go/types: fix multiword data structure alignment on nacl

Fixes #16464

Change-Id: Ibf5625c1b5fa3abd18623023f18664e8f81fa45a
Reviewed-on: https://go-review.googlesource.com/26996
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/types/sizes.go

@@ -64,6 +64,15 @@ func (s *StdSizes) Alignof(T Type) int64 {
 			}
 		}
 		return max
+	case *Slice, *Interface:
+		// Multiword data structures are effectively structs
+		// in which each element has size WordSize.
+		return s.WordSize
+	case *Basic:
+		// Strings are like slices and interfaces.
+		if t.Info()&IsString != 0 {
+			return s.WordSize
+		}
 	}
 	a := s.Sizeof(T) // may be 0
 	// spec: "For a variable x of any type: unsafe.Alignof(x) is at least 1."

src/go/types/sizes_test.go

@@ -58,3 +58,26 @@ type S struct {
 		t.Errorf("Sizeof(%v) with WordSize 8 = %d want 40", ts, got)
 	}
 }
+
+// Issue 16464
+func TestAlignofNaclSlice(t *testing.T) {
+	const src = `
+package main
+
+var s struct {
+	x *int
+	y []byte
+}
+`
+	ts := findStructType(t, src)
+	sizes := &types.StdSizes{WordSize: 4, MaxAlign: 8}
+	var fields []*types.Var
+	// Make a copy manually :(
+	for i := 0; i < ts.NumFields(); i++ {
+		fields = append(fields, ts.Field(i))
+	}
+	offsets := sizes.Offsetsof(fields)
+	if offsets[0] != 0 || offsets[1] != 4 {
+		t.Errorf("OffsetsOf(%v) = %v want %v", ts, offsets, []int{0, 4})
+	}
+}