[dev.regabi] cmd/compile: always use a Field for ODOTPTR expressions

During walk, we create ODOTPTR expressions to access runtime struct
fields. But rather than using an actual Field for the selection, we
were just directly setting the ODOTPTR's Offset field.

This CL changes walk to create proper struct fields (albeit without
the rest of their enclosing struct type) and use them for creating the
ODOTPTR expressions.

Passes toolstash -cmp.

Change-Id: I08dbac3ed29141587feb0905d15adbcbcc4ca49e
Reviewed-on: https://go-review.googlesource.com/c/go/+/280432
Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>

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

@@ -318,15 +318,7 @@ func walkSwitchType(sw *ir.SwitchStmt) {
 	sw.Compiled.Append(ifNil)
 
 	// Load hash from type or itab.
-	dotHash := ir.NewSelectorExpr(base.Pos, ir.ODOTPTR, itab, nil)
-	dotHash.SetType(types.Types[types.TUINT32])
-	dotHash.SetTypecheck(1)
-	if s.facename.Type().IsEmptyInterface() {
-		dotHash.Offset = int64(2 * types.PtrSize) // offset of hash in runtime._type
-	} else {
-		dotHash.Offset = int64(2 * types.PtrSize) // offset of hash in runtime.itab
-	}
-	dotHash.SetBounded(true) // guaranteed not to fault
+	dotHash := typeHashFieldOf(base.Pos, itab)
 	s.hashname = copyExpr(dotHash, dotHash.Type(), &sw.Compiled)
 
 	br := ir.NewBranchStmt(base.Pos, ir.OBREAK, nil)
@@ -409,6 +401,32 @@ func walkSwitchType(sw *ir.SwitchStmt) {
 	walkStmtList(sw.Compiled)
 }
 
+// typeHashFieldOf returns an expression to select the type hash field
+// from an interface's descriptor word (whether a *runtime._type or
+// *runtime.itab pointer).
+func typeHashFieldOf(pos src.XPos, itab *ir.UnaryExpr) *ir.SelectorExpr {
+	if itab.Op() != ir.OITAB {
+		base.Fatalf("expected OITAB, got %v", itab.Op())
+	}
+	var hashField *types.Field
+	if itab.X.Type().IsEmptyInterface() {
+		// runtime._type's hash field
+		if rtypeHashField == nil {
+			rtypeHashField = runtimeField("hash", int64(2*types.PtrSize), types.Types[types.TUINT32])
+		}
+		hashField = rtypeHashField
+	} else {
+		// runtime.itab's hash field
+		if itabHashField == nil {
+			itabHashField = runtimeField("hash", int64(2*types.PtrSize), types.Types[types.TUINT32])
+		}
+		hashField = itabHashField
+	}
+	return boundedDotPtr(pos, itab, hashField)
+}
+
+var rtypeHashField, itabHashField *types.Field
+
 // A typeSwitch walks a type switch.
 type typeSwitch struct {
 	// Temporary variables (i.e., ONAMEs) used by type switch dispatch logic:

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

@@ -539,12 +539,31 @@ func calcHasCall(n ir.Node) bool {
 
 // itabType loads the _type field from a runtime.itab struct.
 func itabType(itab ir.Node) ir.Node {
-	typ := ir.NewSelectorExpr(base.Pos, ir.ODOTPTR, itab, nil)
-	typ.SetType(types.NewPtr(types.Types[types.TUINT8]))
-	typ.SetTypecheck(1)
-	typ.Offset = int64(types.PtrSize) // offset of _type in runtime.itab
-	typ.SetBounded(true)              // guaranteed not to fault
-	return typ
+	if itabTypeField == nil {
+		// runtime.itab's _type field
+		itabTypeField = runtimeField("_type", int64(types.PtrSize), types.NewPtr(types.Types[types.TUINT8]))
+	}
+	return boundedDotPtr(base.Pos, itab, itabTypeField)
+}
+
+var itabTypeField *types.Field
+
+// boundedDotPtr returns a selector expression representing ptr.field
+// and omits nil-pointer checks for ptr.
+func boundedDotPtr(pos src.XPos, ptr ir.Node, field *types.Field) *ir.SelectorExpr {
+	sel := ir.NewSelectorExpr(pos, ir.ODOTPTR, ptr, field.Sym)
+	sel.Selection = field
+	sel.Offset = field.Offset
+	sel.SetType(field.Type)
+	sel.SetTypecheck(1)
+	sel.SetBounded(true) // guaranteed not to fault
+	return sel
+}
+
+func runtimeField(name string, offset int64, typ *types.Type) *types.Field {
+	f := types.NewField(src.NoXPos, ir.Pkgs.Runtime.Lookup(name), typ)
+	f.Offset = offset
+	return f
 }
 
 // ifaceData loads the data field from an interface.