cmd/6g, cmd/8g, liblink: improve handling of float constants

* Enable basic constant propagation for floats.
  The constant propagation is still not as aggressive as it could be.
* Implement MOVSS $(0), Xx and MOVSD $(0), Xx as XORPS Xx, Xx.

Sample code:

func f32() float32 {
	var f float32
	return f
}

func f64() float64 {
	var f float64
	return f
}

Before:

"".f32 t=1 size=32 value=0 args=0x8 locals=0x0
	0x0000 00000 (demo.go:3)	TEXT	"".f32+0(SB),4,$0-8
	0x0000 00000 (demo.go:3)	FUNCDATA	$0,gclocals·a7a3692b8e27e823add69ec4239ba55f+0(SB)
	0x0000 00000 (demo.go:3)	FUNCDATA	$1,gclocals·3280bececceccd33cb74587feedb1f9f+0(SB)
	0x0000 00000 (demo.go:3)	MOVSS	$f32.00000000+0(SB),X0
	0x0008 00008 (demo.go:4)	MOVSS	$f32.00000000+0(SB),X0
	0x0010 00016 (demo.go:5)	MOVSS	X0,"".~r0+8(FP)
	0x0016 00022 (demo.go:5)	RET	,
"".f64 t=1 size=32 value=0 args=0x8 locals=0x0
	0x0000 00000 (demo.go:8)	TEXT	"".f64+0(SB),4,$0-8
	0x0000 00000 (demo.go:8)	FUNCDATA	$0,gclocals·a7a3692b8e27e823add69ec4239ba55f+0(SB)
	0x0000 00000 (demo.go:8)	FUNCDATA	$1,gclocals·3280bececceccd33cb74587feedb1f9f+0(SB)
	0x0000 00000 (demo.go:8)	MOVSD	$f64.0000000000000000+0(SB),X0
	0x0008 00008 (demo.go:9)	MOVSD	$f64.0000000000000000+0(SB),X0
	0x0010 00016 (demo.go:10)	MOVSD	X0,"".~r0+8(FP)
	0x0016 00022 (demo.go:10)	RET	,

After:

"".f32 t=1 size=16 value=0 args=0x8 locals=0x0
	0x0000 00000 (demo.go:3)	TEXT	"".f32+0(SB),4,$0-8
	0x0000 00000 (demo.go:3)	FUNCDATA	$0,gclocals·a7a3692b8e27e823add69ec4239ba55f+0(SB)
	0x0000 00000 (demo.go:3)	FUNCDATA	$1,gclocals·3280bececceccd33cb74587feedb1f9f+0(SB)
	0x0000 00000 (demo.go:3)	XORPS	X0,X0
	0x0003 00003 (demo.go:5)	MOVSS	X0,"".~r0+8(FP)
	0x0009 00009 (demo.go:5)	RET	,
"".f64 t=1 size=16 value=0 args=0x8 locals=0x0
	0x0000 00000 (demo.go:8)	TEXT	"".f64+0(SB),4,$0-8
	0x0000 00000 (demo.go:8)	FUNCDATA	$0,gclocals·a7a3692b8e27e823add69ec4239ba55f+0(SB)
	0x0000 00000 (demo.go:8)	FUNCDATA	$1,gclocals·3280bececceccd33cb74587feedb1f9f+0(SB)
	0x0000 00000 (demo.go:8)	XORPS	X0,X0
	0x0003 00003 (demo.go:10)	MOVSD	X0,"".~r0+8(FP)
	0x0009 00009 (demo.go:10)	RET	,

Change-Id: Ie9eb65e324af4f664153d0a7cd22bb16b0fba16d
Reviewed-on: https://go-review.googlesource.com/2053Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/6g/gsubr.c

@@ -624,7 +624,6 @@ bignodes(void)
  *	t = f
  * hard part is conversions.
  */
-// TODO: lost special constants for floating point.  XORPD for 0.0?
 void
 gmove(Node *f, Node *t)
 {

src/cmd/6g/peep.c

@@ -121,7 +121,7 @@ peep(Prog *firstp)
 		case AMOVSS:
 		case AMOVSD:
 			if(regtyp(&p->to))
-			if(p->from.type == D_CONST)
+			if(p->from.type == D_CONST || p->from.type == D_FCONST)
 				conprop(r);
 			break;
 		}
@@ -384,7 +384,7 @@ regtyp(Adr *a)
 	t = a->type;
 	if(t >= D_AX && t <= D_R15)
 		return 1;
-	if(t >= D_X0 && t <= D_X0+15)
+	if(t >= D_X0 && t <= D_X15)
 		return 1;
 	return 0;
 }

