go_marshal: Implement automatic generation of ABI marshalling code.

This CL implements go_marshal, a code generation utility for
automatically serializing and deserializing ABI structs.

The go_marshal tool automatically generates implementations of the new
marshal interface. Unlike binary.Marshal/Unmarshal, the generated
interface implementations use no runtime reflection, and translates to
a single memcpy for most structs. See go_marshal/README.md for
details.

PiperOrigin-RevId: 268065475

5 files changed, 1447 insertions, 0 deletions

diff --git a/tools/go_marshal/gomarshal/BUILD b/tools/go_marshal/gomarshal/BUILD
new file mode 100644
index 000000000..a0eae6492
--- /dev/null
+++ b/tools/go_marshal/gomarshal/BUILD
@@ -0,0 +1,17 @@
+load("@io_bazel_rules_go//go:def.bzl", "go_library")
+
+package(licenses = ["notice"])
+
+go_library(
+    name = "gomarshal",
+    srcs = [
+        "generator.go",
+        "generator_interfaces.go",
+        "generator_tests.go",
+        "util.go",
+    ],
+    importpath = "gvisor.dev/gvisor/tools/go_marshal/gomarshal",
+    visibility = [
+        "//:sandbox",
+    ],
+)
diff --git a/tools/go_marshal/gomarshal/generator.go b/tools/go_marshal/gomarshal/generator.go
new file mode 100644
index 000000000..641ccd938
--- /dev/null
+++ b/tools/go_marshal/gomarshal/generator.go
@@ -0,0 +1,382 @@
+// Copyright 2019 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package gomarshal implements the go_marshal code generator. See README.md.
+package gomarshal
+
+import (
+	"bytes"
+	"fmt"
+	"go/ast"
+	"go/parser"
+	"go/token"
+	"os"
+	"sort"
+)
+
+const (
+	marshalImport  = "gvisor.dev/gvisor/tools/go_marshal/marshal"
+	usermemImport  = "gvisor.dev/gvisor/pkg/sentry/usermem"
+	safecopyImport = "gvisor.dev/gvisor/pkg/sentry/platform/safecopy"
+)
+
+// List of identifiers we use in generated code, that may conflict a
+// similarly-named source identifier. Avoid problems by refusing the generate
+// code when we see these.
+//
+// This only applies to import aliases at the moment. All other identifiers
+// are qualified by a receiver argument, since they're struct fields.
+//
+// All recievers are single letters, so we don't allow import aliases to be a
+// single letter.
+var badIdents = []string{
+	"src", "srcs", "dst", "dsts", "blk", "buf", "err",
+	// All single-letter identifiers.
+}
+
+// Generator drives code generation for a single invocation of the go_marshal
+// utility.
+//
+// The Generator holds arguments passed to the tool, and drives parsing,
+// processing and code Generator for all types marked with +marshal declared in
+// the input files.
+//
+// See Generator.run() as the entry point.
+type Generator struct {
+	// Paths to input go source files.
+	inputs []string
+	// Output file to write generated go source.
+	output *os.File
+	// Output file to write generated tests.
+	outputTest *os.File
+	// Package name for the generated file.
+	pkg string
+	// Go import path for package we're processing. This package should directly
+	// declare the type we're generating code for.
+	declaration string
+	// Set of extra packages to import in the generated file.
+	imports *importTable
+}
+
+// NewGenerator creates a new code Generator.
+func NewGenerator(srcs []string, out, outTest, pkg, declaration string, imports []string) (*Generator, error) {
+	f, err := os.OpenFile(out, os.O_CREATE|os.O_WRONLY|os.O_TRUNC, 0644)
+	if err != nil {
+		return nil, fmt.Errorf("Couldn't open output file %q: %v", out, err)
+	}
+	fTest, err := os.OpenFile(outTest, os.O_CREATE|os.O_WRONLY|os.O_TRUNC, 0644)
+	if err != nil {
+		return nil, fmt.Errorf("Couldn't open test output file %q: %v", out, err)
+	}
+	g := Generator{
+		inputs:      srcs,
+		output:      f,
+		outputTest:  fTest,
+		pkg:         pkg,
+		declaration: declaration,
+		imports:     newImportTable(),
+	}
+	for _, i := range imports {
+		// All imports on the extra imports list are unconditionally marked as
+		// used, so they're always added to the generated code.
+		g.imports.add(i).markUsed()
+	}
+	g.imports.add(marshalImport).markUsed()
+	// The follow imports may or may not be used by the generated
+	// code, depending what's required for the target types. Don't
+	// mark these imports as used by default.
+	g.imports.add(usermemImport)
+	g.imports.add(safecopyImport)
+	g.imports.add("unsafe")
+
+	return &g, nil
+}
+
+// writeHeader writes the header for the generated source file. The header
+// includes the package name, package level comments and import statements.
+func (g *Generator) writeHeader() error {
+	var b sourceBuffer
+	b.emit("// Automatically generated marshal implementation. See tools/go_marshal.\n\n")
+	b.emit("package %s\n\n", g.pkg)
+	if err := b.write(g.output); err != nil {
+		return err
+	}
+
+	return g.imports.write(g.output)
+}
+
+// writeTypeChecks writes a statement to force the compiler to perform a type
+// check for all Marshallable types referenced by the generated code.
+func (g *Generator) writeTypeChecks(ms map[string]struct{}) error {
+	if len(ms) == 0 {
+		return nil
+	}
+
+	msl := make([]string, 0, len(ms))
+	for m, _ := range ms {
+		msl = append(msl, m)
+	}
+	sort.Strings(msl)
+
+	var buf bytes.Buffer
+	fmt.Fprint(&buf, "// Marshallable types used by this file.\n")
+
+	for _, m := range msl {
+		fmt.Fprintf(&buf, "var _ marshal.Marshallable = (*%s)(nil)\n", m)
+	}
+	fmt.Fprint(&buf, "\n")
+
+	_, err := fmt.Fprint(g.output, buf.String())
+	return err
+}
+
+// parse processes all input files passed this generator and produces a set of
+// parsed go ASTs.
+func (g *Generator) parse() ([]*ast.File, []*token.FileSet, error) {
+	debugf("go_marshal invoked with %d input files:\n", len(g.inputs))
+	for _, path := range g.inputs {
+		debugf("  %s\n", path)
+	}
+
+	files := make([]*ast.File, 0, len(g.inputs))
+	fsets := make([]*token.FileSet, 0, len(g.inputs))
+
+	for _, path := range g.inputs {
+		fset := token.NewFileSet()
+		f, err := parser.ParseFile(fset, path, nil, parser.ParseComments)
+		if err != nil {
+			// Not a valid input file?
