runtime, math/big: allow R0 on s390x to contain values other than 0

The new SSA backend for s390x can use R0 as a general purpose register.
This change modifies assembly code to either avoid using R0 entirely
or explicitly set R0 to 0.

R0 can still be safely used as 0 in address calculations.

Change-Id: I3efa723e9ef322a91a408bd8c31768d7858526c8
Reviewed-on: https://go-review.googlesource.com/28976Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/math/big/arith_s390x.s

@@ -271,6 +271,7 @@ E4:	MOVD R4, c+56(FP)	// return c
 // func shlVU(z, x []Word, s uint) (c Word)
 TEXT ·shlVU(SB),NOSPLIT,$0
 	MOVD z_len+8(FP), R5
+	MOVD $0, R0
 	SUB  $1, R5             // n--
 	BLT  X8b                // n < 0        (n <= 0)
 
@@ -358,6 +359,7 @@ E864:	CMPBGT R5, R0, L864     // i < n-1
 // func shrVU(z, x []Word, s uint) (c Word)
 TEXT ·shrVU(SB),NOSPLIT,$0
 	MOVD z_len+8(FP), R5
+	MOVD $0, R0
 	SUB  $1, R5             // n--
 	BLT  X9b                // n < 0        (n <= 0)
 

src/runtime/asm_s390x.s

@@ -110,22 +110,22 @@ nocgo:
 	MOVD	$runtime·mainPC(SB), R2		// entry
 	SUB     $24, R15
 	MOVD 	R2, 16(R15)
-	MOVD 	R0, 8(R15)
-	MOVD 	R0, 0(R15)
+	MOVD 	$0, 8(R15)
+	MOVD 	$0, 0(R15)
 	BL	runtime·newproc(SB)
 	ADD	$24, R15
 
 	// start this M
 	BL	runtime·mstart(SB)
 
-	MOVD	R0, 1(R0)
+	MOVD	$0, 1(R0)
 	RET
 
 DATA	runtime·mainPC+0(SB)/8,$runtime·main(SB)
 GLOBL	runtime·mainPC(SB),RODATA,$8
 
 TEXT runtime·breakpoint(SB),NOSPLIT|NOFRAME,$0-0
-	MOVD	R0, 2(R0)
+	MOVD	$0, 2(R0)
 	RET
 
 TEXT runtime·asminit(SB),NOSPLIT|NOFRAME,$0-0
@@ -175,7 +175,7 @@ TEXT runtime·mcall(SB), NOSPLIT, $-8-8
 	// Save caller state in g->sched
 	MOVD	R15, (g_sched+gobuf_sp)(g)
 	MOVD	LR, (g_sched+gobuf_pc)(g)
-	MOVD	R0, (g_sched+gobuf_lr)(g)
+	MOVD	$0, (g_sched+gobuf_lr)(g)
 	MOVD	g, (g_sched+gobuf_g)(g)
 
 	// Switch to m->g0 & its stack, call fn.
@@ -232,7 +232,7 @@ switch:
 	ADD	$16, R6	// get past prologue
 	MOVD	R6, (g_sched+gobuf_pc)(g)
 	MOVD	R15, (g_sched+gobuf_sp)(g)
-	MOVD	R0, (g_sched+gobuf_lr)(g)
+	MOVD	$0, (g_sched+gobuf_lr)(g)
 	MOVD	g, (g_sched+gobuf_g)(g)
 
 	// switch to g0
@@ -526,7 +526,7 @@ g0:
 	MOVD	(g_stack+stack_hi)(R5), R5
 	SUB	R2, R5
 	MOVD	R5, 160(R15)             // save depth in old g stack (can't just save SP, as stack might be copied during a callback)
-	MOVD	R0, 0(R15)              // clear back chain pointer (TODO can we give it real back trace information?)
+	MOVD	$0, 0(R15)              // clear back chain pointer (TODO can we give it real back trace information?)
 	MOVD	R4, R2                  // arg in R2
 	BL	R3                      // can clobber: R0-R5, R14, F0-F3, F5, F7-F15
 

