go/gcimporter: update to latest copy from master

Backport of CL 41619.
Generated by copying bimport.go and reverting
the chunk containing the "This file is a copy"
comment near the top.

Fixes golang/go#20121

Change-Id: If24c97d01a550318ab919c37cd0c4a8f34d776c7
Reviewed-on: https://go-review.googlesource.com/41756
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

go/gcimporter15/bimport.go

@@ -21,14 +21,15 @@ import (
 )
 
 type importer struct {
-	imports map[string]*types.Package
+	imports    map[string]*types.Package
-	data    []byte
+	data       []byte
-	path    string
+	importpath string
-	buf     []byte // for reading strings
+	buf        []byte // for reading strings
-	version int    // export format version
+	version    int    // export format version
 
 	// object lists
 	strList       []string           // in order of appearance
+	pathList      []string           // in order of appearance
 	pkgList       []*types.Package   // in order of appearance
 	typList       []types.Type       // in order of appearance
 	interfaceList []*types.Interface // for delayed completion only
@@ -62,13 +63,14 @@ func BImportData(fset *token.FileSet, imports map[string]*types.Package, data []
 	}()
 
 	p := importer{
-		imports: imports,
+		imports:    imports,
-		data:    data,
+		data:       data,
-		path:    path,
+		importpath: path,
-		version: -1,           // unknown version
+		version:    -1,           // unknown version
-		strList: []string{""}, // empty string is mapped to 0
+		strList:    []string{""}, // empty string is mapped to 0
-		fset:    fset,
+		pathList:   []string{""}, // empty string is mapped to 0
-		files:   make(map[string]*token.File),
+		fset:       fset,
+		files:      make(map[string]*token.File),
 	}
 
 	// read version info
@@ -102,10 +104,10 @@ func BImportData(fset *token.FileSet, imports map[string]*types.Package, data []
 
 	// read version specific flags - extend as necessary
 	switch p.version {
-	// case 5:
+	// case 6:
 	// 	...
 	//	fallthrough
-	case 4, 3, 2, 1:
+	case 5, 4, 3, 2, 1:
 		p.debugFormat = p.rawStringln(p.rawByte()) == "debug"
 		p.trackAllTypes = p.int() != 0
 		p.posInfoFormat = p.int() != 0
@@ -171,12 +173,17 @@ func (p *importer) pkg() *types.Package {
 
 	// otherwise, i is the package tag (< 0)
 	if i != packageTag {
-		errorf("unexpected package tag %d", i)
+		errorf("unexpected package tag %d version %d", i, p.version)
 	}
 
 	// read package data
 	name := p.string()
-	path := p.string()
+	var path string
+	if p.version >= 5 {
+		path = p.path()
+	} else {
+		path = p.string()
+	}
 
 	// we should never see an empty package name
 	if name == "" {
@@ -191,7 +198,7 @@ func (p *importer) pkg() *types.Package {
 
 	// if the package was imported before, use that one; otherwise create a new one
 	if path == "" {
-		path = p.path
+		path = p.importpath
 	}
 	pkg := p.imports[path]
 	if pkg == nil {
@@ -285,6 +292,8 @@ func (p *importer) obj(tag int) {
 	}
 }
 
+const deltaNewFile = -64 // see cmd/compile/internal/gc/bexport.go
+
 func (p *importer) pos() token.Pos {
 	if !p.posInfoFormat {
 		return token.NoPos
@@ -292,15 +301,26 @@ func (p *importer) pos() token.Pos {
 
 	file := p.prevFile
 	line := p.prevLine
-	if delta := p.int(); delta != 0 {
+	delta := p.int()
-		// line changed
+	line += delta
-		line += delta
+	if p.version >= 5 {
-	} else if n := p.int(); n >= 0 {
+		if delta == deltaNewFile {
-		// file changed
+			if n := p.int(); n >= 0 {
-		file = p.prevFile[:n] + p.string()
+				// file changed
-		p.prevFile = file
+				file = p.path()
-		line = p.int()
+				line = n
+			}
+		}
+	} else {
+		if delta == 0 {
+			if n := p.int(); n >= 0 {
+				// file changed
+				file = p.prevFile[:n] + p.string()
+				line = p.int()
+			}
+		}
 	}
+	p.prevFile = file
 	p.prevLine = line
 
 	// Synthesize a token.Pos
@@ -778,6 +798,26 @@ func (p *importer) int64() int64 {
 	return p.rawInt64()
 }
 
+func (p *importer) path() string {
+	if p.debugFormat {
+		p.marker('p')
+	}
+	// if the path was seen before, i is its index (>= 0)
+	// (the empty string is at index 0)
+	i := p.rawInt64()
+	if i >= 0 {
+		return p.pathList[i]
+	}
+	// otherwise, i is the negative path length (< 0)
+	a := make([]string, -i)
+	for n := range a {
+		a[n] = p.string()
+	}
+	s := strings.Join(a, "/")
+	p.pathList = append(p.pathList, s)
+	return s
+}
+
 func (p *importer) string() string {
 	if p.debugFormat {
 		p.marker('s')