+			return nil, nil, fmt.Errorf("Input %q can't be parsed: %v", path, err)
+		}
+
+		if debugEnabled() {
+			debugf("AST for %q:\n", path)
+			ast.Print(fset, f)
+		}
+
+		files = append(files, f)
+		fsets = append(fsets, fset)
+	}
+
+	return files, fsets, nil
+}
+
+// collectMarshallabeTypes walks the parsed AST and collects a list of type
+// declarations for which we need to generate the Marshallable interface.
+func (g *Generator) collectMarshallabeTypes(a *ast.File, f *token.FileSet) []*ast.TypeSpec {
+	var types []*ast.TypeSpec
+	for _, decl := range a.Decls {
+		gdecl, ok := decl.(*ast.GenDecl)
+		// Type declaration?
+		if !ok || gdecl.Tok != token.TYPE {
+			debugfAt(f.Position(decl.Pos()), "Skipping declaration since it's not a type declaration.\n")
+			continue
+		}
+		// Does it have a comment?
+		if gdecl.Doc == nil {
+			debugfAt(f.Position(gdecl.Pos()), "Skipping declaration since it doesn't have a comment.\n")
+			continue
+		}
+		// Does the comment contain a "+marshal" line?
+		marked := false
+		for _, c := range gdecl.Doc.List {
+			if c.Text == "// +marshal" {
+				marked = true
+				break
+			}
+		}
+		if !marked {
+			debugfAt(f.Position(gdecl.Pos()), "Skipping declaration since it doesn't have a comment containing +marshal line.\n")
+			continue
+		}
+		for _, spec := range gdecl.Specs {
+			// We already confirmed we're in a type declaration earlier.
+			t := spec.(*ast.TypeSpec)
+			if _, ok := t.Type.(*ast.StructType); ok {
+				debugfAt(f.Position(t.Pos()), "Collected marshallable type %s.\n", t.Name.Name)
+				types = append(types, t)
+				continue
+			}
+			debugf("Skipping declaration %v since it's not a struct declaration.\n", gdecl)
+		}
+	}
+	return types
+}
+
+// collectImports collects all imports from all input source files. Some of
+// these imports are copied to the generated output, if they're referenced by
+// the generated code.
+//
+// collectImports de-duplicates imports while building the list, and ensures
+// identifiers in the generated code don't conflict with any imported package
+// names.
+func (g *Generator) collectImports(a *ast.File, f *token.FileSet) map[string]importStmt {
+	badImportNames := make(map[string]bool)
+	for _, i := range badIdents {
+		badImportNames[i] = true
+	}
+
+	is := make(map[string]importStmt)
+	for _, decl := range a.Decls {
+		gdecl, ok := decl.(*ast.GenDecl)
+		// Import statement?
+		if !ok || gdecl.Tok != token.IMPORT {
+			continue
+		}
+		for _, spec := range gdecl.Specs {
+			i := g.imports.addFromSpec(spec.(*ast.ImportSpec), f)
+			debugf("Collected import '%s' as '%s'\n", i.path, i.name)
+
+			// Make sure we have an import that doesn't use any local names that
+			// would conflict with identifiers in the generated code.
+			if len(i.name) == 1 {
+				abortAt(f.Position(spec.Pos()), fmt.Sprintf("Import has a single character local name '%s'; this may conflict with code generated by go_marshal, use a multi-character import alias", i.name))
+			}
+			if badImportNames[i.name] {
+				abortAt(f.Position(spec.Pos()), fmt.Sprintf("Import name '%s' is likely to conflict with code generated by go_marshal, use a different import alias", i.name))
+			}
+		}
+	}
+	return is
+
+}
+
+func (g *Generator) generateOne(t *ast.TypeSpec, fset *token.FileSet) *interfaceGenerator {
+	// We're guaranteed to have only struct type specs by now. See
+	// Generator.collectMarshallabeTypes.
+	i := newInterfaceGenerator(t, fset)
+	i.validate()
+	i.emitMarshallable()
+	return i
+}
+
+// generateOneTestSuite generates a test suite for the automatically generated
+// implementations type t.
+func (g *Generator) generateOneTestSuite(t *ast.TypeSpec) *testGenerator {
+	i := newTestGenerator(t, g.declaration)
+	i.emitTests()
+	return i
+}
+
+// Run is the entry point to code generation using g.
+//
+// Run parses all input source files specified in g and emits generated code.
+func (g *Generator) Run() error {
+	// Parse our input source files into ASTs and token sets.
+	asts, fsets, err := g.parse()
+	if err != nil {
+		return err
+	}
+
+	if len(asts) != len(fsets) {
+		panic("ASTs and FileSets don't match")
+	}
+
+	// Map of imports in source files; key = local package name, value = import
+	// path.
+	is := make(map[string]importStmt)
+	for i, a := range asts {
+		// Collect all imports from the source files. We may need to copy some
+		// of these to the generated code if they're referenced. This has to be
+		// done before the loop below because we need to process all ASTs before
+		// we start requesting imports to be copied one by one as we encounter
+		// them in each generated source.
+		for name, i := range g.collectImports(a, fsets[i]) {
+			is[name] = i
+		}
+	}
+
+	var impls []*interfaceGenerator
+	var ts []*testGenerator
+	// Set of Marshallable types referenced by generated code.
+	ms := make(map[string]struct{})
+	for i, a := range asts {
+		// Collect type declarations marked for code generation and generate
+		// Marshallable interfaces.
+		for _, t := range g.collectMarshallabeTypes(a, fsets[i]) {
+			impl := g.generateOne(t, fsets[i])
+			// Collect Marshallable types referenced by the generated code.
+			for ref, _ := range impl.ms {
+				ms[ref] = struct{}{}
+			}
+			impls = append(impls, impl)
+			// Collect imports referenced by the generated code and add them to
+			// the list of imports we need to copy to the generated code.
+			for name, _ := range impl.is {
+				if !g.imports.markUsed(name) {
+					panic(fmt.Sprintf("Generated code for '%s' referenced a non-existent import with local name '%s'", impl.typeName(), name))
+				}
+			}
+			ts = append(ts, g.generateOneTestSuite(t))
+		}
+	}
+
+	// Tool was invoked with input files with no data structures marked for code
+	// generation. This is probably not what the user intended.
+	if len(impls) == 0 {
+		var buf bytes.Buffer
+		fmt.Fprintf(&buf, "go_marshal invoked on these files, but they don't contain any types requiring code generation. Perhaps mark some with \"// +marshal\"?:\n")
+		for _, i := range g.inputs {
+			fmt.Fprintf(&buf, "  %s\n", i)
+		}
+		abort(buf.String())
+	}
+
+	// Write output file header. These include things like package name and
+	// import statements.
