diff --git a/cmd/ssadump/main.go b/cmd/ssadump/main.go
index 3a135b02..def7215b 100644
--- a/cmd/ssadump/main.go
+++ b/cmd/ssadump/main.go
@@ -14,6 +14,7 @@ import (
 	"runtime"
 	"runtime/pprof"
 
+	"code.google.com/p/go.tools/go/types"
 	"code.google.com/p/go.tools/importer"
 	"code.google.com/p/go.tools/ssa"
 	"code.google.com/p/go.tools/ssa/interp"
@@ -73,6 +74,17 @@ func main() {
 	args := flag.Args()
 
 	impctx := importer.Config{Build: &build.Default}
+	// TODO(adonovan): make go/types choose its default Sizes from
+	// build.Default or a specified *build.Context.
+	var wordSize int64 = 8
+	switch impctx.Build.GOARCH {
+	case "386", "arm":
+		wordSize = 4
+	}
+	impctx.TypeChecker.Sizes = &types.StdSizes{
+		MaxAlign: 8,
+		WordSize: wordSize,
+	}
 
 	var debugMode bool
 	var mode ssa.BuilderMode
@@ -173,6 +185,12 @@ func main() {
 		if main == nil {
 			log.Fatal("No main package and no tests")
 		}
-		interp.Interpret(main, interpMode, main.Object.Path(), args)
+
+		if runtime.GOARCH != impctx.Build.GOARCH {
+			log.Fatalf("Cross-interpretation is not yet supported (target has GOARCH %s, interpreter has %s).",
+				impctx.Build.GOARCH, runtime.GOARCH)
+		}
+
+		interp.Interpret(main, interpMode, impctx.TypeChecker.Sizes, main.Object.Path(), args)
 	}
 }
diff --git a/ssa/interp/interp.go b/ssa/interp/interp.go
index b9be370b..82ed0e03 100644
--- a/ssa/interp/interp.go
+++ b/ssa/interp/interp.go
@@ -622,18 +622,22 @@ func setGlobal(i *interpreter, pkg *ssa.Package, name string, v value) {
 	panic("no global variable: " + pkg.Object.Path() + "." + name)
 }
 
-var stdSizes = types.StdSizes{WordSize: 8, MaxAlign: 8}
+// _sizes is the effective type-sizing function.
+// TODO(adonovan): avoid global state.
+var _sizes types.Sizes
 
 // Interpret interprets the Go program whose main package is mainpkg.
 // mode specifies various interpreter options.  filename and args are
-// the initial values of os.Args for the target program.
+// the initial values of os.Args for the target program.  sizes is the
+// effective type-sizing function for this program.
 //
 // Interpret returns the exit code of the program: 2 for panic (like
 // gc does), or the argument to os.Exit for normal termination.
 //
 // The SSA program must include the "runtime" package.
 //
-func Interpret(mainpkg *ssa.Package, mode Mode, filename string, args []string) (exitCode int) {
+func Interpret(mainpkg *ssa.Package, mode Mode, sizes types.Sizes, filename string, args []string) (exitCode int) {
+	_sizes = sizes
 	i := &interpreter{
 		prog:    mainpkg.Prog,
 		globals: make(map[ssa.Value]*value),
@@ -669,8 +673,7 @@ func Interpret(mainpkg *ssa.Package, mode Mode, filename string, args []string)
 			setGlobal(i, pkg, "envs", envs)
 
 		case "runtime":
-			// (Assumes no custom Sizeof used during SSA construction.)
-			sz := stdSizes.Sizeof(pkg.Object.Scope().Lookup("MemStats").Type())
+			sz := sizes.Sizeof(pkg.Object.Scope().Lookup("MemStats").Type())
 			setGlobal(i, pkg, "sizeof_C_MStats", uintptr(sz))
 
 		case "os":
diff --git a/ssa/interp/interp_test.go b/ssa/interp/interp_test.go
index 9587dbe8..ceacbbe6 100644
--- a/ssa/interp/interp_test.go
+++ b/ssa/interp/interp_test.go
@@ -16,6 +16,7 @@ import (
 	"testing"
 	"time"
 
