diff --git a/go/ssa/interp/external.go b/go/ssa/interp/external.go
index 2c4dcc76..735a14d9 100644
--- a/go/ssa/interp/external.go
+++ b/go/ssa/interp/external.go
@@ -8,16 +8,13 @@ package interp
 // external or because they use "unsafe" or "reflect" operations.
 
 import (
-	"go/types"
+	"bytes"
 	"math"
 	"os"
 	"runtime"
 	"strings"
-	"sync/atomic"
 	"time"
-	"unsafe"
-
-	"golang.org/x/tools/go/ssa"
+	"unicode/utf8"
 )
 
 type externalFn func(fr *frame, args []value) value
@@ -32,144 +29,81 @@ var externals = make(map[string]externalFn)
 func init() {
 	// That little dot ۰ is an Arabic zero numeral (U+06F0), categories [Nd].
 	for k, v := range map[string]externalFn{
-		"(*sync.Pool).Get":                    ext۰sync۰Pool۰Get,
-		"(*sync.Pool).Put":                    ext۰nop,
-		"(reflect.Value).Bool":                ext۰reflect۰Value۰Bool,
-		"(reflect.Value).CanAddr":             ext۰reflect۰Value۰CanAddr,
-		"(reflect.Value).CanInterface":        ext۰reflect۰Value۰CanInterface,
-		"(reflect.Value).Elem":                ext۰reflect۰Value۰Elem,
-		"(reflect.Value).Field":               ext۰reflect۰Value۰Field,
-		"(reflect.Value).Float":               ext۰reflect۰Value۰Float,
-		"(reflect.Value).Index":               ext۰reflect۰Value۰Index,
-		"(reflect.Value).Int":                 ext۰reflect۰Value۰Int,
-		"(reflect.Value).Interface":           ext۰reflect۰Value۰Interface,
-		"(reflect.Value).IsNil":               ext۰reflect۰Value۰IsNil,
-		"(reflect.Value).IsValid":             ext۰reflect۰Value۰IsValid,
-		"(reflect.Value).Kind":                ext۰reflect۰Value۰Kind,
-		"(reflect.Value).Len":                 ext۰reflect۰Value۰Len,
-		"(reflect.Value).MapIndex":            ext۰reflect۰Value۰MapIndex,
-		"(reflect.Value).MapKeys":             ext۰reflect۰Value۰MapKeys,
-		"(reflect.Value).NumField":            ext۰reflect۰Value۰NumField,
-		"(reflect.Value).NumMethod":           ext۰reflect۰Value۰NumMethod,
-		"(reflect.Value).Pointer":             ext۰reflect۰Value۰Pointer,
-		"(reflect.Value).Set":                 ext۰reflect۰Value۰Set,
-		"(reflect.Value).String":              ext۰reflect۰Value۰String,
-		"(reflect.Value).Type":                ext۰reflect۰Value۰Type,
-		"(reflect.Value).Uint":                ext۰reflect۰Value۰Uint,
-		"(reflect.error).Error":               ext۰reflect۰error۰Error,
-		"(reflect.rtype).Bits":                ext۰reflect۰rtype۰Bits,
-		"(reflect.rtype).Elem":                ext۰reflect۰rtype۰Elem,
-		"(reflect.rtype).Field":               ext۰reflect۰rtype۰Field,
-		"(reflect.rtype).In":                  ext۰reflect۰rtype۰In,
-		"(reflect.rtype).Kind":                ext۰reflect۰rtype۰Kind,
-		"(reflect.rtype).NumField":            ext۰reflect۰rtype۰NumField,
-		"(reflect.rtype).NumIn":               ext۰reflect۰rtype۰NumIn,
-		"(reflect.rtype).NumMethod":           ext۰reflect۰rtype۰NumMethod,
-		"(reflect.rtype).NumOut":              ext۰reflect۰rtype۰NumOut,
-		"(reflect.rtype).Out":                 ext۰reflect۰rtype۰Out,
-		"(reflect.rtype).Size":                ext۰reflect۰rtype۰Size,
-		"(reflect.rtype).String":              ext۰reflect۰rtype۰String,
-		"bytes.init":                          ext۰nop, // avoid asm dependency
-		"bytes.Equal":                         ext۰bytes۰Equal,
-		"bytes.IndexByte":                     ext۰bytes۰IndexByte,
-		"hash/crc32.haveSSE42":                ext۰crc32۰haveSSE42,
-		"internal/cpu.cpuid":                  ext۰cpu۰cpuid,
-		"internal/syscall/unix.syscall_fcntl": ext۰syscall۰unix۰syscall_fcntl,
-		"math.Abs":                            ext۰math۰Abs,
-		"math.Exp":                            ext۰math۰Exp,
-		"math.Float32bits":                    ext۰math۰Float32bits,
-		"math.Float32frombits":                ext۰math۰Float32frombits,
-		"math.Float64bits":                    ext۰math۰Float64bits,
-		"math.Float64frombits":                ext۰math۰Float64frombits,
-		"math.Ldexp":                          ext۰math۰Ldexp,
-		"math.Log":                            ext۰math۰Log,
-		"math.Min":                            ext۰math۰Min,
