cmd/compile: fix storeType to handle pointers to go:notinheap types

storeType splits compound stores up into a scalar parts and a pointer parts.
The scalar part happens unconditionally, and the pointer part happens
under the guard of a write barrier check.

Types which are declared as pointers, but are represented as scalars because
they might have "bad" values, were not handled correctly here. They ended
up not getting stored in either set.

Fixes #42032

Change-Id: I46f6600075c0c370e640b807066247237f93c7ac
Reviewed-on: https://go-review.googlesource.com/c/go/+/264300
Trust: Keith Randall <khr@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

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

@@ -5222,7 +5222,10 @@ func (s *state) storeTypeScalars(t *types.Type, left, right *ssa.Value, skip ski
 	case t.IsBoolean() || t.IsInteger() || t.IsFloat() || t.IsComplex():
 		s.store(t, left, right)
 	case t.IsPtrShaped():
-		// no scalar fields.
+		if t.IsPtr() && t.Elem().NotInHeap() {
+			s.store(t, left, right) // see issue 42032
+		}
+		// otherwise, no scalar fields.
 	case t.IsString():
 		if skip&skipLen != 0 {
 			return
@@ -5266,6 +5269,9 @@ func (s *state) storeTypeScalars(t *types.Type, left, right *ssa.Value, skip ski
 func (s *state) storeTypePtrs(t *types.Type, left, right *ssa.Value) {
 	switch {
 	case t.IsPtrShaped():
+		if t.IsPtr() && t.Elem().NotInHeap() {
+			break // see issue 42032
+		}
 		s.store(t, left, right)
 	case t.IsString():
 		ptr := s.newValue1(ssa.OpStringPtr, s.f.Config.Types.BytePtr, right)

test/fixedbugs/issue42032.go

@@ -0,0 +1,27 @@
+// run
+
+// Copyright 2020 The Go Authors. All rights reserved.  Use of this
+// source code is governed by a BSD-style license that can be found in
+// the LICENSE file.
+
+package main
+
+//go:notinheap
+type NIH struct {
+}
+
+type T struct {
+	x *NIH
+	p *int
+}
+
+var y NIH
+var z int
+
+func main() {
+	a := []T{{&y, &z}}
+	a = append(a, T{&y, &z})
+	if a[1].x == nil {
+		panic("pointer not written")
+	}
+}