+	if err := g.writeHeader(); err != nil {
+		return err
+	}
+
+	// Write type checks for referenced marshallable types to output file.
+	if err := g.writeTypeChecks(ms); err != nil {
+		return err
+	}
+
+	// Write generated interfaces to output file.
+	for _, i := range impls {
+		if err := i.write(g.output); err != nil {
+			return err
+		}
+	}
+
+	// Write generated tests to test file.
+	return g.writeTests(ts)
+}
+
+// writeTests outputs tests for the generated interface implementations to a go
+// source file.
+func (g *Generator) writeTests(ts []*testGenerator) error {
+	var b sourceBuffer
+	b.emit("package %s_test\n\n", g.pkg)
+	if err := b.write(g.outputTest); err != nil {
+		return err
+	}
+
+	imports := newImportTable()
+	for _, t := range ts {
+		imports.merge(t.imports)
+	}
+
+	if err := imports.write(g.outputTest); err != nil {
+		return err
+	}
+
+	for _, t := range ts {
+		if err := t.write(g.outputTest); err != nil {
+			return err
+		}
+	}
+	return nil
+}
diff --git a/tools/go_marshal/gomarshal/generator_interfaces.go b/tools/go_marshal/gomarshal/generator_interfaces.go
new file mode 100644
index 000000000..a712c14dc
--- /dev/null
+++ b/tools/go_marshal/gomarshal/generator_interfaces.go
@@ -0,0 +1,507 @@
+// Copyright 2019 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package gomarshal
+
+import (
+	"fmt"
+	"go/ast"
+	"go/token"
+	"strings"
+)
+
+// interfaceGenerator generates marshalling interfaces for a single type.
+//
+// getState is not thread-safe.
+type interfaceGenerator struct {
+	sourceBuffer
+
+	// The type we're serializing.
+	t *ast.TypeSpec
+
+	// Receiver argument for generated methods.
+	r string
+
+	// FileSet containing the tokens for the type we're processing.
+	f *token.FileSet
+
+	// is records external packages referenced by the generated implementation.
+	is map[string]struct{}
+
+	// ms records Marshallable types referenced by the generated implementation
+	// of t's interfaces.
+	ms map[string]struct{}
+
+	// as records embedded fields in t that are potentially not packed. The key
+	// is the accessor for the field.
+	as map[string]struct{}
+}
+
+// typeName returns the name of the type this g represents.
+func (g *interfaceGenerator) typeName() string {
+	return g.t.Name.Name
+}
+
+// newinterfaceGenerator creates a new interface generator.
+func newInterfaceGenerator(t *ast.TypeSpec, fset *token.FileSet) *interfaceGenerator {
+	if _, ok := t.Type.(*ast.StructType); !ok {
+		panic(fmt.Sprintf("Attempting to generate code for a not struct type %v", t))
+	}
+	g := &interfaceGenerator{
+		t:  t,
+		r:  receiverName(t),
+		f:  fset,
+		is: make(map[string]struct{}),
+		ms: make(map[string]struct{}),
+		as: make(map[string]struct{}),
+	}
+	g.recordUsedMarshallable(g.typeName())
+	return g
+}
+
+func (g *interfaceGenerator) recordUsedMarshallable(m string) {
+	g.ms[m] = struct{}{}
+
+}
+
+func (g *interfaceGenerator) recordUsedImport(i string) {
+	g.is[i] = struct{}{}
+
+}
+
+func (g *interfaceGenerator) recordPotentiallyNonPackedField(fieldName string) {
+	g.as[fieldName] = struct{}{}
+}
+
+func (g *interfaceGenerator) forEachField(fn func(f *ast.Field)) {
+	// This is guaranteed to succeed because g.t is always a struct.
+	st := g.t.Type.(*ast.StructType)
+	for _, field := range st.Fields.List {
+		fn(field)
+	}
+}
+
+func (g *interfaceGenerator) fieldAccessor(n *ast.Ident) string {
+	return fmt.Sprintf("%s.%s", g.r, n.Name)
+}
+
+// abortAt aborts the go_marshal tool with the given error message, with a
+// reference position to the input source. Same as abortAt, but uses g to
+// resolve p to position.
+func (g *interfaceGenerator) abortAt(p token.Pos, msg string) {
+	abortAt(g.f.Position(p), msg)
+}
+
+// validate ensures the type we're working with can be marshalled. These checks
+// are done ahead of time and in one place so we can make assumptions later.
+func (g *interfaceGenerator) validate() {
+	g.forEachField(func(f *ast.Field) {
+		if len(f.Names) == 0 {
+			g.abortAt(f.Pos(), "Cannot marshal structs with embedded fields, give the field a name; use '_' for anonymous fields such as padding fields")
+		}
+	})
+
+	g.forEachField(func(f *ast.Field) {
+		fieldDispatcher{
+			primitive: func(_, t *ast.Ident) {
+				switch t.Name {
+				case "int8", "uint8", "byte", "int16", "uint16", "int32", "uint32", "int64", "uint64":
+					// These are the only primitive types we're allow. Below, we
+					// provide suggestions for some disallowed types and reject
+					// them, then attempt to marshal any remaining types by
+					// invoking the marshal.Marshallable interface on them. If
+					// these types don't actually implement
+					// marshal.Marshallable, compilation of the generated code
+					// will fail with an appropriate error message.
+					return
+				case "int":
+					g.abortAt(f.Pos(), "Type 'int' has ambiguous width, use int32 or int64")
+				case "uint":
+					g.abortAt(f.Pos(), "Type 'uint' has ambiguous width, use uint32 or uint64")
+				case "string":
+					g.abortAt(f.Pos(), "Type 'string' is dynamically-sized and cannot be marshalled, use a fixed size byte array '[...]byte' instead")
+				default:
+					debugfAt(g.f.Position(f.Pos()), fmt.Sprintf("Found derived type '%s', will attempt dispatch via marshal.Marshallable.\n", t.Name))
+				}
+			},
+			selector: func(_, _, _ *ast.Ident) {
+				// No validation to perform on selector fields. However this
+				// callback must still be provided.
+			},
+			array: func(n, _ *ast.Ident, len int) {
+				a := f.Type.(*ast.ArrayType)
+				if a.Len == nil {
+					g.abortAt(f.Pos(), fmt.Sprintf("Dynamically sized slice '%s' cannot be marshalled, arrays must be statically sized", n.Name))
+				}
+
+				if _, ok := a.Len.(*ast.BasicLit); !ok {
+					g.abortAt(a.Len.Pos(), fmt.Sprintf("Array size must be a literal, don's use consts or expressions"))
+				}
+
+				if _, ok := a.Elt.(*ast.Ident); !ok {
+					g.abortAt(a.Elt.Pos(), fmt.Sprintf("Marshalling not supported for arrays with %s elements, array elements must be primitive types", kindString(a.Elt)))
+				}
+
+				if len <= 0 {
+					g.abortAt(a.Len.Pos(), fmt.Sprintf("Marshalling not supported for zero length arrays, why does an ABI struct have one?"))