-		"math.hasSSE4":                        ext۰math۰hasSSE4,
-		"math.hasVectorFacility":              ext۰math۰hasVectorFacility,
-		"os.runtime_args":                     ext۰os۰runtime_args,
-		"os.runtime_beforeExit":               ext۰nop,
-		"os/signal.init":                      ext۰nop,
-		"reflect.New":                         ext۰reflect۰New,
-		"reflect.SliceOf":                     ext۰reflect۰SliceOf,
-		"reflect.TypeOf":                      ext۰reflect۰TypeOf,
-		"reflect.ValueOf":                     ext۰reflect۰ValueOf,
-		"reflect.Zero":                        ext۰reflect۰Zero,
-		"reflect.init":                        ext۰reflect۰Init,
-		"reflect.valueInterface":              ext۰reflect۰valueInterface,
-		"runtime.Breakpoint":                  ext۰runtime۰Breakpoint,
-		"runtime.Caller":                      ext۰runtime۰Caller,
-		"runtime.Callers":                     ext۰runtime۰Callers,
-		"runtime.FuncForPC":                   ext۰runtime۰FuncForPC,
-		"runtime.GC":                          ext۰runtime۰GC,
-		"runtime.GOMAXPROCS":                  ext۰runtime۰GOMAXPROCS,
-		"runtime.Goexit":                      ext۰runtime۰Goexit,
-		"runtime.Gosched":                     ext۰runtime۰Gosched,
-		"runtime.init":                        ext۰nop,
-		"runtime.KeepAlive":                   ext۰nop,
-		"runtime.NumCPU":                      ext۰runtime۰NumCPU,
-		"runtime.NumGoroutine":                ext۰runtime۰NumGoroutine,
-		"runtime.ReadMemStats":                ext۰runtime۰ReadMemStats,
-		"runtime.SetFinalizer":                ext۰nop, // ignore
-		"(*runtime.Func).Entry":               ext۰runtime۰Func۰Entry,
-		"(*runtime.Func).FileLine":            ext۰runtime۰Func۰FileLine,
-		"(*runtime.Func).Name":                ext۰runtime۰Func۰Name,
-		"runtime.environ":                     ext۰runtime۰environ,
-		"runtime.getgoroot":                   ext۰runtime۰getgoroot,
-		"strings.init":                        ext۰nop, // avoid asm dependency
-		"strings.Count":                       ext۰strings۰Count,
-		"strings.Index":                       ext۰strings۰Index,
-		"strings.IndexByte":                   ext۰strings۰IndexByte,
-		"sync.runtime_Semacquire":             ext۰nop, // unimplementable
-		"sync.runtime_Semrelease":             ext۰nop, // unimplementable
-		"sync.runtime_Syncsemcheck":           ext۰nop, // unimplementable
-		"sync.runtime_notifyListCheck":        ext۰nop,
-		"sync.runtime_registerPoolCleanup":    ext۰nop,
-		"sync/atomic.AddInt32":                ext۰atomic۰AddInt32,
-		"sync/atomic.AddUint32":               ext۰atomic۰AddUint32,
-		"sync/atomic.CompareAndSwapInt32":     ext۰atomic۰CompareAndSwapInt32,
-		"sync/atomic.CompareAndSwapUint32":    ext۰atomic۰CompareAndSwapUint32,
-		"sync/atomic.LoadInt32":               ext۰atomic۰LoadInt32,
-		"sync/atomic.LoadUint32":              ext۰atomic۰LoadUint32,
-		"sync/atomic.StoreInt32":              ext۰atomic۰StoreInt32,
-		"sync/atomic.StoreUint32":             ext۰atomic۰StoreUint32,
-		"sync/atomic.AddInt64":                ext۰atomic۰AddInt64,
-		"sync/atomic.AddUint64":               ext۰atomic۰AddUint64,
-		"sync/atomic.CompareAndSwapInt64":     ext۰atomic۰CompareAndSwapInt64,
-		"sync/atomic.CompareAndSwapUint64":    ext۰atomic۰CompareAndSwapUint64,
-		"sync/atomic.LoadInt64":               ext۰atomic۰LoadInt64,
-		"sync/atomic.LoadUint64":              ext۰atomic۰LoadUint64,
-		"sync/atomic.StoreInt64":              ext۰atomic۰StoreInt64,
-		"sync/atomic.StoreUint64":             ext۰atomic۰StoreUint64,
-		"(*sync/atomic.Value).Load":           ext۰atomic۰ValueLoad,
-		"(*sync/atomic.Value).Store":          ext۰atomic۰ValueStore,
-		"testing.MainStart":                   ext۰testing۰MainStart,
-		"time.Sleep":                          ext۰time۰Sleep,
-		"time.now":                            ext۰time۰now,
+		"(reflect.Value).Bool":            ext۰reflect۰Value۰Bool,
+		"(reflect.Value).CanAddr":         ext۰reflect۰Value۰CanAddr,
+		"(reflect.Value).CanInterface":    ext۰reflect۰Value۰CanInterface,
