cmd/compile: []T where T is go:notinheap does not need write barriers

Currently, assigning a []T where T is a go:notinheap type generates an
unnecessary write barrier for storing the slice pointer.

This fixes this by teaching HasHeapPointer that this type does not
have a heap pointer, and tweaking the lowering of slice assignments so
the pointer store retains the correct type rather than simply lowering
it to a *uint8 store.

Change-Id: I8bf7c66e64a7fefdd14f2bd0de8a5a3596340bab
Reviewed-on: https://go-review.googlesource.com/76027
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/gc/ssa.go

@@ -3736,8 +3736,9 @@ func (s *state) storeTypePtrs(t *types.Type, left, right *ssa.Value) {
 		ptr := s.newValue1(ssa.OpStringPtr, s.f.Config.Types.BytePtr, right)
 		s.vars[&memVar] = s.newValue3A(ssa.OpStore, types.TypeMem, s.f.Config.Types.BytePtr, left, ptr, s.mem())
 	case t.IsSlice():
-		ptr := s.newValue1(ssa.OpSlicePtr, s.f.Config.Types.BytePtr, right)
-		s.vars[&memVar] = s.newValue3A(ssa.OpStore, types.TypeMem, s.f.Config.Types.BytePtr, left, ptr, s.mem())
+		elType := types.NewPtr(t.Elem())
+		ptr := s.newValue1(ssa.OpSlicePtr, elType, right)
+		s.vars[&memVar] = s.newValue3A(ssa.OpStore, types.TypeMem, elType, left, ptr, s.mem())
 	case t.IsInterface():
 		// itab field is treated as a scalar.
 		idata := s.newValue1(ssa.OpIData, s.f.Config.Types.BytePtr, right)

src/cmd/compile/internal/types/type.go

@@ -1391,7 +1391,7 @@ func Haspointers1(t *Type, ignoreNotInHeap bool) bool {
 		}
 		return false
 
-	case TPTR32, TPTR64:
+	case TPTR32, TPTR64, TSLICE:
 		return !(ignoreNotInHeap && t.Elem().NotInHeap())
 	}
 

test/notinheap3.go

@@ -10,11 +10,13 @@ package p
 
 type t1 struct {
 	x *nih
+	s []nih
 	y [1024]byte // Prevent write decomposition
 }
 
 type t2 struct {
 	x *ih
+	s []ih
 	y [1024]byte
 }
 
@@ -39,6 +41,8 @@ func f() {
 	// Test direct writes
 	v1.x = nil        // no barrier
 	v2.x = nil        // ERROR "write barrier"
+	v1.s = []nih(nil) // no barrier
+	v2.s = []ih(nil)  // ERROR "write barrier"
 }
 
 func g() {