cmd/compile: use symbolic offsets of fields in internal/abi.ITab

After this CL, we can reorder or pad internal/abi.ITab fields at will
(keeping Fun last, and updating ITabTypeOff correctly) without
breaking anything.

Change-Id: Ib7bb5828519813e0d1aa36be5092f96fcd62b3be
Reviewed-on: https://go-review.googlesource.com/c/go/+/549516
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>

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

@@ -22,6 +22,7 @@ import (
 	"cmd/compile/internal/liveness"
 	"cmd/compile/internal/objw"
 	"cmd/compile/internal/reflectdata"
+	"cmd/compile/internal/rttype"
 	"cmd/compile/internal/ssa"
 	"cmd/compile/internal/staticdata"
 	"cmd/compile/internal/typecheck"
@@ -5537,7 +5538,7 @@ func (s *state) getClosureAndRcvr(fn *ir.SelectorExpr) (*ssa.Value, *ssa.Value)
 	i := s.expr(fn.X)
 	itab := s.newValue1(ssa.OpITab, types.Types[types.TUINTPTR], i)
 	s.nilCheck(itab)
-	itabidx := fn.Offset() + 2*int64(types.PtrSize) + 8 // offset of fun field in runtime.itab
+	itabidx := fn.Offset() + rttype.ITab.OffsetOf("Fun")
 	closure := s.newValue1I(ssa.OpOffPtr, s.f.Config.Types.UintptrPtr, itabidx, itab)
 	rcvr := s.newValue1(ssa.OpIData, s.f.Config.Types.BytePtr, i)
 	return closure, rcvr
@@ -6522,7 +6523,7 @@ func (s *state) dynamicDottype(n *ir.DynamicTypeAssertExpr, commaok bool) (res,
 		targetItab = s.expr(n.ITab)
 		// TODO(mdempsky): Investigate whether compiling n.RType could be
 		// better than loading itab.typ.
-		target = s.load(byteptr, s.newValue1I(ssa.OpOffPtr, byteptr, int64(types.PtrSize), targetItab)) // itab.typ
+		target = s.load(byteptr, s.newValue1I(ssa.OpOffPtr, byteptr, rttype.ITab.OffsetOf("Type"), targetItab))
 	} else {
 		target = s.expr(n.RType)
 	}
@@ -6580,7 +6581,7 @@ func (s *state) dottype1(pos src.XPos, src, dst *types.Type, iface, source, targ
 					return
 				}
 				// Load type out of itab, build interface with existing idata.
-				off := s.newValue1I(ssa.OpOffPtr, byteptr, int64(types.PtrSize), itab)
+				off := s.newValue1I(ssa.OpOffPtr, byteptr, rttype.ITab.OffsetOf("Type"), itab)
 				typ := s.load(byteptr, off)
 				idata := s.newValue1(ssa.OpIData, byteptr, iface)
 				res = s.newValue2(ssa.OpIMake, dst, typ, idata)
@@ -6590,7 +6591,7 @@ func (s *state) dottype1(pos src.XPos, src, dst *types.Type, iface, source, targ
 			s.startBlock(bOk)
 			// nonempty -> empty
 			// Need to load type from itab
-			off := s.newValue1I(ssa.OpOffPtr, byteptr, int64(types.PtrSize), itab)
+			off := s.newValue1I(ssa.OpOffPtr, byteptr, rttype.ITab.OffsetOf("Type"), itab)
 			s.vars[typVar] = s.load(byteptr, off)
 			s.endBlock()
 
@@ -6644,7 +6645,7 @@ func (s *state) dottype1(pos src.XPos, src, dst *types.Type, iface, source, targ
 		s.startBlock(bNonNil)
 		typ := itab
 		if !src.IsEmptyInterface() {
-			typ = s.load(byteptr, s.newValue1I(ssa.OpOffPtr, byteptr, int64(types.PtrSize), itab))
+			typ = s.load(byteptr, s.newValue1I(ssa.OpOffPtr, byteptr, rttype.ITab.OffsetOf("Type"), itab))
 		}
 
 		// Check the cache first.
@@ -6685,9 +6686,9 @@ func (s *state) dottype1(pos src.XPos, src, dst *types.Type, iface, source, targ
 				// Load hash from type or itab.
 				var hash *ssa.Value
 				if src.IsEmptyInterface() {
-					hash = s.newValue2(ssa.OpLoad, typs.UInt32, s.newValue1I(ssa.OpOffPtr, typs.UInt32Ptr, 2*s.config.PtrSize, typ), s.mem())
+					hash = s.newValue2(ssa.OpLoad, typs.UInt32, s.newValue1I(ssa.OpOffPtr, typs.UInt32Ptr, rttype.Type.OffsetOf("Hash"), typ), s.mem())
 				} else {
-					hash = s.newValue2(ssa.OpLoad, typs.UInt32, s.newValue1I(ssa.OpOffPtr, typs.UInt32Ptr, 2*s.config.PtrSize, itab), s.mem())
+					hash = s.newValue2(ssa.OpLoad, typs.UInt32, s.newValue1I(ssa.OpOffPtr, typs.UInt32Ptr, rttype.ITab.OffsetOf("Hash"), itab), s.mem())
 				}
 				hash = s.newValue1(zext, typs.Uintptr, hash)
 				s.vars[hashVar] = hash

src/cmd/compile/internal/walk/switch.go

@@ -700,7 +700,7 @@ func typeHashFieldOf(pos src.XPos, itab *ir.UnaryExpr) *ir.SelectorExpr {
 	} else {
 		// runtime.itab's hash field
 		if itabHashField == nil {
-			itabHashField = runtimeField("hash", int64(2*types.PtrSize), types.Types[types.TUINT32])
+			itabHashField = runtimeField("hash", rttype.ITab.OffsetOf("Hash"), types.Types[types.TUINT32])
 		}
 		hashField = itabHashField
 	}

src/cmd/compile/internal/walk/walk.go

@@ -10,6 +10,7 @@ import (
 	"cmd/compile/internal/base"
 	"cmd/compile/internal/ir"
 	"cmd/compile/internal/reflectdata"
+	"cmd/compile/internal/rttype"
 	"cmd/compile/internal/ssagen"
 	"cmd/compile/internal/typecheck"
 	"cmd/compile/internal/types"
@@ -345,8 +346,8 @@ func mayCall(n ir.Node) bool {
 // itabType loads the _type field from a runtime.itab struct.
 func itabType(itab ir.Node) ir.Node {
 	if itabTypeField == nil {
-		// runtime.itab's _type field
-		itabTypeField = runtimeField("_type", int64(types.PtrSize), types.NewPtr(types.Types[types.TUINT8]))
+		// internal/abi.ITab's Type field
+		itabTypeField = runtimeField("Type", rttype.ITab.OffsetOf("Type"), types.NewPtr(types.Types[types.TUINT8]))
 	}
 	return boundedDotPtr(base.Pos, itab, itabTypeField)
 }

src/internal/abi/iface.go

@@ -8,10 +8,7 @@ package abi
 // It records the underlying concrete type (Type), the interface type it
 // is implementing (Inter), and some ancillary information.
 //
-// layout of ITab known to compilers
 // allocated in non-garbage-collected memory
-// Needs to be in sync with
-// cmd/compile/internal/reflectdata/reflect.go:/^func.WritePluginTable.
 type ITab struct {
 	Inter *InterfaceType
 	Type  *Type