cmd/cgo: readability improvements to generated _cgo_export.h

Also copy doc comments from Go code to _cgo_export.h.

This is a step toward installing this generated file when using
-buildmode=c-archive or c-shared, so that C code can #include it.

Change-Id: I3a243f7b386b58ec5c5ddb9a246bb9f9eddc5fb8
Reviewed-on: https://go-review.googlesource.com/9790
Reviewed-by: Minux Ma <minux@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>

src/cmd/cgo/ast.go

@@ -235,9 +235,17 @@ func (f *File) saveExport(x interface{}, context string) {
 			error_(c.Pos(), "export comment has wrong name %q, want %q", name, n.Name.Name)
 		}
 
+		doc := ""
+		for _, c1 := range n.Doc.List {
+			if c1 != c {
+				doc += c1.Text + "\n"
+			}
+		}
+
 		f.ExpFunc = append(f.ExpFunc, &ExpFunc{
 			Func:    n,
 			ExpName: name,
+			Doc:     doc,
 		})
 		break
 	}

src/cmd/cgo/doc.go

@@ -239,6 +239,9 @@ The following options are available when running cgo directly:
 		syscall package when bootstrapping a new target.
 	-objdir directory
 		Put all generated files in directory.
+	-importpath string
+		The import path for the Go package. Optional; used for
+		nicer comments in the generated files.
 	-gccgo
 		Generate output for the gccgo compiler rather than the
 		gc compiler.

src/cmd/cgo/main.go

@@ -99,6 +99,7 @@ func (n *Name) IsVar() bool {
 type ExpFunc struct {
 	Func    *ast.FuncDecl
 	ExpName string // name to use from C
+	Doc     string
 }
 
 // A TypeRepr contains the string representation of a type.
@@ -161,11 +162,13 @@ var dynout = flag.String("dynout", "", "write -dynimport output to this file")
 var dynpackage = flag.String("dynpackage", "main", "set Go package for -dynimport output")
 var dynlinker = flag.Bool("dynlinker", false, "record dynamic linker information in -dynimport mode")
 
-// These flags are for bootstrapping a new Go implementation,
+// This flag is for bootstrapping a new Go implementation,
 // to generate Go types that match the data layout and
 // constant values used in the host's C libraries and system calls.
 var godefs = flag.Bool("godefs", false, "for bootstrap: write Go definitions for C file to standard output")
+
 var objDir = flag.String("objdir", "", "object directory")
+var importPath = flag.String("importpath", "", "import path of package being built (for comments in generated files)")
 
 var gccgo = flag.Bool("gccgo", false, "generate files for use with gccgo")
 var gccgoprefix = flag.String("gccgoprefix", "", "-fgo-prefix option used with gccgo")

src/cmd/cgo/out.go

@@ -638,9 +638,7 @@ func (p *Package) writeExports(fgo2, fm io.Writer) {
 	fgcc := creat(*objDir + "_cgo_export.c")
 	fgcch := creat(*objDir + "_cgo_export.h")
 
-	fmt.Fprintf(fgcch, "/* Created by cgo - DO NOT EDIT. */\n/*  This file is arch-specific.  */\n")
+	p.writeExportHeader(fgcch)
-	fmt.Fprintf(fgcch, "%s\n", p.Preamble)
-	fmt.Fprintf(fgcch, "%s\n", p.gccExportHeaderProlog())
 
 	fmt.Fprintf(fgcc, "/* Created by cgo - DO NOT EDIT. */\n")
 	fmt.Fprintf(fgcc, "#include \"_cgo_export.h\"\n\n")
@@ -737,6 +735,10 @@ func (p *Package) writeExports(fgo2, fm io.Writer) {
 				s += fmt.Sprintf("%s p%d", p.cgoType(atype).C, i)
 			})
 		s += ")"
+
+		if len(exp.Doc) > 0 {
+			fmt.Fprintf(fgcch, "\n%s", exp.Doc)
+		}
 		fmt.Fprintf(fgcch, "\nextern %s;\n", s)
 
 		fmt.Fprintf(fgcc, "extern void _cgoexp%s_%s(void *, int);\n", cPrefix, exp.ExpName)
@@ -833,9 +835,7 @@ func (p *Package) writeGccgoExports(fgo2, fm io.Writer) {
 
 	gccgoSymbolPrefix := p.gccgoSymbolPrefix()
 
-	fmt.Fprintf(fgcch, "/* Created by cgo - DO NOT EDIT. */\n")
+	p.writeExportHeader(fgcch)
-	fmt.Fprintf(fgcch, "%s\n", p.Preamble)
-	fmt.Fprintf(fgcch, "%s\n", p.gccExportHeaderProlog())
 
 	fmt.Fprintf(fgcc, "/* Created by cgo - DO NOT EDIT. */\n")
 	fmt.Fprintf(fgcc, "#include \"_cgo_export.h\"\n")
@@ -861,6 +861,7 @@ func (p *Package) writeGccgoExports(fgo2, fm io.Writer) {
 				})
 		default:
 			// Declare a result struct.
+			fmt.Fprintf(fgcch, "\n/* Return type for %s */\n", exp.ExpName)
 			fmt.Fprintf(fgcch, "struct %s_result {\n", exp.ExpName)
 			forFieldList(fntype.Results,
 				func(i int, atype ast.Expr) {
@@ -890,6 +891,10 @@ func (p *Package) writeGccgoExports(fgo2, fm io.Writer) {
 		fmt.Fprintf(cdeclBuf, ")")
 		cParams := cdeclBuf.String()
 
+		if len(exp.Doc) > 0 {
+			fmt.Fprintf(fgcch, "\n%s", exp.Doc)
+		}
+
 		// We need to use a name that will be exported by the
 		// Go code; otherwise gccgo will make it static and we
 		// will not be able to link against it from the C
@@ -989,6 +994,22 @@ func (p *Package) writeGccgoExports(fgo2, fm io.Writer) {
 	}
 }
 
+// writeExportHeader writes out the start of the _cgo_export.h file.
+func (p *Package) writeExportHeader(fgcch io.Writer) {
+	fmt.Fprintf(fgcch, "/* Created by \"go tool cgo\" - DO NOT EDIT. */\n\n")
+	pkg := *importPath
+	if pkg == "" {
+		pkg = p.PackagePath
+	}
+	fmt.Fprintf(fgcch, "/* package %s */\n\n", pkg)
+
+	fmt.Fprintf(fgcch, "/* Start of preamble from import \"C\" comments.  */\n\n")
+	fmt.Fprintf(fgcch, "%s\n", p.Preamble)
+	fmt.Fprintf(fgcch, "\n/* End of preamble from import \"C\" comments.  */\n\n")
+
+	fmt.Fprintf(fgcch, "%s\n", p.gccExportHeaderProlog())
+}
+
 // Return the package prefix when using gccgo.
 func (p *Package) gccgoSymbolPrefix() string {
 	if !*gccgo {
@@ -1302,6 +1323,8 @@ func (p *Package) gccExportHeaderProlog() string {
 }
 
 const gccExportHeaderProlog = `
+/* Start of boilerplate cgo prologue.  */
+
 typedef signed char GoInt8;
 typedef unsigned char GoUint8;
 typedef short GoInt16;
@@ -1327,6 +1350,8 @@ typedef void *GoMap;
 typedef void *GoChan;
 typedef struct { void *t; void *v; } GoInterface;
 typedef struct { void *data; GoInt len; GoInt cap; } GoSlice;
+
+/* End of boilerplate cgo prologue.  */
 `
 
 // gccgoExportFileProlog is written to the _cgo_export.c file when