src/cmd/8g/peep.c

@@ -120,7 +120,7 @@ peep(Prog *firstp)
 		case AMOVSS:
 		case AMOVSD:
 			if(regtyp(&p->to))
-			if(p->from.type == D_CONST)
+			if(p->from.type == D_CONST || p->from.type == D_FCONST)
 				conprop(r);
 			break;
 		}

src/liblink/obj6.c

@@ -241,6 +241,19 @@ progedit(Link *ctxt, Prog *p)
 
 	// Rewrite float constants to values stored in memory.
 	switch(p->as) {
+	case AMOVSS:
+		// Convert AMOVSS $(0), Xx to AXORPS Xx, Xx
+		if(p->from.type == D_FCONST)
+		if(p->from.u.dval == 0)
+		if(p->to.type >= D_X0)
+		if(p->to.type <= D_X15) {
+			p->as = AXORPS;
+			p->from.type = p->to.type;
+			p->from.index = p->to.index;
+			break;
+		}
+		// fallthrough
+
 	case AFMOVF:
 	case AFADDF:
 	case AFSUBF:
@@ -250,7 +263,6 @@ progedit(Link *ctxt, Prog *p)
 	case AFDIVRF:
 	case AFCOMF:
 	case AFCOMFP:
-	case AMOVSS:
 	case AADDSS:
 	case ASUBSS:
 	case AMULSS:
@@ -274,6 +286,19 @@ progedit(Link *ctxt, Prog *p)
 			p->from.offset = 0;
 		}
 		break;
+
+	case AMOVSD:
+		// Convert AMOVSD $(0), Xx to AXORPS Xx, Xx
+		if(p->from.type == D_FCONST)
+		if(p->from.u.dval == 0)
+		if(p->to.type >= D_X0)
+		if(p->to.type <= D_X15) {
+			p->as = AXORPS;
+			p->from.type = p->to.type;
+			p->from.index = p->to.index;
+			break;
+		}
+		// fallthrough
 	
 	case AFMOVD:
 	case AFADDD:
@@ -284,7 +309,6 @@ progedit(Link *ctxt, Prog *p)
 	case AFDIVRD:
 	case AFCOMD:
 	case AFCOMDP:
-	case AMOVSD:
 	case AADDSD:
 	case ASUBSD:
 	case AMULSD:

src/liblink/obj8.c

@@ -177,6 +177,19 @@ progedit(Link *ctxt, Prog *p)
 
 	// Rewrite float constants to values stored in memory.
 	switch(p->as) {
+	case AMOVSS:
+		// Convert AMOVSS $(0), Xx to AXORPS Xx, Xx
+		if(p->from.type == D_FCONST)
+		if(p->from.u.dval == 0)
+		if(p->to.type >= D_X0)
+		if(p->to.type <= D_X7) {
+			p->as = AXORPS;
+			p->from.type = p->to.type;
+			p->from.index = p->to.index;
+			break;
+		}
+		// fallthrough
+
 	case AFMOVF:
 	case AFADDF:
 	case AFSUBF:
@@ -186,7 +199,6 @@ progedit(Link *ctxt, Prog *p)
 	case AFDIVRF:
 	case AFCOMF:
 	case AFCOMFP:
-	case AMOVSS:
 	case AADDSS:
 	case ASUBSS:
 	case AMULSS:
@@ -211,6 +223,19 @@ progedit(Link *ctxt, Prog *p)
 		}
 		break;
 
+	case AMOVSD:
+		// Convert AMOVSD $(0), Xx to AXORPS Xx, Xx
+		if(p->from.type == D_FCONST)
+		if(p->from.u.dval == 0)
+		if(p->to.type >= D_X0)
+		if(p->to.type <= D_X7) {
+			p->as = AXORPS;
+			p->from.type = p->to.type;
+			p->from.index = p->to.index;
+			break;
+		}
+		// fallthrough
+
 	case AFMOVD:
 	case AFADDD:
 	case AFSUBD:
@@ -220,7 +245,6 @@ progedit(Link *ctxt, Prog *p)
 	case AFDIVRD:
 	case AFCOMD:
 	case AFCOMDP:
-	case AMOVSD:
 	case AADDSD:
 	case ASUBSD:
 	case AMULSD: