cmd/compile: move compiler-specific flags into compiler-spec. export data section

Also: Adjust go/importer accordingly.

Change-Id: Ia6669563793e218946af45b9fba1cf986a21c031
Reviewed-on: https://go-review.googlesource.com/21896Reviewed-by: Alan Donovan <adonovan@google.com>

src/cmd/compile/internal/gc/bexport.go

@@ -182,22 +182,12 @@ func export(out *bufio.Writer, trace bool) int {
 		Fatalf("exporter: local package path not empty: %q", localpkg.Path)
 	}
 	p.pkg(localpkg)
-
-	// write compiler-specific flags
-	// TODO(gri) move this into the compiler-specific export data section
-	{
-		var flags string
-		if safemode != 0 {
-			flags = "safe"
-		}
-		p.string(flags)
-	}
 	if p.trace {
 		p.tracef("\n")
 	}
 
 	// export objects
-
+	//
 	// First, export all exported (package-level) objects; i.e., all objects
 	// in the current exportlist. These objects represent all information
 	// required to import this package and type-check against it; i.e., this
@@ -270,6 +260,12 @@ func export(out *bufio.Writer, trace bool) int {
 		}
 	}
 
+	// write compiler-specific flags
+	p.bool(safemode != 0)
+	if p.trace {
+		p.tracef("\n")
+	}
+
 	// Phase 2: Export objects added to exportlist during phase 1.
 	// Don't use range since exportlist may grow during this phase
 	// and we want to export all remaining objects.

src/cmd/compile/internal/gc/bimport.go

@@ -62,9 +62,6 @@ func Import(in *bufio.Reader) {
 		Fatalf("importer: imported package not found in pkgList[0]")
 	}
 
-	// read compiler-specific flags
-	importpkg.Safe = p.string() == "safe"
-
 	// defer some type-checking until all types are read in completely
 	// (parser.go:import_package)
 	tcok := typecheckok
@@ -73,7 +70,7 @@ func Import(in *bufio.Reader) {
 
 	// read objects
 
-	// Phase 1
+	// phase 1
 	objcount := 0
 	for {
 		tag := p.tagOrIndex()
@@ -91,7 +88,10 @@ func Import(in *bufio.Reader) {
 
 	// --- compiler-specific export data ---
 
-	// Phase 2
+	// read compiler-specific flags
+	importpkg.Safe = p.bool()
+
+	// phase 2
 	objcount = 0
 	for {
 		tag := p.tagOrIndex()
@@ -264,7 +264,7 @@ func (p *importer) obj(tag int) {
 		}
 
 	default:
-		Fatalf("importer: unexpected object tag")
+		Fatalf("importer: unexpected object (tag = %d)", tag)
 	}
 }
 

src/go/internal/gcimporter/bimport.go

@@ -78,9 +78,6 @@ func BImportData(imports map[string]*types.Package, data []byte, path string) (i
 		panic("imported packaged not found in pkgList[0]")
 	}
 
-	// read compiler-specific flags
-	p.string() // discard
-
 	// read objects of phase 1 only (see cmd/compiler/internal/gc/bexport.go)
 	objcount := 0
 	for {
@@ -193,7 +190,7 @@ func (p *importer) obj(tag int) {
 		p.declare(types.NewFunc(token.NoPos, pkg, name, sig))
 
 	default:
-		panic("unexpected object tag")
+		panic(fmt.Sprintf("unexpected object tag %d", tag))
 	}
 }