+				}
+			},
+			unhandled: func(_ *ast.Ident) {
+				g.abortAt(f.Pos(), fmt.Sprintf("Marshalling not supported for %s fields", kindString(f.Type)))
+			},
+		}.dispatch(f)
+	})
+}
+
+// scalarSize returns the size of type identified by t. If t isn't a primitive
+// type, the size isn't known at code generation time, and must be resolved via
+// the marshal.Marshallable interface.
+func (g *interfaceGenerator) scalarSize(t *ast.Ident) (size int, unknownSize bool) {
+	switch t.Name {
+	case "int8", "uint8", "byte":
+		return 1, false
+	case "int16", "uint16":
+		return 2, false
+	case "int32", "uint32":
+		return 4, false
+	case "int64", "uint64":
+		return 8, false
+	default:
+		return 0, true
+	}
+}
+
+func (g *interfaceGenerator) shift(bufVar string, n int) {
+	g.emit("%s = %s[%d:]\n", bufVar, bufVar, n)
+}
+
+func (g *interfaceGenerator) shiftDynamic(bufVar, name string) {
+	g.emit("%s = %s[%s.SizeBytes():]\n", bufVar, bufVar, name)
+}
+
+func (g *interfaceGenerator) marshalScalar(accessor, typ string, bufVar string) {
+	switch typ {
+	case "int8", "uint8", "byte":
+		g.emit("%s[0] = byte(%s)\n", bufVar, accessor)
+		g.shift(bufVar, 1)
+	case "int16", "uint16":
+		g.recordUsedImport("usermem")
+		g.emit("usermem.ByteOrder.PutUint16(%s[:2], uint16(%s))\n", bufVar, accessor)
+		g.shift(bufVar, 2)
+	case "int32", "uint32":
+		g.recordUsedImport("usermem")
+		g.emit("usermem.ByteOrder.PutUint32(%s[:4], uint32(%s))\n", bufVar, accessor)
+		g.shift(bufVar, 4)
+	case "int64", "uint64":
+		g.recordUsedImport("usermem")
+		g.emit("usermem.ByteOrder.PutUint64(%s[:8], uint64(%s))\n", bufVar, accessor)
+		g.shift(bufVar, 8)
+	default:
+		g.emit("%s.MarshalBytes(%s[:%s.SizeBytes()])\n", accessor, bufVar, accessor)
+		g.shiftDynamic(bufVar, accessor)
+	}
+}
+
+func (g *interfaceGenerator) unmarshalScalar(accessor, typ string, bufVar string) {
+	switch typ {
+	case "int8":
+		g.emit("%s = int8(%s[0])\n", accessor, bufVar)
+		g.shift(bufVar, 1)
+	case "uint8":
+		g.emit("%s = uint8(%s[0])\n", accessor, bufVar)
+		g.shift(bufVar, 1)
+	case "byte":
+		g.emit("%s = %s[0]\n", accessor, bufVar)
+		g.shift(bufVar, 1)
+
+	case "int16":
+		g.recordUsedImport("usermem")
+		g.emit("%s = int16(usermem.ByteOrder.Uint16(%s[:2]))\n", accessor, bufVar)
+		g.shift(bufVar, 2)
+	case "uint16":
+		g.recordUsedImport("usermem")
+		g.emit("%s = usermem.ByteOrder.Uint16(%s[:2])\n", accessor, bufVar)
+		g.shift(bufVar, 2)
+
+	case "int32":
+		g.recordUsedImport("usermem")
+		g.emit("%s = int32(usermem.ByteOrder.Uint32(%s[:4]))\n", accessor, bufVar)
+		g.shift(bufVar, 4)
+	case "uint32":
+		g.recordUsedImport("usermem")
+		g.emit("%s = usermem.ByteOrder.Uint32(%s[:4])\n", accessor, bufVar)
+		g.shift(bufVar, 4)
+
+	case "int64":
+		g.recordUsedImport("usermem")
+		g.emit("%s = int64(usermem.ByteOrder.Uint64(%s[:8]))\n", accessor, bufVar)
+		g.shift(bufVar, 8)
+	case "uint64":
+		g.recordUsedImport("usermem")
+		g.emit("%s = usermem.ByteOrder.Uint64(%s[:8])\n", accessor, bufVar)
+		g.shift(bufVar, 8)
+	default:
+		g.emit("%s.UnmarshalBytes(%s[:%s.SizeBytes()])\n", accessor, bufVar, accessor)
+		g.shiftDynamic(bufVar, accessor)
+		g.recordPotentiallyNonPackedField(accessor)
+	}
+}
+
+// areFieldsPackedExpression returns a go expression checking whether g.t's fields are
+// packed. Returns "", false if g.t has no fields that may be potentially
+// packed, otherwise returns <clause>, true, where <clause> is an expression
+// like "t.a.Packed() && t.b.Packed() && t.c.Packed()".
+func (g *interfaceGenerator) areFieldsPackedExpression() (string, bool) {
+	if len(g.as) == 0 {
+		return "", false
+	}
+
+	cs := make([]string, 0, len(g.as))
+	for accessor, _ := range g.as {
+		cs = append(cs, fmt.Sprintf("%s.Packed()", accessor))
+	}
+	return strings.Join(cs, " && "), true
+}
+
+func (g *interfaceGenerator) emitMarshallable() {
+	// Is g.t a packed struct without consideing field types?
