cmd/compile: add package height to export data

A package's height is defined as the length of the longest import path
between itself and a leaf package (i.e., package with no imports).

We can't rely on knowing the path of the package being compiled, so
package height is useful for defining a package ordering.

Updates #24693.

Change-Id: I965162c440b6c5397db91b621ea0be7fa63881f1
Reviewed-on: https://go-review.googlesource.com/105038
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

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

@@ -135,13 +135,14 @@ import (
 const debugFormat = false // default: false
 
 // Current export format version. Increase with each format change.
+// 6: package height (CL 105038)
 // 5: improved position encoding efficiency (issue 20080, CL 41619)
 // 4: type name objects support type aliases, uses aliasTag
 // 3: Go1.8 encoding (same as version 2, aliasTag defined but never used)
 // 2: removed unused bool in ODCL export (compiler only)
 // 1: header format change (more regular), export package for _ struct fields
 // 0: Go1.7 encoding
-const exportVersion = 5
+const exportVersion = 6
 
 // exportInlined enables the export of inlined function bodies and related
 // dependencies. The compiler should work w/o any loss of functionality with
@@ -428,6 +429,7 @@ func (p *exporter) pkg(pkg *types.Pkg) {
 	p.tag(packageTag)
 	p.string(pkg.Name)
 	p.path(pkg.Path)
+	p.int(pkg.Height)
 }
 
 func unidealType(typ *types.Type, val Val) *types.Type {

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

@@ -96,10 +96,10 @@ func Import(imp *types.Pkg, in *bufio.Reader) {
 
 	// read version specific flags - extend as necessary
 	switch p.version {
-	// case 6:
+	// case 7:
 	// 	...
 	//	fallthrough
-	case 5, 4, 3, 2, 1:
+	case 6, 5, 4, 3, 2, 1:
 		p.debugFormat = p.rawStringln(p.rawByte()) == "debug"
 		p.trackAllTypes = p.bool()
 		p.posInfoFormat = p.bool()
@@ -281,6 +281,10 @@ func (p *importer) pkg() *types.Pkg {
 	} else {
 		path = p.string()
 	}
+	var height int
+	if p.version >= 6 {
+		height = p.int()
+	}
 
 	// we should never see an empty package name
 	if name == "" {
@@ -298,6 +302,18 @@ func (p *importer) pkg() *types.Pkg {
 		p.formatErrorf("package path %q for pkg index %d", path, len(p.pkgList))
 	}
 
+	if p.version >= 6 {
+		if height < 0 || height >= types.MaxPkgHeight {
+			p.formatErrorf("bad package height %v for package %s", height, name)
+		}
+
+		// reexported packages should always have a lower height than
+		// the main package
+		if len(p.pkgList) != 0 && height >= p.imp.Height {
+			p.formatErrorf("package %q (height %d) reexports package %q (height %d)", p.imp.Path, p.imp.Height, path, height)
+		}
+	}
+
 	// add package to pkgList
 	pkg := p.imp
 	if path != "" {
@@ -313,6 +329,7 @@ func (p *importer) pkg() *types.Pkg {
 		yyerror("import %q: package depends on %q (import cycle)", p.imp.Path, path)
 		errorexit()
 	}
+	pkg.Height = height
 	p.pkgList = append(p.pkgList, pkg)
 
 	return pkg

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

@@ -141,6 +141,11 @@ func Main(archInit func(*Arch)) {
 	localpkg = types.NewPkg("", "")
 	localpkg.Prefix = "\"\""
 
+	// We won't know localpkg's height until after import
+	// processing. In the mean time, set to MaxPkgHeight to ensure
+	// height comparisons at least work until then.
+	localpkg.Height = types.MaxPkgHeight
+
 	// pseudo-package, for scoping
 	builtinpkg = types.NewPkg("go.builtin", "") // TODO(gri) name this package go.builtin?
 	builtinpkg.Prefix = "go.builtin"            // not go%2ebuiltin
@@ -925,6 +930,10 @@ func loadsys() {
 	inimport = false
 }
 
+// myheight tracks the local package's height based on packages
+// imported so far.
+var myheight int
+
 func importfile(f *Val) *types.Pkg {
 	path_, ok := f.U.(string)
 	if !ok {
@@ -1117,6 +1126,10 @@ func importfile(f *Val) *types.Pkg {
 		errorexit()
 	}
 
+	if importpkg.Height >= myheight {
+		myheight = importpkg.Height + 1
+	}
+
 	return importpkg
 }
 

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

@@ -65,6 +65,8 @@ func parseFiles(filenames []string) uint {
 		testdclstack()
 	}
 
+	localpkg.Height = myheight
+
 	return lines
 }
 

src/cmd/compile/internal/types/pkg.go

@@ -15,14 +15,24 @@ import (
 // pkgMap maps a package path to a package.
 var pkgMap = make(map[string]*Pkg)
 
+// MaxPkgHeight is a height greater than any likely package height.
+const MaxPkgHeight = 1e9
+
 type Pkg struct {
-	Path     string // string literal used in import statement, e.g. "runtime/internal/sys"
-	Name     string // package name, e.g. "sys"
-	Pathsym  *obj.LSym
-	Prefix   string // escaped path for use in symbol table
-	Imported bool   // export data of this package was parsed
-	Direct   bool   // imported directly
-	Syms     map[string]*Sym
+	Path    string // string literal used in import statement, e.g. "runtime/internal/sys"
+	Name    string // package name, e.g. "sys"
+	Prefix  string // escaped path for use in symbol table
+	Syms    map[string]*Sym
+	Pathsym *obj.LSym
+
+	// Height is the package's height in the import graph. Leaf
+	// packages (i.e., packages with no imports) have height 0,
+	// and all other packages have height 1 plus the maximum
+	// height of their imported packages.
+	Height int
+
+	Imported bool // export data of this package was parsed
+	Direct   bool // imported directly
 }
 
 // NewPkg returns a new Pkg for the given package path and name.

src/go/internal/gcimporter/bimport.go

@@ -102,10 +102,10 @@ func BImportData(fset *token.FileSet, imports map[string]*types.Package, data []
 
 	// read version specific flags - extend as necessary
 	switch p.version {
-	// case 6:
+	// case 7:
 	// 	...
 	//	fallthrough
-	case 5, 4, 3, 2, 1:
+	case 6, 5, 4, 3, 2, 1:
 		p.debugFormat = p.rawStringln(p.rawByte()) == "debug"
 		p.trackAllTypes = p.int() != 0
 		p.posInfoFormat = p.int() != 0
@@ -182,6 +182,9 @@ func (p *importer) pkg() *types.Package {
 	} else {
 		path = p.string()
 	}
+	if p.version >= 6 {
+		p.int() // package height; unused by go/types
+	}
 
 	// we should never see an empty package name
 	if name == "" {