internal/bytealg: share code for IndexByte functions on arm

Move the shared code of IndexByte and IndexByteString into
indexbytebody. This will allow to implement optimizations (e.g.
for #29001) in a single function.

Change-Id: I1d550da8eb65f95e492a460a12058cc35b1162b6
Reviewed-on: https://go-review.googlesource.com/c/go/+/167939
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/internal/bytealg/indexbyte_arm.s

@@ -9,46 +9,38 @@ TEXT ·IndexByte(SB),NOSPLIT,$0-20
 	MOVW	b_base+0(FP), R0
 	MOVW	b_len+4(FP), R1
 	MOVBU	c+12(FP), R2	// byte to find
-	MOVW	R0, R4		// store base for later
-	ADD	R0, R1		// end
-
-_loop:
-	CMP	R0, R1
-	B.EQ	_notfound
-	MOVBU.P	1(R0), R3
-	CMP	R2, R3
-	B.NE	_loop
-
-	SUB	$1, R0		// R0 will be one beyond the position we want
-	SUB	R4, R0		// remove base
-	MOVW    R0, ret+16(FP)
-	RET
-
-_notfound:
-	MOVW	$-1, R0
-	MOVW	R0, ret+16(FP)
-	RET
+	MOVW	$ret+16(FP), R5
+	B	indexbytebody<>(SB)
 
 TEXT ·IndexByteString(SB),NOSPLIT,$0-16
 	MOVW	s_base+0(FP), R0
 	MOVW	s_len+4(FP), R1
 	MOVBU	c+8(FP), R2	// byte to find
+	MOVW	$ret+12(FP), R5
+	B	indexbytebody<>(SB)
+
+// input:
+//  R0: data
+//  R1: data length
+//  R2: byte to find
+//  R5: address to put result
+TEXT indexbytebody<>(SB),NOSPLIT,$0-0
 	MOVW	R0, R4		// store base for later
 	ADD	R0, R1		// end
 
-_sib_loop:
+loop:
 	CMP	R0, R1
-	B.EQ	_sib_notfound
+	B.EQ	notfound
 	MOVBU.P	1(R0), R3
 	CMP	R2, R3
-	B.NE	_sib_loop
+	B.NE	loop
 
 	SUB	$1, R0		// R0 will be one beyond the position we want
 	SUB	R4, R0		// remove base
-	MOVW	R0, ret+12(FP)
+	MOVW	R0, (R5)
 	RET
 
-_sib_notfound:
+notfound:
 	MOVW	$-1, R0
-	MOVW	R0, ret+12(FP)
+	MOVW	R0, (R5)
 	RET