cmd/cgo: use consistent tag for a particular struct

For #31891
Fixes #38408

Change-Id: Ie7498c2cab728ae798e66e7168425e16b063520e
Reviewed-on: https://go-review.googlesource.com/c/go/+/228102
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>

misc/cgo/test/testx.go

@@ -124,6 +124,11 @@ typedef struct {
 } Issue31891B;
 
 void callIssue31891(void);
+
+typedef struct {
+	int i;
+} Issue38408, *PIssue38408;
+
 */
 import "C"
 
@@ -552,3 +557,8 @@ func useIssue31891B(c *C.Issue31891B) {}
 func test31891(t *testing.T) {
 	C.callIssue31891()
 }
+
+// issue 38408
+// A typedef pointer can be used as the element type.
+// No runtime test; just make sure it compiles.
+var _ C.PIssue38408 = &C.Issue38408{i: 1}

src/cmd/cgo/gcc.go

@@ -2060,6 +2060,10 @@ var goIdent = make(map[string]*ast.Ident)
 // that may contain a pointer. This is used for cgo pointer checking.
 var unionWithPointer = make(map[ast.Expr]bool)
 
+// anonymousStructTag provides a consistent tag for an anonymous struct.
+// The same dwarf.StructType pointer will always get the same tag.
+var anonymousStructTag = make(map[*dwarf.StructType]string)
+
 func (c *typeConv) Init(ptrSize, intSize int64) {
 	c.ptrSize = ptrSize
 	c.intSize = intSize
@@ -2408,8 +2412,12 @@ func (c *typeConv) loadType(dtype dwarf.Type, pos token.Pos, parent string) *Typ
 			break
 		}
 		if tag == "" {
-			tag = "__" + strconv.Itoa(tagGen)
+			tag = anonymousStructTag[dt]
-			tagGen++
+			if tag == "" {
+				tag = "__" + strconv.Itoa(tagGen)
+				tagGen++
+				anonymousStructTag[dt] = tag
+			}
 		} else if t.C.Empty() {
 			t.C.Set(dt.Kind + " " + tag)
 		}