+	thisPacked := true
+	g.forEachField(func(f *ast.Field) {
+		if f.Tag != nil {
+			if f.Tag.Value == "`marshal:\"unaligned\"`" {
+				if thisPacked {
+					debugfAt(g.f.Position(g.t.Pos()),
+						fmt.Sprintf("Marking type '%s' as not packed due to tag `marshal:\"unaligned\"`.\n", g.t.Name))
+					thisPacked = false
+				}
+			}
+		}
+	})
+
+	g.emit("// SizeBytes implements marshal.Marshallable.SizeBytes.\n")
+	g.emit("func (%s *%s) SizeBytes() int {\n", g.r, g.typeName())
+	g.inIndent(func() {
+		primitiveSize := 0
+		var dynamicSizeTerms []string
+
+		g.forEachField(fieldDispatcher{
+			primitive: func(n, t *ast.Ident) {
+				if size, dynamic := g.scalarSize(t); !dynamic {
+					primitiveSize += size
+				} else {
+					g.recordUsedMarshallable(t.Name)
+					dynamicSizeTerms = append(dynamicSizeTerms, fmt.Sprintf("%s.SizeBytes()", g.fieldAccessor(n)))
+				}
+			},
+			selector: func(n, tX, tSel *ast.Ident) {
+				tName := fmt.Sprintf("%s.%s", tX.Name, tSel.Name)
+				g.recordUsedImport(tX.Name)
+				g.recordUsedMarshallable(tName)
+				dynamicSizeTerms = append(dynamicSizeTerms, fmt.Sprintf("(*%s)(nil).SizeBytes()", tName))
+			},
+			array: func(n, t *ast.Ident, len int) {
+				if len < 1 {
+					// Zero-length arrays should've been rejected by validate().
+					panic("unreachable")
+				}
+				if size, dynamic := g.scalarSize(t); !dynamic {
+					primitiveSize += size * len
+				} else {
+					g.recordUsedMarshallable(t.Name)
+					dynamicSizeTerms = append(dynamicSizeTerms, fmt.Sprintf("(*%s)(nil).SizeBytes()*%d", t.Name, len))
+				}
+			},
+		}.dispatch)
+		g.emit("return %d", primitiveSize)
+		if len(dynamicSizeTerms) > 0 {
+			g.incIndent()
+		}
+		{
+			for _, d := range dynamicSizeTerms {
+				g.emitNoIndent(" +\n")
+				g.emit(d)
+			}
+		}
+		if len(dynamicSizeTerms) > 0 {
+			g.decIndent()
+		}
+	})
+	g.emit("\n}\n\n")
+
+	g.emit("// MarshalBytes implements marshal.Marshallable.MarshalBytes.\n")
+	g.emit("func (%s *%s) MarshalBytes(dst []byte) {\n", g.r, g.typeName())
+	g.inIndent(func() {
+		g.forEachField(fieldDispatcher{
+			primitive: func(n, t *ast.Ident) {
+				if n.Name == "_" {
+					g.emit("// Padding: dst[:sizeof(%s)] ~= %s(0)\n", t.Name, t.Name)
+					if len, dynamic := g.scalarSize(t); !dynamic {
+						g.shift("dst", len)
+					} else {
+						// We can't use shiftDynamic here because we don't have
+						// an instance of the dynamic type we can referece here
+						// (since the version in this struct is anonymous). Use
+						// a typed nil pointer to call SizeBytes() instead.
+						g.emit("dst = dst[(*%s)(nil).SizeBytes():]\n", t.Name)
+					}
+					return
+				}
+				g.marshalScalar(g.fieldAccessor(n), t.Name, "dst")
+			},
+			selector: func(n, tX, tSel *ast.Ident) {
+				g.marshalScalar(g.fieldAccessor(n), fmt.Sprintf("%s.%s", tX.Name, tSel.Name), "dst")
+			},
+			array: func(n, t *ast.Ident, size int) {
+				if n.Name == "_" {
+					g.emit("// Padding: dst[:sizeof(%s)*%d] ~= [%d]%s{0}\n", t.Name, size, size, t.Name)
+					if len, dynamic := g.scalarSize(t); !dynamic {
+						g.shift("dst", len*size)
+					} else {
+						// We can't use shiftDynamic here because we don't have
+						// an instance of the dynamic type we can reference here
+						// (since the version in this struct is anonymous). Use
+						// a typed nil pointer to call SizeBytes() instead.
+						g.emit("dst = dst[(*%s)(nil).SizeBytes()*%d:]\n", t.Name, size)
+					}
+					return
+				}
+
+				g.emit("for i := 0; i < %d; i++ {\n", size)
+				g.inIndent(func() {
+					g.marshalScalar(fmt.Sprintf("%s[i]", g.fieldAccessor(n)), t.Name, "dst")
+				})
+				g.emit("}\n")
+			},
+		}.dispatch)
+	})
+	g.emit("}\n\n")
+
+	g.emit("// UnmarshalBytes implements marshal.Marshallable.UnmarshalBytes.\n")
+	g.emit("func (%s *%s) UnmarshalBytes(src []byte) {\n", g.r, g.typeName())
+	g.inIndent(func() {
+		g.forEachField(fieldDispatcher{
+			primitive: func(n, t *ast.Ident) {
+				if n.Name == "_" {
+					g.emit("// Padding: var _ %s ~= src[:sizeof(%s)]\n", t.Name, t.Name)
+					if len, dynamic := g.scalarSize(t); !dynamic {
+						g.shift("src", len)
+					} else {
+						// We can't use shiftDynamic here because we don't have
+						// an instance of the dynamic type we can reference here
+						// (since the version in this struct is anonymous). Use
+						// a typed nil pointer to call SizeBytes() instead.
+						g.emit("src = src[(*%s)(nil).SizeBytes():]\n", t.Name)
+						g.recordPotentiallyNonPackedField(fmt.Sprintf("(*%s)(nil)", t.Name))
+					}
+					return
+				}
+				g.unmarshalScalar(g.fieldAccessor(n), t.Name, "src")
+			},
+			selector: func(n, tX, tSel *ast.Ident) {
+				g.unmarshalScalar(g.fieldAccessor(n), fmt.Sprintf("%s.%s", tX.Name, tSel.Name), "src")
+			},
+			array: func(n, t *ast.Ident, size int) {
+				if n.Name == "_" {
+					g.emit("// Padding: ~ copy([%d]%s(%s), src[:sizeof(%s)*%d])\n", size, t.Name, g.fieldAccessor(n), t.Name, size)
+					if len, dynamic := g.scalarSize(t); !dynamic {
+						g.shift("src", len*size)
+					} else {
+						// We can't use shiftDynamic here because we don't have
+						// an instance of the dynamic type we can referece here
+						// (since the version in this struct is anonymous). Use
+						// a typed nil pointer to call SizeBytes() instead.