+		"(reflect.Value).Elem":            ext۰reflect۰Value۰Elem,
+		"(reflect.Value).Field":           ext۰reflect۰Value۰Field,
+		"(reflect.Value).Float":           ext۰reflect۰Value۰Float,
+		"(reflect.Value).Index":           ext۰reflect۰Value۰Index,
+		"(reflect.Value).Int":             ext۰reflect۰Value۰Int,
+		"(reflect.Value).Interface":       ext۰reflect۰Value۰Interface,
+		"(reflect.Value).IsNil":           ext۰reflect۰Value۰IsNil,
+		"(reflect.Value).IsValid":         ext۰reflect۰Value۰IsValid,
+		"(reflect.Value).Kind":            ext۰reflect۰Value۰Kind,
+		"(reflect.Value).Len":             ext۰reflect۰Value۰Len,
+		"(reflect.Value).MapIndex":        ext۰reflect۰Value۰MapIndex,
+		"(reflect.Value).MapKeys":         ext۰reflect۰Value۰MapKeys,
+		"(reflect.Value).NumField":        ext۰reflect۰Value۰NumField,
+		"(reflect.Value).NumMethod":       ext۰reflect۰Value۰NumMethod,
+		"(reflect.Value).Pointer":         ext۰reflect۰Value۰Pointer,
+		"(reflect.Value).Set":             ext۰reflect۰Value۰Set,
+		"(reflect.Value).String":          ext۰reflect۰Value۰String,
+		"(reflect.Value).Type":            ext۰reflect۰Value۰Type,
+		"(reflect.Value).Uint":            ext۰reflect۰Value۰Uint,
+		"(reflect.error).Error":           ext۰reflect۰error۰Error,
+		"(reflect.rtype).Bits":            ext۰reflect۰rtype۰Bits,
+		"(reflect.rtype).Elem":            ext۰reflect۰rtype۰Elem,
+		"(reflect.rtype).Field":           ext۰reflect۰rtype۰Field,
+		"(reflect.rtype).In":              ext۰reflect۰rtype۰In,
+		"(reflect.rtype).Kind":            ext۰reflect۰rtype۰Kind,
+		"(reflect.rtype).NumField":        ext۰reflect۰rtype۰NumField,
+		"(reflect.rtype).NumIn":           ext۰reflect۰rtype۰NumIn,
+		"(reflect.rtype).NumMethod":       ext۰reflect۰rtype۰NumMethod,
+		"(reflect.rtype).NumOut":          ext۰reflect۰rtype۰NumOut,
+		"(reflect.rtype).Out":             ext۰reflect۰rtype۰Out,
+		"(reflect.rtype).Size":            ext۰reflect۰rtype۰Size,
+		"(reflect.rtype).String":          ext۰reflect۰rtype۰String,
+		"bytes.Equal":                     ext۰bytes۰Equal,
+		"bytes.IndexByte":                 ext۰bytes۰IndexByte,
+		"fmt.Sprint":                      ext۰fmt۰Sprint,
+		"math.Abs":                        ext۰math۰Abs,
+		"math.Exp":                        ext۰math۰Exp,
+		"math.Float32bits":                ext۰math۰Float32bits,
+		"math.Float32frombits":            ext۰math۰Float32frombits,
+		"math.Float64bits":                ext۰math۰Float64bits,
+		"math.Float64frombits":            ext۰math۰Float64frombits,
+		"math.Inf":                        ext۰math۰Inf,
+		"math.IsNaN":                      ext۰math۰IsNaN,
+		"math.Ldexp":                      ext۰math۰Ldexp,
+		"math.Log":                        ext۰math۰Log,
+		"math.Min":                        ext۰math۰Min,
+		"math.NaN":                        ext۰math۰NaN,
+		"os.Exit":                         ext۰os۰Exit,
+		"os.Getenv":                       ext۰os۰Getenv,
+		"reflect.New":                     ext۰reflect۰New,
+		"reflect.SliceOf":                 ext۰reflect۰SliceOf,
+		"reflect.TypeOf":                  ext۰reflect۰TypeOf,
+		"reflect.ValueOf":                 ext۰reflect۰ValueOf,
+		"reflect.Zero":                    ext۰reflect۰Zero,
+		"runtime.Breakpoint":              ext۰runtime۰Breakpoint,
+		"runtime.GC":                      ext۰runtime۰GC,
+		"runtime.GOMAXPROCS":              ext۰runtime۰GOMAXPROCS,
+		"runtime.GOROOT":                  ext۰runtime۰GOROOT,
+		"runtime.Goexit":                  ext۰runtime۰Goexit,
+		"runtime.Gosched":                 ext۰runtime۰Gosched,
+		"runtime.NumCPU":                  ext۰runtime۰NumCPU,
+		"strings.Count":                   ext۰strings۰Count,
+		"strings.Index":                   ext۰strings۰Index,
+		"strings.IndexByte":               ext۰strings۰IndexByte,
+		"strings.Replace":                 ext۰strings۰Replace,
+		"time.Sleep":                      ext۰time۰Sleep,
+		"unicode/utf8.DecodeRuneInString": ext۰unicode۰utf8۰DecodeRuneInString,
 	} {
 		externals[k] = v
 	}
 }
 
