runtime: add (unused for now) gc field to type information

R=rsc
CC=golang-dev
https://golang.org/cl/6255074

src/cmd/gc/reflect.c

@@ -546,15 +546,17 @@ dcommontype(Sym *s, int ot, Type *t)
 	// ../../pkg/reflect/type.go:/^type.commonType
 	// actual type structure
 	//	type commonType struct {
-	//		size uintptr;
-	//		hash uint32;
-	//		alg uint8;
-	//		align uint8;
-	//		fieldAlign uint8;
-	//		kind uint8;
-	//		string *string;
-	//		*extraType;
-	//		ptrToThis *Type
+	//		size          uintptr
+	//		hash          uint32
+	//		_             uint8
+	//		align         uint8
+	//		fieldAlign    uint8
+	//		kind          uint8
+	//		alg           unsafe.Pointer
+	//		gc            unsafe.Pointer
+	//		string        *string
+	//		*extraType
+	//		ptrToThis     *Type
 	//	}
 	ot = duintptr(s, ot, t->width);
 	ot = duint32(s, ot, typehash(t));
@@ -579,6 +581,7 @@ dcommontype(Sym *s, int ot, Type *t)
 		ot = dsymptr(s, ot, algarray, alg*sizeofAlg);
 	else
 		ot = dsymptr(s, ot, algsym, 0);
+	ot = duintptr(s, ot, 0);  // gc
 	p = smprint("%-uT", t);
 	//print("dcommontype: %s\n", p);
 	ot = dgostringptr(s, ot, p);	// string

src/cmd/ld/dwarf.c

@@ -775,7 +775,7 @@ enum {
 	KindNoPointers = 1<<7,
 
 	// size of Type interface header + CommonType structure.
-	CommonSize = 2*PtrSize+ 5*PtrSize + 8,
+	CommonSize = 2*PtrSize+ 6*PtrSize + 8,
 };
 
 static Reloc*

src/pkg/reflect/type.go

@@ -244,6 +244,7 @@ type commonType struct {
 	fieldAlign    uint8        // alignment of struct field with this type
 	kind          uint8        // enumeration for C
 	alg           *uintptr     // algorithm table (../runtime/runtime.h:/Alg)
+	gc            uintptr      // garbage collection data
 	string        *string      // string form; unnecessary but undeniably useful
 	*uncommonType              // (relatively) uncommon fields
 	ptrToThis     *runtimeType // pointer to this type, if used in binary or has methods

src/pkg/runtime/type.go

@@ -21,7 +21,8 @@ type commonType struct {
 	align      uint8
 	fieldAlign uint8
 	kind       uint8
-	alg        *uintptr
+	alg        unsafe.Pointer
+	gc         unsafe.Pointer
 	string     *string
 	*uncommonType
 	ptrToThis *interface{}

src/pkg/runtime/type.h

@@ -28,6 +28,7 @@ struct CommonType
 	uint8 fieldAlign;
 	uint8 kind;
 	Alg *alg;
+	void *gc;
 	String *string;
 	UncommonType *x;
 	Type *ptrto;