+						g.emit("src = src[(*%s)(nil).SizeBytes()*%d:]\n", t.Name, size)
+					}
+					return
+				}
+
+				g.emit("for i := 0; i < %d; i++ {\n", size)
+				g.inIndent(func() {
+					g.unmarshalScalar(fmt.Sprintf("%s[i]", g.fieldAccessor(n)), t.Name, "src")
+				})
+				g.emit("}\n")
+			},
+		}.dispatch)
+	})
+	g.emit("}\n\n")
+
+	g.emit("// Packed implements marshal.Marshallable.Packed.\n")
+	g.emit("func (%s *%s) Packed() bool {\n", g.r, g.typeName())
+	g.inIndent(func() {
+		expr, fieldsMaybePacked := g.areFieldsPackedExpression()
+		switch {
+		case !thisPacked:
+			g.emit("return false\n")
+		case fieldsMaybePacked:
+			g.emit("return %s\n", expr)
+		default:
+			g.emit("return true\n")
+
+		}
+	})
+	g.emit("}\n\n")
+
+	g.emit("// MarshalUnsafe implements marshal.Marshallable.MarshalUnsafe.\n")
+	g.emit("func (%s *%s) MarshalUnsafe(dst []byte) {\n", g.r, g.typeName())
+	g.inIndent(func() {
+		if thisPacked {
+			g.recordUsedImport("safecopy")
+			g.recordUsedImport("unsafe")
+			if cond, ok := g.areFieldsPackedExpression(); ok {
+				g.emit("if %s {\n", cond)
+				g.inIndent(func() {
+					g.emit("safecopy.CopyIn(dst, unsafe.Pointer(%s))\n", g.r)
+				})
+				g.emit("} else {\n")
+				g.inIndent(func() {
+					g.emit("%s.MarshalBytes(dst)\n", g.r)
+				})
+				g.emit("}\n")
+			} else {
+				g.emit("safecopy.CopyIn(dst, unsafe.Pointer(%s))\n", g.r)
+			}
+		} else {
+			g.emit("// Type %s doesn't have a packed layout in memory, fallback to MarshalBytes.\n", g.typeName())
+			g.emit("%s.MarshalBytes(dst)\n", g.r)
+		}
+	})
+	g.emit("}\n\n")
+
+	g.emit("// UnmarshalUnsafe implements marshal.Marshallable.UnmarshalUnsafe.\n")
+	g.emit("func (%s *%s) UnmarshalUnsafe(src []byte) {\n", g.r, g.typeName())
+	g.inIndent(func() {
+		if thisPacked {
+			g.recordUsedImport("safecopy")
+			g.recordUsedImport("unsafe")
+			if cond, ok := g.areFieldsPackedExpression(); ok {
+				g.emit("if %s {\n", cond)
+				g.inIndent(func() {
+					g.emit("safecopy.CopyOut(unsafe.Pointer(%s), src)\n", g.r)
+				})
+				g.emit("} else {\n")
+				g.inIndent(func() {
+					g.emit("%s.UnmarshalBytes(src)\n", g.r)
+				})
+				g.emit("}\n")
+			} else {
+				g.emit("safecopy.CopyOut(unsafe.Pointer(%s), src)\n", g.r)
+			}
+		} else {
+			g.emit("// Type %s doesn't have a packed layout in memory, fall back to UnmarshalBytes.\n", g.typeName())
+			g.emit("%s.UnmarshalBytes(src)\n", g.r)
+		}
+	})
+	g.emit("}\n\n")
+
+}
diff --git a/tools/go_marshal/gomarshal/generator_tests.go b/tools/go_marshal/gomarshal/generator_tests.go
new file mode 100644
index 000000000..df25cb5b2
--- /dev/null
+++ b/tools/go_marshal/gomarshal/generator_tests.go
@@ -0,0 +1,154 @@
+// Copyright 2019 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package gomarshal
+
+import (
+	"fmt"
+	"go/ast"
+	"io"
+	"strings"
+)
+
+var standardImports = []string{
+	"fmt",
+	"reflect",
+	"testing",
+	"gvisor.dev/gvisor/tools/go_marshal/analysis",
+}
+
+type testGenerator struct {
+	sourceBuffer
+
+	// The type we're serializing.
+	t *ast.TypeSpec
+
+	// Receiver argument for generated methods.
+	r string
+
+	// Imports used by generated code.
+	imports *importTable
+
+	// Import statement for the package declaring the type we generated code
+	// for. We need this to construct test instances for the type, since the
+	// tests aren't written in the same package.
+	decl *importStmt
+}
+
+func newTestGenerator(t *ast.TypeSpec, declaration string) *testGenerator {
+	if _, ok := t.Type.(*ast.StructType); !ok {
+		panic(fmt.Sprintf("Attempting to generate code for a not struct type %v", t))
+	}
+	g := &testGenerator{
+		t:       t,
+		r:       receiverName(t),
+		imports: newImportTable(),
+	}
+
+	for _, i := range standardImports {
+		g.imports.add(i).markUsed()
+	}
+	g.decl = g.imports.add(declaration)
+	g.decl.markUsed()
+
+	return g
+}
+
+func (g *testGenerator) typeName() string {
+	return fmt.Sprintf("%s.%s", g.decl.name, g.t.Name.Name)
+}
+
+func (g *testGenerator) forEachField(fn func(f *ast.Field)) {
+	// This is guaranteed to succeed because g.t is always a struct.
