runtime: convert int64tofloat64, uint64tofloat64 to Go

I noticed that 5g doesn't flush the float64 result back to the stack, hence the change in the function signature. I'm wondering if I should also change the signature for the other two functions.

LGTM=rsc
R=minux, josharian, rsc
CC=golang-codereviews
https://golang.org/cl/132990044

src/pkg/runtime/vlrt.go

@@ -29,18 +29,34 @@ package runtime
 
 import "unsafe"
 
-func float64toint64(d float64, y uint64) {
-	_d2v(&y, d)
-}
-
-func float64touint64(d float64, y uint64) {
-	_d2v(&y, d)
-}
-
 const (
 	sign64 = 1 << (64 - 1)
 )
 
+func float64toint64(d float64) (y uint64) {
+	_d2v(&y, d)
+	return
+}
+
+func float64touint64(d float64) (y uint64) {
+	_d2v(&y, d)
+	return
+}
+
+func int64tofloat64(y int64) float64 {
+	if y < 0 {
+		return -uint64tofloat64(-uint64(y))
+	}
+	return uint64tofloat64(uint64(y))
+}
+
+func uint64tofloat64(y uint64) float64 {
+	hi := float64(uint32(y >> 32))
+	lo := float64(uint32(y))
+	d := hi*(1<<32) + lo
+	return d
+}
+
 func _d2v(y *uint64, d float64) {
 	x := *(*uint64)(unsafe.Pointer(&d))
 

src/pkg/runtime/vlrt_arm.c

@@ -159,20 +159,6 @@ _v2f(Vlong x)
 	return _v2d(x);
 }
 
-void
-runtime·int64tofloat64(Vlong y, double d)
-{
-	d = _v2d(y);
-	USED(&d); // FLUSH
-}
-
-void
-runtime·uint64tofloat64(Vlong y, double d)
-{
-	d = _ul2d(y.hi)*4294967296. + _ul2d(y.lo);
-	USED(&d); // FLUSH
-}
-
 static void
 dodiv(Vlong num, Vlong den, Vlong *q, Vlong *r)
 {