reflect: expose reflect.call argument slice to the garbage collector

The argument slice was kept hidden from the garbage collector
by destroying its referent in an unsafe.Pointer to uintptr
conversion.  This change preserves the unsafe.Pointer referent
and only performs an unsafe.Pointer to uintptr conversions
within expressions that construct new unsafe.Pointer values.

R=golang-dev, khr, rsc
CC=golang-dev
https://golang.org/cl/14008043

src/pkg/reflect/value.go

@@ -446,11 +446,11 @@ func (v Value) call(op string, in []Value) []Value {
 	// For now make everything look like a pointer by allocating
 	// a []unsafe.Pointer.
 	args := make([]unsafe.Pointer, size/ptrSize)
-	ptr := uintptr(unsafe.Pointer(&args[0]))
+	ptr := unsafe.Pointer(&args[0])
 	off := uintptr(0)
 	if v.flag&flagMethod != 0 {
 		// Hard-wired first argument.
-		*(*iword)(unsafe.Pointer(ptr)) = rcvr
+		*(*iword)(ptr) = rcvr
 		off = ptrSize
 	}
 	for i, v := range in {
@@ -459,7 +459,7 @@ func (v Value) call(op string, in []Value) []Value {
 		a := uintptr(targ.align)
 		off = (off + a - 1) &^ (a - 1)
 		n := targ.size
-		addr := unsafe.Pointer(ptr + off)
+		addr := unsafe.Pointer(uintptr(ptr) + off)
 		v = v.assignTo("reflect.Value.Call", targ, (*interface{})(addr))
 		if v.flag&flagIndir == 0 {
 			storeIword(addr, iword(v.val), n)
@@ -471,7 +471,7 @@ func (v Value) call(op string, in []Value) []Value {
 	off = (off + ptrSize - 1) &^ (ptrSize - 1)
 
 	// Call.
-	call(fn, unsafe.Pointer(ptr), uint32(size))
+	call(fn, ptr, uint32(size))
 
 	// Copy return values out of args.
 	//
@@ -482,7 +482,7 @@ func (v Value) call(op string, in []Value) []Value {
 		a := uintptr(tv.Align())
 		off = (off + a - 1) &^ (a - 1)
 		fl := flagIndir | flag(tv.Kind())<<flagKindShift
-		ret[i] = Value{tv.common(), unsafe.Pointer(ptr + off), fl}
+		ret[i] = Value{tv.common(), unsafe.Pointer(uintptr(ptr) + off), fl}
 		off += tv.Size()
 	}