src/runtime/memclr_s390x.s

@@ -16,8 +16,8 @@ start:
 	CMPBLE	R5, $15, clear12to15
 	CMP	R5, $32
 	BGE	clearmt32
-	MOVD	R0, 0(R4)
-	MOVD	R0, 8(R4)
+	MOVD	$0, 0(R4)
+	MOVD	$0, 8(R4)
 	ADD	$16, R4
 	SUB	$16, R5
 	BR	start
@@ -25,79 +25,79 @@ start:
 clear0to3:
 	CMPBEQ	R5, $0, done
 	CMPBNE	R5, $1, clear2
-	MOVB	R0, 0(R4)
+	MOVB	$0, 0(R4)
 	RET
 clear2:
 	CMPBNE	R5, $2, clear3
-	MOVH	R0, 0(R4)
+	MOVH	$0, 0(R4)
 	RET
 clear3:
-	MOVH	R0, 0(R4)
-	MOVB	R0, 2(R4)
+	MOVH	$0, 0(R4)
+	MOVB	$0, 2(R4)
 	RET
 
 clear4to7:
 	CMPBNE	R5, $4, clear5
-	MOVW	R0, 0(R4)
+	MOVW	$0, 0(R4)
 	RET
 clear5:
 	CMPBNE	R5, $5, clear6
-	MOVW	R0, 0(R4)
-	MOVB	R0, 4(R4)
+	MOVW	$0, 0(R4)
+	MOVB	$0, 4(R4)
 	RET
 clear6:
 	CMPBNE	R5, $6, clear7
-	MOVW	R0, 0(R4)
-	MOVH	R0, 4(R4)
+	MOVW	$0, 0(R4)
+	MOVH	$0, 4(R4)
 	RET
 clear7:
-	MOVW	R0, 0(R4)
-	MOVH	R0, 4(R4)
-	MOVB	R0, 6(R4)
+	MOVW	$0, 0(R4)
+	MOVH	$0, 4(R4)
+	MOVB	$0, 6(R4)
 	RET
 
 clear8to11:
 	CMPBNE	R5, $8, clear9
-	MOVD	R0, 0(R4)
+	MOVD	$0, 0(R4)
 	RET
 clear9:
 	CMPBNE	R5, $9, clear10
-	MOVD	R0, 0(R4)
-	MOVB	R0, 8(R4)
+	MOVD	$0, 0(R4)
+	MOVB	$0, 8(R4)
 	RET
 clear10:
 	CMPBNE	R5, $10, clear11
-	MOVD	R0, 0(R4)
-	MOVH	R0, 8(R4)
+	MOVD	$0, 0(R4)
+	MOVH	$0, 8(R4)
 	RET
 clear11:
-	MOVD	R0, 0(R4)
-	MOVH	R0, 8(R4)
-	MOVB	R0, 10(R4)
+	MOVD	$0, 0(R4)
+	MOVH	$0, 8(R4)
+	MOVB	$0, 10(R4)
 	RET
 
 clear12to15:
 	CMPBNE	R5, $12, clear13
-	MOVD	R0, 0(R4)
-	MOVW	R0, 8(R4)
+	MOVD	$0, 0(R4)
+	MOVW	$0, 8(R4)
 	RET
 clear13:
 	CMPBNE	R5, $13, clear14
-	MOVD	R0, 0(R4)
-	MOVW	R0, 8(R4)
-	MOVB	R0, 12(R4)
+	MOVD	$0, 0(R4)
+	MOVW	$0, 8(R4)
+	MOVB	$0, 12(R4)
 	RET
 clear14:
 	CMPBNE	R5, $14, clear15
-	MOVD	R0, 0(R4)
-	MOVW	R0, 8(R4)
-	MOVH	R0, 12(R4)
+	MOVD	$0, 0(R4)
+	MOVW	$0, 8(R4)
+	MOVH	$0, 12(R4)
 	RET
 clear15:
-	MOVD	R0, 0(R4)
-	MOVW	R0, 8(R4)
-	MOVH	R0, 12(R4)
-	MOVB	R0, 14(R4)
+	MOVD	$0, 0(R4)
+	MOVW	$0, 8(R4)
+	MOVH	$0, 12(R4)
+	MOVB	$0, 14(R4)
 	RET
 
 clearmt32:
@@ -117,6 +117,6 @@ done:
 // DO NOT CALL - target for exrl (execute relative long) instruction.
 TEXT runtime·memclr_s390x_exrl_xc(SB),NOSPLIT|NOFRAME,$0-0
 	XC	$1, 0(R4), 0(R4)
-	MOVD	R0, 0(R0)
+	MOVD	$0, 0(R0)
 	RET