cmd/compile: remove rtype *uncommonType field

Instead of a pointer on every rtype, use a bit flag to indicate that
the contents of uncommonType directly follows the rtype value when it
is needed.

This requires a bit of juggling in the compiler's rtype encoder. The
backing arrays for fields in the rtype are presently encoded directly
after the slice header. This packing requires separating the encoding
of the uncommonType slice headers from their backing arrays.

Reduces binary size of godoc by ~180KB (1.5%).
No measurable change in all.bash time.
For #6853.

Change-Id: I60205948ceb5c0abba76fdf619652da9c465a597
Reviewed-on: https://go-review.googlesource.com/19790
Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/type.go

@@ -8,6 +8,11 @@ package runtime
 
 import "unsafe"
 
+// tflag is documented in ../reflect/type.go.
+type tflag uint8
+
+const tflagUncommon tflag = 1
+
 // Needs to be in sync with ../cmd/compile/internal/ld/decodesym.go:/^func.commonsize,
 // ../cmd/compile/internal/gc/reflect.go:/^func.dcommontype and
 // ../reflect/type.go:/^type.rtype.
@@ -15,7 +20,7 @@ type _type struct {
 	size       uintptr
 	ptrdata    uintptr // size of memory prefix holding all pointers
 	hash       uint32
-	_unused    uint8
+	tflag      tflag
 	align      uint8
 	fieldalign uint8
 	kind       uint8
@@ -25,7 +30,68 @@ type _type struct {
 	// Otherwise it is a ptrmask bitmap. See mbitmap.go for details.
 	gcdata  *byte
 	_string string
-	x       *uncommontype
+}
+
+func (t *_type) uncommon() *uncommontype {
+	if t.tflag&tflagUncommon == 0 {
+		return nil
+	}
+	switch t.kind & kindMask {
+	case kindStruct:
+		type u struct {
+			structtype
+			u uncommontype
+		}
+		return &(*u)(unsafe.Pointer(t)).u
+	case kindPtr:
+		type u struct {
+			ptrtype
+			u uncommontype
+		}
+		return &(*u)(unsafe.Pointer(t)).u
+	case kindFunc:
+		type u struct {
+			functype
+			u uncommontype
+		}
+		return &(*u)(unsafe.Pointer(t)).u
+	case kindSlice:
+		type u struct {
+			slicetype
+			u uncommontype
+		}
+		return &(*u)(unsafe.Pointer(t)).u
+	case kindArray:
+		type u struct {
+			arraytype
+			u uncommontype
+		}
+		return &(*u)(unsafe.Pointer(t)).u
+	case kindChan:
+		type u struct {
+			chantype
+			u uncommontype
+		}
+		return &(*u)(unsafe.Pointer(t)).u
+	case kindMap:
+		type u struct {
+			maptype
+			u uncommontype
+		}
+		return &(*u)(unsafe.Pointer(t)).u
+	case kindInterface:
+		type u struct {
+			interfacetype
+			u uncommontype
+		}
+		return &(*u)(unsafe.Pointer(t)).u
+	default:
+		type u struct {
+			_type
+			u uncommontype
+		}
+		return &(*u)(unsafe.Pointer(t)).u
+	}
 }
 
 func hasPrefix(s, prefix string) bool {