reflect: mark mapassign as noescape

The lack of this annotation causes Value.SetMapIndex to allocate
when it doesn't need to.

Add comments about why it's safe to do so.

Add a test to make sure we stay allocation-free.

Change-Id: I00826e0d73e317a31bdeae5c7e46bf95b0c6ae6a
Reviewed-on: https://go-review.googlesource.com/17060Reviewed-by: David Chase <drchase@google.com>

src/reflect/all_test.go

@@ -4965,3 +4965,32 @@ func TestPtrToMethods(t *testing.T) {
 		t.Fatal("does not implement Stringer, but should")
 	}
 }
+
+func TestMapAlloc(t *testing.T) {
+	m := ValueOf(make(map[int]int, 10))
+	k := ValueOf(5)
+	v := ValueOf(7)
+	allocs := testing.AllocsPerRun(100, func() {
+		m.SetMapIndex(k, v)
+	})
+	if allocs > 0.5 {
+		t.Errorf("allocs per map assignment: want 0 got %f", allocs)
+	}
+}
+
+func TestChanAlloc(t *testing.T) {
+	// Note: for a chan int, the return Value must be allocated, so we
+	// use a chan *int instead.
+	c := ValueOf(make(chan *int, 1))
+	v := ValueOf(new(int))
+	allocs := testing.AllocsPerRun(100, func() {
+		c.Send(v)
+		_, _ = c.Recv()
+	})
+	if allocs < 0.5 || allocs > 1.5 {
+		t.Errorf("allocs per chan send/recv: want 1 got %f", allocs)
+	}
+	// Note: there is one allocation in reflect.recv which seems to be
+	// a limitation of escape analysis.  If that is ever fixed the
+	// allocs < 0.5 condition will trigger and this test should be fixed.
+}

src/reflect/value.go

@@ -142,7 +142,7 @@ func unpackEface(i interface{}) Value {
 	if ifaceIndir(t) {
 		f |= flagIndir
 	}
-	return Value{t, unsafe.Pointer(e.word), f}
+	return Value{t, e.word, f}
 }
 
 // A ValueError occurs when a Value method is invoked on
@@ -590,7 +590,7 @@ func storeRcvr(v Value, p unsafe.Pointer) {
 	if t.Kind() == Interface {
 		// the interface data word becomes the receiver word
 		iface := (*nonEmptyInterface)(v.ptr)
-		*(*unsafe.Pointer)(p) = unsafe.Pointer(iface.word)
+		*(*unsafe.Pointer)(p) = iface.word
 	} else if v.flag&flagIndir != 0 && !ifaceIndir(t) {
 		*(*unsafe.Pointer)(p) = *(*unsafe.Pointer)(v.ptr)
 	} else {
@@ -2086,11 +2086,10 @@ func ValueOf(i interface{}) Value {
 		return Value{}
 	}
 
-	// TODO(rsc): Eliminate this terrible hack.
-	// In the call to unpackEface, i.typ doesn't escape,
-	// and i.word is an integer.  So it looks like
-	// i doesn't escape.  But really it does,
-	// because i.word is actually a pointer.
+	// TODO: Maybe allow contents of a Value to live on the stack.
+	// For now we make the contents always escape to the heap.  It
+	// makes life easier in a few places (see chanrecv/mapassign
+	// comment below).
 	escapes(i)
 
 	return unpackEface(i)
@@ -2446,6 +2445,14 @@ func chancap(ch unsafe.Pointer) int
 func chanclose(ch unsafe.Pointer)
 func chanlen(ch unsafe.Pointer) int
 
+// Note: some of the noescape annotations below are technically a lie,
+// but safe in the context of this package.  Functions like chansend
+// and mapassign don't escape the referent, but may escape anything
+// the referent points to (they do shallow copies of the referent).
+// It is safe in this package because the referent may only point
+// to something a Value may point to, and that is always in the heap
+// (due to the escapes() call in ValueOf).
+
 //go:noescape
 func chanrecv(t *rtype, ch unsafe.Pointer, nb bool, val unsafe.Pointer) (selected, received bool)
 
@@ -2458,6 +2465,7 @@ func makemap(t *rtype) (m unsafe.Pointer)
 //go:noescape
 func mapaccess(t *rtype, m unsafe.Pointer, key unsafe.Pointer) (val unsafe.Pointer)
 
+//go:noescape
 func mapassign(t *rtype, m unsafe.Pointer, key, val unsafe.Pointer)
 
 //go:noescape