+	"code.google.com/p/go.tools/go/types"
 	"code.google.com/p/go.tools/importer"
 	"code.google.com/p/go.tools/ssa"
 	"code.google.com/p/go.tools/ssa/interp"
@@ -222,7 +223,7 @@ func run(t *testing.T, dir, input string, success successPredicate) bool {
 	interp.CapturedOutput = &out
 
 	hint = fmt.Sprintf("To trace execution, run:\n%% go build code.google.com/p/go.tools/cmd/ssadump && ./ssadump -build=C -run --interp=T %s\n", input)
-	exitCode := interp.Interpret(mainPkg, 0, inputs[0], []string{})
+	exitCode := interp.Interpret(mainPkg, 0, &types.StdSizes{8, 8}, inputs[0], []string{})
 
 	// The definition of success varies with each file.
 	if err := success(exitCode, out.String()); err != nil {
diff --git a/ssa/interp/ops.go b/ssa/interp/ops.go
index e32bdf9c..2b9fea9e 100644
--- a/ssa/interp/ops.go
+++ b/ssa/interp/ops.go
@@ -108,7 +108,7 @@ func constValue(c *ssa.Const) value {
 		}
 	}
 
-	panic(fmt.Sprintf("constValue: Value.(type)=%T Type()=%s", c.Value, c.Type()))
+	panic(fmt.Sprintf("constValue: %s", c))
 }
 
 // asInt converts x, which must be an integer, to an int suitable for
diff --git a/ssa/interp/reflect.go b/ssa/interp/reflect.go
index 5735cc92..556d8036 100644
--- a/ssa/interp/reflect.go
+++ b/ssa/interp/reflect.go
@@ -81,33 +81,7 @@ func ext۰reflect۰rtype۰Bits(fn *ssa.Function, args []value) value {
 	if !ok {
 		panic(fmt.Sprintf("reflect.Type.Bits(%T): non-basic type", rt))
 	}
-	switch basic.Kind() {
-	case types.Int8, types.Uint8:
-		return 8
-	case types.Int16, types.Uint16:
-		return 16
-	case types.Int, types.UntypedInt:
-		// Assume sizeof(int) is same on host and target; ditto uint.
-		return reflect.TypeOf(int(0)).Bits()
-	case types.Uintptr:
-		// Assume sizeof(uintptr) is same on host and target.
-		return reflect.TypeOf(uintptr(0)).Bits()
-	case types.Int32, types.Uint32:
-		return 32
-	case types.Int64, types.Uint64:
-		return 64
-	case types.Float32:
-		return 32
-	case types.Float64, types.UntypedFloat:
-		return 64
-	case types.Complex64:
-		return 64
-	case types.Complex128, types.UntypedComplex:
-		return 128
-	default:
-		panic(fmt.Sprintf("reflect.Type.Bits(%s)", basic))
-	}
-	return nil
+	return _sizes.Sizeof(basic) * 8
 }
 
 func ext۰reflect۰rtype۰Elem(fn *ssa.Function, args []value) value {
@@ -161,8 +135,7 @@ func ext۰reflect۰rtype۰Out(fn *ssa.Function, args []value) value {
 
 func ext۰reflect۰rtype۰Size(fn *ssa.Function, args []value) value {
 	// Signature: func (t reflect.rtype) uintptr
-	// (Assumes no custom Sizeof used during SSA construction.)
-	return uintptr(stdSizes.Sizeof(args[0].(rtype).t))
+	return uint64(_sizes.Sizeof(args[0].(rtype).t))
 }
 
 func ext۰reflect۰rtype۰String(fn *ssa.Function, args []value) value {
diff --git a/ssa/util.go b/ssa/util.go
index a2bd9a76..3c661c36 100644
--- a/ssa/util.go
+++ b/ssa/util.go
@@ -62,7 +62,7 @@ func deref(typ types.Type) types.Type {
 // it returns the incoming type for all other types.  The default type
 // for untyped nil is untyped nil.
 //
-// Exported to exp/ssa/interp.
+// Exported to ssa/interp.
 //
 // TODO(gri): this is a copy of go/types.defaultType; export that function.
 //