3, ip, lr}
-	stmgeia	r0!, {r1, r3, ip, lr}
+	stmgeia	ip!, {r1, r3, r8, lr}	@ 64 bytes at a time.
+	stmgeia	ip!, {r1, r3, r8, lr}
+	stmgeia	ip!, {r1, r3, r8, lr}
+	stmgeia	ip!, {r1, r3, r8, lr}
 	bgt	2b
-	ldmeqfd	sp!, {pc}		@ Now <64 bytes to go.
+	ldmeqfd	sp!, {r8, pc}		@ Now <64 bytes to go.
 /*
  * No need to correct the count; we're only testing bits from now on
  */
 	tst	r2, #32
-	stmneia	r0!, {r1, r3, ip, lr}
-	stmneia	r0!, {r1, r3, ip, lr}
+	stmneia	ip!, {r1, r3, r8, lr}
+	stmneia	ip!, {r1, r3, r8, lr}
 	tst	r2, #16
-	stmneia	r0!, {r1, r3, ip, lr}
-	ldr	lr, [sp], #4
+	stmneia	ip!, {r1, r3, r8, lr}
+	ldmfd	sp!, {r8, lr}
 
 #else
 
@@ -74,54 +77,54 @@ ENTRY(memset)
  * whole cache lines at once.
  */
 
-	stmfd	sp!, {r4-r7, lr}
+	stmfd	sp!, {r4-r8, lr}
 	mov	r4, r1
 	mov	r5, r1
 	mov	r6, r1
 	mov	r7, r1
-	mov	ip, r1
+	mov	r8, r1
 	mov	lr, r1
 
 	cmp	r2, #96
-	tstgt	r0, #31
+	tstgt	ip, #31
 	ble	3f
 
-	and	ip, r0, #31
-	rsb	ip, ip, #32
-	sub	r2, r2, ip
-	movs	ip, ip, lsl #(32 - 4)
-	stmcsia	r0!, {r4, r5, r6, r7}
-	stmmiia	r0!, {r4, r5}
-	tst	ip, #(1 << 30)
-	mov	ip, r1
-	strne	r1, [r0], #4
+	and	r8, ip, #31
+	rsb	r8, r8, #32
+	sub	r2, r2, r8
+	movs	r8, r8, lsl #(32 - 4)
+	stmcsia	ip!, {r4, r5, r6, r7}
+	stmmiia	ip!, {r4, r5}
+	tst	r8, #(1 << 30)
+	mov	r8, r1
+	strne	r1, [ip], #4
 
 3:	subs	r2, r2, #64
-	stmgeia	r0!, {r1, r3-r7, ip, lr}
-	stmgeia	r0!, {r1, r3-r7, ip, lr}
+	stmgeia	ip!, {r1, r3-r8, lr}
+	stmgeia	ip!, {r1, r3-r8, lr}
 	bgt	3b
-	ldmeqfd	sp!, {r4-r7, pc}
+	ldmeqfd	sp!, {r4-r8, pc}
 
 	tst	r2, #32
-	stmneia	r0!, {r1, r3-r7, ip, lr}
+	stmneia	ip!, {r1, r3-r8, lr}
 	tst	r2, #16
-	stmneia	r0!, {r4-r7}
-	ldmfd	sp!, {r4-r7, lr}
+	stmneia	ip!, {r4-r7}
+	ldmfd	sp!, {r4-r8, lr}
 
 #endif
 
 4:	tst	r2, #8
-	stmneia	r0!, {r1, r3}
+	stmneia	ip!, {r1, r3}
 	tst	r2, #4
-	strne	r1, [r0], #4
+	strne	r1, [ip], #4
 /*
  * When we get here, we've got less than 4 bytes to zero.  We
  * may have an unaligned pointer as well.
  */
 5:	tst	r2, #2
-	strneb	r1, [r0], #1
-	strneb	r1, [r0], #1
+	strneb	r1, [ip], #1
+	strneb	r1, [ip], #1
 	tst	r2, #1
-	strneb	r1, [r0], #1
+	strneb	r1, [ip], #1
 	mov	pc, lr
 ENDPROC(memset)