-// wrapError returns an interpreted 'error' interface value for err.
-func wrapError(err error) value {
-	if err == nil {
-		return iface{}
-	}
-	return iface{t: errorType, v: err.Error()}
-}
-
-func ext۰nop(fr *frame, args []value) value { return nil }
-
-func ext۰sync۰Pool۰Get(fr *frame, args []value) value {
-	Pool := fr.i.prog.ImportedPackage("sync").Type("Pool").Object()
-	_, newIndex, _ := types.LookupFieldOrMethod(Pool.Type(), false, Pool.Pkg(), "New")
-
-	if New := (*args[0].(*value)).(structure)[newIndex[0]]; New != nil {
-		return call(fr.i, fr, 0, New, nil)
-	}
-	return nil
-}
-
 func ext۰bytes۰Equal(fr *frame, args []value) value {
 	// func Equal(a, b []byte) bool
 	a := args[0].([]value)
@@ -197,10 +131,6 @@ func ext۰bytes۰IndexByte(fr *frame, args []value) value {
 	return -1
 }
 
-func ext۰crc32۰haveSSE42(fr *frame, args []value) value {
-	return false
-}
-
 func ext۰math۰Float64frombits(fr *frame, args []value) value {
 	return math.Float64frombits(args[0].(uint64))
 }
@@ -229,12 +159,16 @@ func ext۰math۰Min(fr *frame, args []value) value {
 	return math.Min(args[0].(float64), args[1].(float64))
 }
 
-func ext۰math۰hasSSE4(fr *frame, args []value) value {
-	return false
+func ext۰math۰NaN(fr *frame, args []value) value {
+	return math.NaN()
 }
 
-func ext۰math۰hasVectorFacility(fr *frame, args []value) value {
-	return false
+func ext۰math۰IsNaN(fr *frame, args []value) value {
+	return math.IsNaN(args[0].(float64))
+}
+
+func ext۰math۰Inf(fr *frame, args []value) value {
+	return math.Inf(args[0].(int))
 }
 
 func ext۰math۰Ldexp(fr *frame, args []value) value {
@@ -245,94 +179,32 @@ func ext۰math۰Log(fr *frame, args []value) value {
 	return math.Log(args[0].(float64))
 }
 
-func ext۰os۰runtime_args(fr *frame, args []value) value {
-	return fr.i.osArgs
-}
-
 func ext۰runtime۰Breakpoint(fr *frame, args []value) value {
 	runtime.Breakpoint()
 	return nil
 }
 
-func ext۰runtime۰Caller(fr *frame, args []value) value {
-	// func Caller(skip int) (pc uintptr, file string, line int, ok bool)
-	skip := 1 + args[0].(int)
-	for i := 0; i < skip; i++ {
-		if fr != nil {
-			fr = fr.caller
-		}
-	}
-	var pc uintptr
-	var file string
-	var line int
-	var ok bool
-	if fr != nil {
-		fn := fr.fn
-		// TODO(adonovan): use pc/posn of current instruction, not start of fn.
-		// (Required to interpret the log package's tests.)
-		pc = uintptr(unsafe.Pointer(fn))
-		posn := fn.Prog.Fset.Position(fn.Pos())
-		file = posn.Filename
-		line = posn.Line
-		ok = true
-	}
-	return tuple{pc, file, line, ok}
-}
-
-func ext۰runtime۰Callers(fr *frame, args []value) value {
-	// Callers(skip int, pc []uintptr) int
-	skip := args[0].(int)
-	pc := args[1].([]value)
-	for i := 0; i < skip; i++ {
-		if fr != nil {
-			fr = fr.caller
-		}
-	}
-	i := 0
-	for fr != nil && i < len(pc) {
-		pc[i] = uintptr(unsafe.Pointer(fr.fn))
-		i++
-		fr = fr.caller
-	}
-	return i
-}
-
-func ext۰runtime۰FuncForPC(fr *frame, args []value) value {
-	// FuncForPC(pc uintptr) *Func
-	pc := args[0].(uintptr)
-	var fn *ssa.Function
-	if pc != 0 {
-		fn = (*ssa.Function)(unsafe.Pointer(pc)) // indeed unsafe!
-	}
-	var Func value
-	Func = structure{fn} // a runtime.Func
-	return &Func
-}
-
-func ext۰runtime۰environ(fr *frame, args []value) value {
-	// This function also implements syscall.runtime_envs.
-	return environ
-}
-
-func ext۰runtime۰getgoroot(fr *frame, args []value) value {
-	return os.Getenv("GOROOT")
-}
-
 func ext۰strings۰Count(fr *frame, args []value) value {
-	// Call compiled version to avoid asm dependency.
 	return strings.Count(args[0].(string), args[1].(string))
 }
 
 func ext۰strings۰IndexByte(fr *frame, args []value) value {
-	// Call compiled version to avoid asm dependency.
 	return strings.IndexByte(args[0].(string), args[1].(byte))
 }
 
 func ext۰strings۰Index(fr *frame, args []value) value {
-	// Call compiled version to avoid asm dependency.
 	return strings.Index(args[0].(string), args[1].(string))
 }
 
