[dev.typeparams] cmd/compile: keep export format unchanged if no type params are exported

Added new export tags 'G' and 'U' to export parameterized
functions/methods and parameterized types respectively. This has the
advantage that the Go 1.18 format remains backward-compatible with the
Go 1.17 format if no type parameters are exported.

Change-Id: I9dba8faaa65609eb3f9c693bd0c79daee98bd865
Reviewed-on: https://go-review.googlesource.com/c/go/+/340989
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/importer/iimport.go

@@ -308,19 +308,18 @@ func (r *importReader) obj(name string) {
 
 		r.declare(types2.NewConst(pos, r.currPkg, name, typ, val))
 
-	case 'F':
+	case 'F', 'G':
 		var tparams []*types2.TypeName
-		if r.p.exportVersion >= iexportVersionGenerics {
+		if tag == 'G' {
 			tparams = r.tparamList()
 		}
 		sig := r.signature(nil)
 		sig.SetTParams(tparams)
-
 		r.declare(types2.NewFunc(pos, r.currPkg, name, sig))
 
-	case 'T':
+	case 'T', 'U':
 		var tparams []*types2.TypeName
-		if r.p.exportVersion >= iexportVersionGenerics {
+		if tag == 'U' {
 			tparams = r.tparamList()
 		}
 
@@ -328,7 +327,9 @@ func (r *importReader) obj(name string) {
 		// declaration before recursing.
 		obj := types2.NewTypeName(pos, r.currPkg, name, nil)
 		named := types2.NewNamed(obj, nil, nil)
-		named.SetTParams(tparams)
+		if tag == 'U' {
+			named.SetTParams(tparams)
+		}
 		r.declare(obj)
 
 		underlying := r.p.typAt(r.uint64(), named).Underlying()

src/cmd/compile/internal/typecheck/iexport.go

@@ -314,12 +314,7 @@ func WriteExports(out io.Writer, extensions bool) {
 	// Assemble header.
 	var hdr intWriter
 	hdr.WriteByte('i')
-	if base.Flag.G > 0 {
+	hdr.uint64(iexportVersionCurrent)
-		hdr.uint64(iexportVersionCurrent)
-	} else {
-		// Use old export format if doing -G=0 (no generics)
-		hdr.uint64(iexportVersionPosCol)
-	}
 	hdr.uint64(uint64(p.strings.Len()))
 	hdr.uint64(dataLen)
 
@@ -487,7 +482,11 @@ func (p *iexporter) doDecl(n *ir.Name) {
 			}
 
 			// Function.
-			w.tag('F')
+			if n.Type().TParams().NumFields() == 0 {
+				w.tag('F')
+			} else {
+				w.tag('G')
+			}
 			w.pos(n.Pos())
 			// The tparam list of the function type is the
 			// declaration of the type params. So, write out the type
@@ -495,7 +494,7 @@ func (p *iexporter) doDecl(n *ir.Name) {
 			// referenced via their type offset (via typOff) in all
 			// other places in the signature and function that they
 			// are used.
-			if base.Flag.G > 0 {
+			if n.Type().TParams().NumFields() > 0 {
 				w.tparamList(n.Type().TParams().FieldSlice())
 			}
 			w.signature(n.Type())
@@ -544,10 +543,14 @@ func (p *iexporter) doDecl(n *ir.Name) {
 		}
 
 		// Defined type.
-		w.tag('T')
+		if len(n.Type().RParams()) == 0 {
+			w.tag('T')
+		} else {
+			w.tag('U')
+		}
 		w.pos(n.Pos())
 
-		if base.Flag.G > 0 {
+		if len(n.Type().RParams()) > 0 {
 			// Export type parameters, if any, needed for this type
 			w.typeList(n.Type().RParams())
 		}

src/cmd/compile/internal/typecheck/iimport.go

@@ -305,9 +305,9 @@ func (r *importReader) doDecl(sym *types.Sym) *ir.Name {
 		r.constExt(n)
 		return n
 
-	case 'F':
+	case 'F', 'G':
 		var tparams []*types.Field
-		if r.p.exportVersion >= iexportVersionGenerics {
+		if tag == 'G' {
 			tparams = r.tparamList()
 		}
 		typ := r.signature(nil, tparams)
@@ -316,9 +316,9 @@ func (r *importReader) doDecl(sym *types.Sym) *ir.Name {
 		r.funcExt(n)
 		return n
 
-	case 'T':
+	case 'T', 'U':
 		var rparams []*types.Type
-		if r.p.exportVersion >= iexportVersionGenerics {
+		if tag == 'U' {
 			rparams = r.typeList()
 		}
 
@@ -326,7 +326,7 @@ func (r *importReader) doDecl(sym *types.Sym) *ir.Name {
 		// declaration before recursing.
 		n := importtype(pos, sym)
 		t := n.Type()
-		if rparams != nil {
+		if tag == 'U' {
 			t.SetRParams(rparams)
 		}
 

src/go/internal/gcimporter/iimport.go

@@ -290,24 +290,14 @@ func (r *importReader) obj(name string) {
 		r.declare(types.NewConst(pos, r.currPkg, name, typ, val))
 
 	case 'F':
-		if r.p.exportVersion >= iexportVersionGenerics {
-			numTparams := r.uint64()
-			if numTparams > 0 {
-				errorf("unexpected tparam")
-			}
-		}
 		sig := r.signature(nil)
 
 		r.declare(types.NewFunc(pos, r.currPkg, name, sig))
 
-	case 'T':
+	case 'G':
-		if r.p.exportVersion >= iexportVersionGenerics {
+		errorf("unexpected parameterized function/method")
-			numTparams := r.uint64()
-			if numTparams > 0 {
-				errorf("unexpected tparam")
-			}
-		}
 
+	case 'T':
 		// Types can be recursive. We need to setup a stub
 		// declaration before recursing.
 		obj := types.NewTypeName(pos, r.currPkg, name, nil)
@@ -328,6 +318,9 @@ func (r *importReader) obj(name string) {
 			}
 		}
 
+	case 'U':
+		errorf("unexpected parameterized type")
+
 	case 'V':
 		typ := r.typ()