4 files changed, 84 insertions, 6 deletions
diff --git a/tools/go_marshal/gomarshal/generator.go b/tools/go_marshal/gomarshal/generator.go
index fbec7bb9a..0294ba5ba 100644
--- a/tools/go_marshal/gomarshal/generator.go
+++ b/tools/go_marshal/gomarshal/generator.go
@@ -101,14 +101,16 @@ func NewGenerator(srcs []string, out, outTest, pkg string, imports []string) (*G
 		// used, so that they're always added to the generated code.
 		g.imports.add(i).markUsed()
 	}
-	g.imports.add(marshalImport).markUsed()
+
 	// The following imports may or may not be used by the generated code,
 	// depending on what's required for the target types. Don't mark these as
 	// used by default.
+	g.imports.add("io")
 	g.imports.add("reflect")
 	g.imports.add("runtime")
-	g.imports.add(safecopyImport)
 	g.imports.add("unsafe")
+	g.imports.add(marshalImport)
+	g.imports.add(safecopyImport)
 	g.imports.add(usermemImport)
 
 	return &g, nil
diff --git a/tools/go_marshal/gomarshal/generator_interfaces.go b/tools/go_marshal/gomarshal/generator_interfaces.go
index f25331ac5..22aae0f6b 100644
--- a/tools/go_marshal/gomarshal/generator_interfaces.go
+++ b/tools/go_marshal/gomarshal/generator_interfaces.go
@@ -602,4 +602,50 @@ func (g *interfaceGenerator) emitMarshallable() {
 		}
 	})
 	g.emit("}\n\n")
+
+	g.emit("// WriteTo implements io.WriterTo.WriteTo.\n")
+	g.recordUsedImport("io")
+	g.emit("func (%s *%s) WriteTo(w io.Writer) (int64, error) {\n", g.r, g.typeName())
+	g.inIndent(func() {
+		fallback := func() {
+			g.emit("// Type %s doesn't have a packed layout in memory, fall back to MarshalBytes.\n", g.typeName())
+			g.emit("buf := make([]byte, %s.SizeBytes())\n", g.r)
+			g.emit("%s.MarshalBytes(buf)\n", g.r)
+			g.emit("n, err := w.Write(buf)\n")
+			g.emit("return int64(n), err\n")
+		}
+		if thisPacked {
+			g.recordUsedImport("reflect")
+			g.recordUsedImport("runtime")
+			g.recordUsedImport("unsafe")
+			if cond, ok := g.areFieldsPackedExpression(); ok {
+				g.emit("if !%s {\n", cond)
+				g.inIndent(fallback)
+				g.emit("}\n\n")
+			}
+			// Fast serialization.
+			g.emit("// Bypass escape analysis on %s. The no-op arithmetic operation on the\n", g.r)
+			g.emit("// pointer makes the compiler think val doesn't depend on %s.\n", g.r)
+			g.emit("// See src/runtime/stubs.go:noescape() in the golang toolchain.\n")
+			g.emit("ptr := unsafe.Pointer(%s)\n", g.r)
+			g.emit("val := uintptr(ptr)\n")
+			g.emit("val = val^0\n\n")
+
+			g.emit("// Construct a slice backed by %s's underlying memory.\n", g.r)
+			g.emit("var buf []byte\n")
+			g.emit("hdr := (*reflect.SliceHeader)(unsafe.Pointer(&buf))\n")
+			g.emit("hdr.Data = val\n")
+			g.emit("hdr.Len = %s.SizeBytes()\n", g.r)
+			g.emit("hdr.Cap = %s.SizeBytes()\n\n", g.r)
+
+			g.emit("len, err := w.Write(buf)\n")
+			g.emit("// Since we bypassed the compiler's escape analysis, indicate that %s\n", g.r)
+			g.emit("// must live until after the Write.\n")
+			g.emit("runtime.KeepAlive(%s)\n", g.r)
+			g.emit("return int64(len), err\n")
+		} else {
+			fallback()
+		}
+	})
+	g.emit("}\n\n")
 }
diff --git a/tools/go_marshal/gomarshal/generator_tests.go b/tools/go_marshal/gomarshal/generator_tests.go
index cc760b6d0..5ad97af14 100644
--- a/tools/go_marshal/gomarshal/generator_tests.go
+++ b/tools/go_marshal/gomarshal/generator_tests.go
@@ -22,6 +22,7 @@ import (
 )
 