+func ext۰strings۰Replace(fr *frame, args []value) value {
+	// func Replace(s, old, new string, n int) string
+	s := args[0].(string)
+	new := args[1].(string)
+	old := args[2].(string)
+	n := args[3].(int)
+	return strings.Replace(s, old, new, n)
+}
+
 func ext۰runtime۰GOMAXPROCS(fr *frame, args []value) value {
 	// Ignore args[0]; don't let the interpreted program
 	// set the interpreter's GOMAXPROCS!
@@ -345,6 +217,10 @@ func ext۰runtime۰Goexit(fr *frame, args []value) value {
 	return nil
 }
 
+func ext۰runtime۰GOROOT(fr *frame, args []value) value {
+	return runtime.GOROOT()
+}
+
 func ext۰runtime۰GC(fr *frame, args []value) value {
 	runtime.GC()
 	return nil
@@ -359,187 +235,6 @@ func ext۰runtime۰NumCPU(fr *frame, args []value) value {
 	return runtime.NumCPU()
 }
 
-func ext۰runtime۰NumGoroutine(fr *frame, args []value) value {
-	return int(atomic.LoadInt32(&fr.i.goroutines))
-}
-
-func ext۰runtime۰ReadMemStats(fr *frame, args []value) value {
-	// TODO(adonovan): populate args[0].(Struct)
-	return nil
-}
-
-func ext۰atomic۰LoadUint32(fr *frame, args []value) value {
-	// TODO(adonovan): fix: not atomic!
-	return (*args[0].(*value)).(uint32)
-}
-
-func ext۰atomic۰StoreUint32(fr *frame, args []value) value {
-	// TODO(adonovan): fix: not atomic!
-	*args[0].(*value) = args[1].(uint32)
-	return nil
-}
-
-func ext۰atomic۰LoadInt32(fr *frame, args []value) value {
-	// TODO(adonovan): fix: not atomic!
-	return (*args[0].(*value)).(int32)
-}
-
-func ext۰atomic۰StoreInt32(fr *frame, args []value) value {
-	// TODO(adonovan): fix: not atomic!
-	*args[0].(*value) = args[1].(int32)
-	return nil
-}
-
-func ext۰atomic۰CompareAndSwapInt32(fr *frame, args []value) value {
-	// TODO(adonovan): fix: not atomic!
-	p := args[0].(*value)
-	if (*p).(int32) == args[1].(int32) {
-		*p = args[2].(int32)
-		return true
-	}
-	return false
-}
-
-func ext۰atomic۰CompareAndSwapUint32(fr *frame, args []value) value {
-	// TODO(adonovan): fix: not atomic!
-	p := args[0].(*value)
-	if (*p).(uint32) == args[1].(uint32) {
-		*p = args[2].(uint32)
-		return true
-	}
-	return false
-}
-
-func ext۰atomic۰AddInt32(fr *frame, args []value) value {
-	// TODO(adonovan): fix: not atomic!
-	p := args[0].(*value)
-	newv := (*p).(int32) + args[1].(int32)
-	*p = newv
-	return newv
-}
-
-func ext۰atomic۰AddUint32(fr *frame, args []value) value {
-	// TODO(adonovan): fix: not atomic!
-	p := args[0].(*value)
-	newv := (*p).(uint32) + args[1].(uint32)
-	*p = newv
-	return newv
-}
-
-func ext۰atomic۰LoadUint64(fr *frame, args []value) value {
-	// TODO(adonovan): fix: not atomic!
-	return (*args[0].(*value)).(uint64)
-}
-
-func ext۰atomic۰StoreUint64(fr *frame, args []value) value {
-	// TODO(adonovan): fix: not atomic!
-	*args[0].(*value) = args[1].(uint64)
-	return nil
-}
-
-func ext۰atomic۰LoadInt64(fr *frame, args []value) value {
-	// TODO(adonovan): fix: not atomic!
-	return (*args[0].(*value)).(int64)