+	st := g.t.Type.(*ast.StructType)
+	for _, field := range st.Fields.List {
+		fn(field)
+	}
+}
+
+func (g *testGenerator) testFuncName(base string) string {
+	return fmt.Sprintf("%s%s", base, strings.Title(g.t.Name.Name))
+}
+
+func (g *testGenerator) inTestFunction(name string, body func()) {
+	g.emit("func %s(t *testing.T) {\n", g.testFuncName(name))
+	g.inIndent(body)
+	g.emit("}\n\n")
+}
+
+func (g *testGenerator) emitTestNonZeroSize() {
+	g.inTestFunction("TestSizeNonZero", func() {
+		g.emit("x := &%s{}\n", g.typeName())
+		g.emit("if x.SizeBytes() == 0 {\n")
+		g.inIndent(func() {
+			g.emit("t.Fatal(\"Marshallable.Size() should not return zero\")\n")
+		})
+		g.emit("}\n")
+	})
+}
+
+func (g *testGenerator) emitTestSuspectAlignment() {
+	g.inTestFunction("TestSuspectAlignment", func() {
+		g.emit("x := %s{}\n", g.typeName())
+		g.emit("analysis.AlignmentCheck(t, reflect.TypeOf(x))\n")
+	})
+}
+
+func (g *testGenerator) emitTestMarshalUnmarshalPreservesData() {
+	g.inTestFunction("TestSafeMarshalUnmarshalPreservesData", func() {
+		g.emit("var x, y, z, yUnsafe, zUnsafe %s\n", g.typeName())
+		g.emit("analysis.RandomizeValue(&x)\n\n")
+
+		g.emit("buf := make([]byte, x.SizeBytes())\n")
+		g.emit("x.MarshalBytes(buf)\n")
+		g.emit("bufUnsafe := make([]byte, x.SizeBytes())\n")
+		g.emit("x.MarshalUnsafe(bufUnsafe)\n\n")
+
+		g.emit("y.UnmarshalBytes(buf)\n")
+		g.emit("if !reflect.DeepEqual(x, y) {\n")
+		g.inIndent(func() {
+			g.emit("t.Fatal(fmt.Sprintf(\"Data corrupted across Marshal/Unmarshal cycle:\\nBefore: %%+v\\nAfter: %%+v\\n\", x, y))\n")
+		})
+		g.emit("}\n")
+		g.emit("yUnsafe.UnmarshalBytes(bufUnsafe)\n")
+		g.emit("if !reflect.DeepEqual(x, yUnsafe) {\n")
+		g.inIndent(func() {
+			g.emit("t.Fatal(fmt.Sprintf(\"Data corrupted across MarshalUnsafe/Unmarshal cycle:\\nBefore: %%+v\\nAfter: %%+v\\n\", x, yUnsafe))\n")
+		})
+		g.emit("}\n\n")
+
+		g.emit("z.UnmarshalUnsafe(buf)\n")
+		g.emit("if !reflect.DeepEqual(x, z) {\n")
+		g.inIndent(func() {
+			g.emit("t.Fatal(fmt.Sprintf(\"Data corrupted across Marshal/UnmarshalUnsafe cycle:\\nBefore: %%+v\\nAfter: %%+v\\n\", x, z))\n")
+		})
+		g.emit("}\n")
+		g.emit("zUnsafe.UnmarshalUnsafe(bufUnsafe)\n")
+		g.emit("if !reflect.DeepEqual(x, zUnsafe) {\n")
+		g.inIndent(func() {
+			g.emit("t.Fatal(fmt.Sprintf(\"Data corrupted across MarshalUnsafe/UnmarshalUnsafe cycle:\\nBefore: %%+v\\nAfter: %%+v\\n\", x, zUnsafe))\n")
+		})
+		g.emit("}\n")
+	})
+}
+
+func (g *testGenerator) emitTests() {
+	g.emitTestNonZeroSize()
+	g.emitTestSuspectAlignment()
+	g.emitTestMarshalUnmarshalPreservesData()
+}
+
+func (g *testGenerator) write(out io.Writer) error {
+	return g.sourceBuffer.write(out)
+}
diff --git a/tools/go_marshal/gomarshal/util.go b/tools/go_marshal/gomarshal/util.go
new file mode 100644
index 000000000..967537abf
--- /dev/null
+++ b/tools/go_marshal/gomarshal/util.go
@@ -0,0 +1,387 @@
+// Copyright 2019 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package gomarshal
+
+import (
+	"bytes"
+	"flag"
+	"fmt"
+	"go/ast"
+	"go/token"
+	"io"
+	"os"
+	"path"
+	"reflect"
+	"sort"
+	"strconv"
+	"strings"
+)
+
+var debug = flag.Bool("debug", false, "enables debugging output")
+
+// receiverName returns an appropriate receiver name given a type spec.
+func receiverName(t *ast.TypeSpec) string {
+	if len(t.Name.Name) < 1 {
+		// Zero length type name?
+		panic("unreachable")
+	}
+	return strings.ToLower(t.Name.Name[:1])
+}
+
+// kindString returns a user-friendly representation of an AST expr type.
+func kindString(e ast.Expr) string {
+	switch e.(type) {
+	case *ast.Ident:
+		return "scalar"
+	case *ast.ArrayType:
+		return "array"
+	case *ast.StructType:
+		return "struct"
+	case *ast.StarExpr:
+		return "pointer"
+	case *ast.FuncType:
+		return "function"
+	case *ast.InterfaceType:
+		return "interface"
+	case *ast.MapType:
+		return "map"
+	case *ast.ChanType:
+		return "channel"
+	default:
+		return reflect.TypeOf(e).String()
+	}
+}
+
+// fieldDispatcher is a collection of callbacks for handling different types of
+// fields in a struct declaration.
+type fieldDispatcher struct {
+	primitive func(n, t *ast.Ident)
+	selector  func(n, tX, tSel *ast.Ident)
+	array     func(n, t *ast.Ident, size int)
+	unhandled func(n *ast.Ident)
+}
+
+// Precondition: All dispatch callbacks that will be invoked must be
+// provided. Embedded fields are not allowed, len(f.Names) >= 1.
+func (fd fieldDispatcher) dispatch(f *ast.Field) {
+	// Each field declaration may actually be multiple declarations of the same
+	// type. For example, consider:
+	//
+	// type Point struct {
+	//     x, y, z int
+	// }
+	//
+	// We invoke the call-backs once per such instance. Embedded fields are not
+	// allowed, and results in a panic.
+	if len(f.Names) < 1 {
+		panic("Precondition not met: attempted to dispatch on embedded field")
+	}
+
+	for _, name := range f.Names {
+		switch v := f.Type.(type) {
+		case *ast.Ident:
+			fd.primitive(name, v)
+		case *ast.SelectorExpr:
+			fd.selector(name, v.X.(*ast.Ident), v.Sel)
+		case *ast.ArrayType:
+			len := 0
+			if v.Len != nil {
+				// Non-literal array length is handled by generatorInterfaces.validate().
+				if lenLit, ok := v.Len.(*ast.BasicLit); ok {
+					var err error
+					len, err = strconv.Atoi(lenLit.Value)
+					if err != nil {
+						panic(err)
+					}
+				}
+			}
+			switch t := v.Elt.(type) {
+			case *ast.Ident:
+				fd.array(name, t, len)
+			default:
+				fd.array(name, nil, len)
+			}
+		default:
+			fd.unhandled(name)
+		}
+	}
+}
+
+// debugEnabled indicates whether debugging is enabled for gomarshal.
+func debugEnabled() bool {
+	return *debug
+}
+
+// abort aborts the go_marshal tool with the given error message.
+func abort(msg string) {
+	if !strings.HasSuffix(msg, "\n") {
+		msg += "\n"
+	}
+	fmt.Print(msg)
+	os.Exit(1)
+}
+
+// abortAt aborts the go_marshal tool with the given error message, with
+// a reference position to the input source.
+func abortAt(p token.Position, msg string) {
+	abort(fmt.Sprintf("%v:\n  %s\n", p, msg))
+}
+
+// debugf conditionally prints a debug message.
+func debugf(f string, a ...interface{}) {
+	if debugEnabled() {
+		fmt.Printf(f, a...)
+	}
+}
+
+// debugfAt conditionally prints a debug message with a reference to a position
+// in the input source.
+func debugfAt(p token.Position, f string, a ...interface{}) {
+	if debugEnabled() {
+		fmt.Printf("%s:\n  %s", p, fmt.Sprintf(f, a...))