 var standardImports = []string{
+	"bytes",
 	"fmt",
 	"reflect",
 	"testing",
@@ -117,26 +118,50 @@ func (g *testGenerator) emitTestMarshalUnmarshalPreservesData() {
 		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("t.Fatal(fmt.Sprintf(\"Data corrupted across MarshalBytes/UnmarshalBytes 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("t.Fatal(fmt.Sprintf(\"Data corrupted across MarshalUnsafe/UnmarshalBytes 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("t.Fatal(fmt.Sprintf(\"Data corrupted across MarshalBytes/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("t.Fatal(fmt.Sprintf(\"Data corrupted across MarshalUnsafe/UnmarshalUnsafe cycle:\\nBefore: %+v\\nAfter: %+v\\n\", x, zUnsafe))\n")
+		})
+		g.emit("}\n")
+	})
+}
+
+func (g *testGenerator) emitTestWriteToUnmarshalPreservesData() {
+	g.inTestFunction("TestWriteToUnmarshalPreservesData", func() {
+		g.emit("var x, y, yUnsafe %s\n", g.typeName())
+		g.emit("analysis.RandomizeValue(&x)\n\n")
+
+		g.emit("var buf bytes.Buffer\n\n")
+
+		g.emit("x.WriteTo(&buf)\n")
+		g.emit("y.UnmarshalBytes(buf.Bytes())\n\n")
+		g.emit("yUnsafe.UnmarshalUnsafe(buf.Bytes())\n\n")
+
+		g.emit("if !reflect.DeepEqual(x, y) {\n")
+		g.inIndent(func() {
+			g.emit("t.Fatal(fmt.Sprintf(\"Data corrupted across WriteTo/UnmarshalBytes cycle:\\nBefore: %+v\\nAfter: %+v\\n\", x, y))\n")
+		})
+		g.emit("}\n")
+		g.emit("if !reflect.DeepEqual(x, yUnsafe) {\n")
+		g.inIndent(func() {
+			g.emit("t.Fatal(fmt.Sprintf(\"Data corrupted across WriteTo/UnmarshalUnsafe cycle:\\nBefore: %+v\\nAfter: %+v\\n\", x, yUnsafe))\n")
 		})
 		g.emit("}\n")
 	})
@@ -146,6 +171,7 @@ func (g *testGenerator) emitTests() {
 	g.emitTestNonZeroSize()
 	g.emitTestSuspectAlignment()
 	g.emitTestMarshalUnmarshalPreservesData()
+	g.emitTestWriteToUnmarshalPreservesData()
 }
 
 func (g *testGenerator) write(out io.Writer) error {
diff --git a/tools/go_marshal/marshal/marshal.go b/tools/go_marshal/marshal/marshal.go
index 10614ec4d..e521b50bd 100644
--- a/tools/go_marshal/marshal/marshal.go
+++ b/tools/go_marshal/marshal/marshal.go
@@ -21,6 +21,8 @@
 package marshal
 
 import (
+	"io"
+
 	"gvisor.dev/gvisor/pkg/usermem"
 )
 
@@ -42,6 +44,8 @@ type Task interface {
 
 // Marshallable represents a type that can be marshalled to and from memory.
 type Marshallable interface {
+	io.WriterTo
+
 	// SizeBytes is the size of the memory representation of a type in
 	// marshalled form.
 	SizeBytes() int