-}
-
-func ext۰atomic۰StoreInt64(fr *frame, args []value) value {
-	// TODO(adonovan): fix: not atomic!
-	*args[0].(*value) = args[1].(int64)
-	return nil
-}
-
-func ext۰atomic۰CompareAndSwapInt64(fr *frame, args []value) value {
-	// TODO(adonovan): fix: not atomic!
-	p := args[0].(*value)
-	if (*p).(int64) == args[1].(int64) {
-		*p = args[2].(int64)
-		return true
-	}
-	return false
-}
-
-func ext۰atomic۰CompareAndSwapUint64(fr *frame, args []value) value {
-	// TODO(adonovan): fix: not atomic!
-	p := args[0].(*value)
-	if (*p).(uint64) == args[1].(uint64) {
-		*p = args[2].(uint64)
-		return true
-	}
-	return false
-}
-
-func ext۰atomic۰AddInt64(fr *frame, args []value) value {
-	// TODO(adonovan): fix: not atomic!
-	p := args[0].(*value)
-	newv := (*p).(int64) + args[1].(int64)
-	*p = newv
-	return newv
-}
-
-func ext۰atomic۰AddUint64(fr *frame, args []value) value {
-	// TODO(adonovan): fix: not atomic!
-	p := args[0].(*value)
-	newv := (*p).(uint64) + args[1].(uint64)
-	*p = newv
-	return newv
-}
-
-func ext۰atomic۰ValueLoad(fr *frame, args []value) value {
-	// TODO(adonovan): fix: not atomic!
-	// Receiver is *struct{v interface{}}.
-	return (*args[0].(*value)).(structure)[0]
-}
-
-func ext۰atomic۰ValueStore(fr *frame, args []value) value {
-	// TODO(adonovan): fix: not atomic!
-	// Receiver is *struct{v interface{}}.
-	(*args[0].(*value)).(structure)[0] = args[1]
-	return nil
-}
-
-func ext۰cpu۰cpuid(fr *frame, args []value) value {
-	return tuple{uint32(0), uint32(0), uint32(0), uint32(0)}
-}
-
-func ext۰syscall۰unix۰syscall_fcntl(fr *frame, args []value) value {
-	return tuple{int(0), wrapError(nil)}
-}
-
-// Pretend: type runtime.Func struct { entry *ssa.Function }
-
-func ext۰runtime۰Func۰FileLine(fr *frame, args []value) value {
-	// func (*runtime.Func) FileLine(uintptr) (string, int)
-	f, _ := (*args[0].(*value)).(structure)[0].(*ssa.Function)
-	pc := args[1].(uintptr)
-	_ = pc
-	if f != nil {
-		// TODO(adonovan): use position of current instruction, not fn.
-		posn := f.Prog.Fset.Position(f.Pos())
-		return tuple{posn.Filename, posn.Line}
-	}
-	return tuple{"", 0}
-}
-
-func ext۰runtime۰Func۰Name(fr *frame, args []value) value {
-	// func (*runtime.Func) Name() string
-	f, _ := (*args[0].(*value)).(structure)[0].(*ssa.Function)
-	if f != nil {
-		return f.String()
-	}
-	return ""
-}
-
-func ext۰runtime۰Func۰Entry(fr *frame, args []value) value {
-	// func (*runtime.Func) Entry() uintptr
-	f, _ := (*args[0].(*value)).(structure)[0].(*ssa.Function)
-	return uintptr(unsafe.Pointer(f))
-}
-
-func ext۰time۰now(fr *frame, args []value) value {
-	nano := time.Now().UnixNano()
-	return tuple{int64(nano / 1e9), int32(nano % 1e9), int64(0)}
-}
-
 func ext۰time۰Sleep(fr *frame, args []value) value {
 	time.Sleep(time.Duration(args[0].(int64)))
 	return nil
@@ -554,9 +249,42 @@ func valueToBytes(v value) []byte {
 	return b
 }
 