+	}
+}
+
+// emit generates a line of code in the output file.
+//
+// emit is a wrapper around writing a formatted string to the output
+// buffer. emit can be invoked in one of two ways:
+//
+// (1) emit("some string")
+//     When emit is called with a single string argument, it is simply copied to
+//     the output buffer without any further formatting.
+// (2) emit(fmtString, args...)
+//     emit can also be invoked in a similar fashion to *Printf() functions,
+//     where the first argument is a format string.
+//
+// Calling emit with a single argument that is not a string will result in a
+// panic, as the caller's intent is ambiguous.
+func emit(out io.Writer, indent int, a ...interface{}) {
+	const spacesPerIndentLevel = 4
+
+	if len(a) < 1 {
+		panic("emit() called with no arguments")
+	}
+
+	if indent > 0 {
+		if _, err := fmt.Fprint(out, strings.Repeat(" ", indent*spacesPerIndentLevel)); err != nil {
+			// Writing to the emit output should not fail. Typically the output
+			// is a byte.Buffer; writes to these never fail.
+			panic(err)
+		}
+	}
+
+	first, ok := a[0].(string)
+	if !ok {
+		// First argument must be either the string to emit (case 1 from
+		// function-level comment), or a format string (case 2).
+		panic(fmt.Sprintf("First argument to emit() is not a string: %+v", a[0]))
+	}
+
+	if len(a) == 1 {
+		// Single string argument. Assume no formatting requested.
+		if _, err := fmt.Fprint(out, first); err != nil {
+			// Writing to out should not fail.
+			panic(err)
+		}
+		return
+
+	}
+
+	// Formatting requested.
+	if _, err := fmt.Fprintf(out, first, a[1:]...); err != nil {
+		// Writing to out should not fail.
+		panic(err)
+	}
+}
+
+// sourceBuffer represents fragments of generated go source code.
+//
+// sourceBuffer provides a convenient way to build up go souce fragments in
+// memory. May be safely zero-value initialized. Not thread-safe.
+type sourceBuffer struct {
+	// Current indentation level.
+	indent int
+
+	// Memory buffer containing contents while they're being generated.
+	b bytes.Buffer
+}
+
+func (b *sourceBuffer) incIndent() {
+	b.indent++
+}
+
+func (b *sourceBuffer) decIndent() {
+	if b.indent <= 0 {
+		panic("decIndent() without matching incIndent()")
+	}
+	b.indent--
+}
+
+func (b *sourceBuffer) emit(a ...interface{}) {
+	emit(&b.b, b.indent, a...)
+}
+
+func (b *sourceBuffer) emitNoIndent(a ...interface{}) {
+	emit(&b.b, 0 /*indent*/, a...)
+}
+
+func (b *sourceBuffer) inIndent(body func()) {
+	b.incIndent()
+	body()
+	b.decIndent()
+}
+
+func (b *sourceBuffer) write(out io.Writer) error {
+	_, err := fmt.Fprint(out, b.b.String())
+	return err
+}
+
+// Write implements io.Writer.Write.
+func (b *sourceBuffer) Write(buf []byte) (int, error) {
+	return (b.b.Write(buf))
+}
+
+// importStmt represents a single import statement.
+type importStmt struct {
+	// Local name of the imported package.
+	name string
+	// Import path.
+	path string
+	// Indicates whether the local name is an alias, or simply the final
+	// component of the path.
+	aliased bool
+	// Indicates whether this import was referenced by generated code.
+	used bool
+}
+
+func newImport(p string) *importStmt {
+	name := path.Base(p)
+	return &importStmt{
+		name:    name,
+		path:    p,
+		aliased: false,
+	}
+}
+
+func newImportFromSpec(spec *ast.ImportSpec, f *token.FileSet) *importStmt {
+	p := spec.Path.Value[1 : len(spec.Path.Value)-1] // Strip the " quotes around path.
+	name := path.Base(p)
+	if name == "" || name == "/" || name == "." {
+		panic(fmt.Sprintf("Couldn't process local package name for import at %s, (processed as %s)",
+			f.Position(spec.Path.Pos()), name))
+	}
+	if spec.Name != nil {
+		name = spec.Name.Name
+	}
+	return &importStmt{
+		name:    name,
+		path:    p,
+		aliased: spec.Name != nil,
+	}
+}
+
+func (i *importStmt) String() string {
+	if i.aliased {
+		return fmt.Sprintf("%s \"%s\"", i.name, i.path)
+	}
+	return fmt.Sprintf("\"%s\"", i.path)
+}
+
+func (i *importStmt) markUsed() {
+	i.used = true
+}
+
+func (i *importStmt) equivalent(other *importStmt) bool {
+	return i == other
+}
+
+// importTable represents a collection of importStmts.
+type importTable struct {
+	// Map of imports and whether they should be copied to the output.
+	is map[string]*importStmt
+}
+
+func newImportTable() *importTable {
+	return &importTable{
+		is: make(map[string]*importStmt),
+	}
+}
+
+// Merges import statements from other into i. Collisions in import statements
+// result in a panic.
+func (i *importTable) merge(other *importTable) {
+	for name, im := range other.is {
+		if dup, ok := i.is[name]; ok && dup.equivalent(im) {
+			panic(fmt.Sprintf("Found colliding import statements: ours: %+v, other's: %+v", dup, im))
+		}
+
+		i.is[name] = im
+	}
+}
+
+func (i *importTable) add(s string) *importStmt {
+	n := newImport(s)
+	i.is[n.name] = n
+	return n
+}
+
+func (i *importTable) addFromSpec(spec *ast.ImportSpec, f *token.FileSet) *importStmt {
+	n := newImportFromSpec(spec, f)
+	i.is[n.name] = n
+	return n
+}
+
+// Marks the import named n as used. If no such import is in the table, returns
+// false.
+func (i *importTable) markUsed(n string) bool {
+	if n, ok := i.is[n]; ok {
+		n.markUsed()
+		return true
+	}
+	return false
+}
+
+func (i *importTable) clear() {
+	for _, i := range i.is {
+		i.used = false
+	}
+}
+
+func (i *importTable) write(out io.Writer) error {
+	if len(i.is) == 0 {
+		// Nothing to import, we're done.
+		return nil
+	}
+
+	imports := make([]string, 0, len(i.is))
+	for _, i := range i.is {
+		if i.used {
+			imports = append(imports, i.String())
+		}
+	}
+	sort.Strings(imports)
+
+	var b sourceBuffer
+	b.emit("import (\n")
+	b.incIndent()
+	for _, i := range imports {
+		b.emit("%s\n", i)
+	}
+	b.decIndent()
+	b.emit(")\n\n")
+
+	return b.write(out)
+}