-func ext۰testing۰MainStart(fr *frame, args []value) value {
-	// We no longer support interpretation of the "testing" package
-	// because it changes too often and uses low-level features that
-	// are a pain to emulate.
-	panic(`interpretation of the "testing" package is no longer supported`)
+func ext۰os۰Getenv(fr *frame, args []value) value {
+	name := args[0].(string)
+	switch name {
+	case "GOSSAINTERP":
+		return "1"
+	case "GOARCH":
+		return "amd64"
+	case "GOOS":
+		return "linux"
+	}
+	return os.Getenv(name)
+}
+
+func ext۰os۰Exit(fr *frame, args []value) value {
+	panic(exitPanic(args[0].(int)))
+}
+
+func ext۰unicode۰utf8۰DecodeRuneInString(fr *frame, args []value) value {
+	r, n := utf8.DecodeRuneInString(args[0].(string))
+	return tuple{r, n}
+}
+
+// A fake function for turning an arbitrary value into a string.
+// Handles only the cases needed by the tests.
+// Uses same logic as 'print' built-in.
+func ext۰fmt۰Sprint(fr *frame, args []value) value {
+	buf := new(bytes.Buffer)
+	wasStr := false
+	for i, arg := range args[0].([]value) {
+		x := arg.(iface).v
+		_, isStr := x.(string)
+		if i > 0 && !wasStr && !isStr {
+			buf.WriteByte(' ')
+		}
+		wasStr = isStr
+		buf.WriteString(toString(x))
+	}
+	return buf.String()
 }
diff --git a/go/ssa/interp/external_darwin.go b/go/ssa/interp/external_darwin.go
deleted file mode 100644
index 47130093..00000000
--- a/go/ssa/interp/external_darwin.go
+++ /dev/null
@@ -1,35 +0,0 @@
-// Copyright 2014 The Go Authors.  All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-// +build darwin
-
-package interp
-
-import "syscall"
-
-func init() {
-	externals["syscall.Sysctl"] = ext۰syscall۰Sysctl
-
-	fillStat = func(st *syscall.Stat_t, stat structure) {
-		stat[0] = st.Dev
-		stat[1] = st.Mode
-		stat[2] = st.Nlink
-		stat[3] = st.Ino
-		stat[4] = st.Uid
-		stat[5] = st.Gid
-		stat[6] = st.Rdev
-		// TODO(adonovan): fix: copy Timespecs.
-		// stat[8] = st.Atim
-		// stat[9] = st.Mtim
-		// stat[10] = st.Ctim
-		stat[12] = st.Size
-		stat[13] = st.Blocks
-		stat[14] = st.Blksize
-	}
-}
-
-func ext۰syscall۰Sysctl(fr *frame, args []value) value {
-	r, err := syscall.Sysctl(args[0].(string))
-	return tuple{r, wrapError(err)}
-}
diff --git a/go/ssa/interp/external_unix.go b/go/ssa/interp/external_unix.go
deleted file mode 100644
index bfa39f68..00000000
--- a/go/ssa/interp/external_unix.go
+++ /dev/null
@@ -1,256 +0,0 @@
-// Copyright 2013 The Go Authors.  All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-// +build darwin linux
-
-package interp
-
-import "syscall"
-
-func init() {
-	for k, v := range map[string]externalFn{
-		"os.Pipe":              ext۰os۰Pipe,
-		"syscall.Close":        ext۰syscall۰Close,
-		"syscall.Exit":         ext۰syscall۰Exit,
-		"syscall.Fchown":       ext۰syscall۰Fchown,
-		"syscall.Fstat":        ext۰syscall۰Fstat,
-		"syscall.Ftruncate":    ext۰syscall۰Ftruncate,
-		"syscall.Getpid":       ext۰syscall۰Getpid,
-		"syscall.Getwd":        ext۰syscall۰Getwd,
-		"syscall.Kill":         ext۰syscall۰Kill,
-		"syscall.Link":         ext۰syscall۰Link,
-		"syscall.Lstat":        ext۰syscall۰Lstat,
-		"syscall.Mkdir":        ext۰syscall۰Mkdir,
-		"syscall.Open":         ext۰syscall۰Open,
-		"syscall.ParseDirent":  ext۰syscall۰ParseDirent,
-		"syscall.RawSyscall":   ext۰syscall۰RawSyscall,
-		"syscall.Read":         ext۰syscall۰Read,
-		"syscall.ReadDirent":   ext۰syscall۰ReadDirent,
-		"syscall.Readlink":     ext۰syscall۰Readlink,
-		"syscall.Rmdir":        ext۰syscall۰Rmdir,
-		"syscall.Seek":         ext۰syscall۰Seek,
-		"syscall.Stat":         ext۰syscall۰Stat,
-		"syscall.Symlink":      ext۰syscall۰Symlink,
-		"syscall.Write":        ext۰syscall۰Write,
-		"syscall.Unlink":       ext۰syscall۰Unlink,
-		"syscall۰UtimesNano":   ext۰syscall۰UtimesNano,
-		"syscall.setenv_c":     ext۰nop,
-		"syscall.unsetenv_c":   ext۰nop,
-		"syscall.runtime_envs": ext۰runtime۰environ,
-	} {
-		externals[k] = v
-	}
-
-	syswrite = syscall.Write
-}
-
-func ext۰os۰Pipe(fr *frame, args []value) value {
-	// func os.Pipe() (r *File, w *File, err error)
-
-	// The portable POSIX pipe(2) call is good enough for our needs.
-	var p [2]int
-	if err := syscall.Pipe(p[:]); err != nil {
-		// TODO(adonovan): fix: return an *os.SyscallError.
-		return tuple{nil, nil, wrapError(err)}
-	}
-
-	NewFile := fr.i.prog.ImportedPackage("os").Func("NewFile")
-	r := call(fr.i, fr, 0, NewFile, []value{uintptr(p[0]), "|0"})
-	w := call(fr.i, fr, 0, NewFile, []value{uintptr(p[1]), "|1"})
-	return tuple{r, w, wrapError(nil)}
-}
-
-// overridden on darwin
-var fillStat = func(st *syscall.Stat_t, stat structure) {
-	stat[0] = st.Dev
-	stat[1] = st.Ino
-	stat[2] = st.Nlink
-	stat[3] = st.Mode
-	stat[4] = st.Uid
-	stat[5] = st.Gid
-	stat[7] = st.Rdev
-	stat[8] = st.Size
-	stat[9] = st.Blksize
-	stat[10] = st.Blocks
-	// TODO(adonovan): fix: copy Timespecs.
-	// stat[11] = st.Atim
-	// stat[12] = st.Mtim
-	// stat[13] = st.Ctim
-}
-
-func ext۰syscall۰Close(fr *frame, args []value) value {
-	// func Close(fd int) (err error)
-	return wrapError(syscall.Close(args[0].(int)))
-}
-
-func ext۰syscall۰Exit(fr *frame, args []value) value {
-	panic(exitPanic(args[0].(int)))
-}
-
-func ext۰syscall۰Fchown(fr *frame, args []value) value {
-	fd := args[0].(int)
-	uid := args[1].(int)
-	gid := args[2].(int)
-	return wrapError(syscall.Fchown(fd, uid, gid))
-}
-
-func ext۰syscall۰Fstat(fr *frame, args []value) value {
-	// func Fstat(fd int, stat *Stat_t) (err error)
-	fd := args[0].(int)
-	stat := (*args[1].(*value)).(structure)
-
-	var st syscall.Stat_t
-	err := syscall.Fstat(fd, &st)
-	fillStat(&st, stat)
-	return wrapError(err)
-}
-
-func ext۰syscall۰Ftruncate(fr *frame, args []value) value {
-	fd := args[0].(int)
-	length := args[1].(int64)
-	return wrapError(syscall.Ftruncate(fd, length))
-}
-
-func ext۰syscall۰Getpid(fr *frame, args []value) value {
-	return syscall.Getpid()
-}
-
-func ext۰syscall۰Getwd(fr *frame, args []value) value {
-	s, err := syscall.Getwd()
-	return tuple{s, wrapError(err)}
-}
-
-func ext۰syscall۰Kill(fr *frame, args []value) value {
-	// func Kill(pid int, sig Signal) (err error)
-	return wrapError(syscall.Kill(args[0].(int), syscall.Signal(args[1].(int))))
-}
-
-func ext۰syscall۰Link(fr *frame, args []value) value {
-	path := args[0].(string)
-	link := args[1].(string)
-	return wrapError(syscall.Link(path, link))
-}
-
-func ext۰syscall۰Lstat(fr *frame, args []value) value {
-	// func Lstat(name string, stat *Stat_t) (err error)
-	name := args[0].(string)
-	stat := (*args[1].(*value)).(structure)
-
-	var st syscall.Stat_t
-	err := syscall.Lstat(name, &st)
-	fillStat(&st, stat)
-	return wrapError(err)
-}
-
-func ext۰syscall۰Mkdir(fr *frame, args []value) value {
-	path := args[0].(string)
-	mode := args[1].(uint32)
-	return wrapError(syscall.Mkdir(path, mode))
-}
-
-func ext۰syscall۰Open(fr *frame, args []value) value {
-	// func Open(path string, mode int, perm uint32) (fd int, err error) {
-	path := args[0].(string)
-	mode := args[1].(int)
-	perm := args[2].(uint32)
-	fd, err := syscall.Open(path, mode, perm)
-	return tuple{fd, wrapError(err)}
-}
-
-func ext۰syscall۰ParseDirent(fr *frame, args []value) value {
-	// func ParseDirent(buf []byte, max int, names []string) (consumed int, count int, newnames []string)
-	max := args[1].(int)
-	var names []string
-	for _, iname := range args[2].([]value) {
-		names = append(names, iname.(string))
-	}
-	consumed, count, newnames := syscall.ParseDirent(valueToBytes(args[0]), max, names)
-	var inewnames []value
-	for _, newname := range newnames {
-		inewnames = append(inewnames, newname)
-	}
-	return tuple{consumed, count, inewnames}
-}
-
-func ext۰syscall۰RawSyscall(fr *frame, args []value) value {
-	return tuple{uintptr(0), uintptr(0), uintptr(syscall.ENOSYS)}
-}
-
-func ext۰syscall۰Read(fr *frame, args []value) value {
-	// func Read(fd int, p []byte) (n int, err error)
-	fd := args[0].(int)
-	p := args[1].([]value)
-	b := make([]byte, len(p))
-	n, err := syscall.Read(fd, b)
-	for i := 0; i < n; i++ {
-		p[i] = b[i]
-	}
-	return tuple{n, wrapError(err)}
-}
-
-func ext۰syscall۰ReadDirent(fr *frame, args []value) value {
-	// func ReadDirent(fd int, buf []byte) (n int, err error)
-	fd := args[0].(int)
-	p := args[1].([]value)
-	b := make([]byte, len(p))
-	n, err := syscall.ReadDirent(fd, b)
-	for i := 0; i < n; i++ {
-		p[i] = b[i]
-	}
-	return tuple{n, wrapError(err)}
-}
-
-func ext۰syscall۰Readlink(fr *frame, args []value) value {
-	path := args[0].(string)
-	buf := valueToBytes(args[1])
-	n, err := syscall.Readlink(path, buf)
-	return tuple{n, wrapError(err)}
-}
-
-func ext۰syscall۰Rmdir(fr *frame, args []value) value {
-	return wrapError(syscall.Rmdir(args[0].(string)))
-}
-
-func ext۰syscall۰Seek(fr *frame, args []value) value {
-	fd := args[0].(int)
-	offset := args[1].(int64)
-	whence := args[2].(int)
-	new, err := syscall.Seek(fd, offset, whence)
-	return tuple{new, wrapError(err)}
-}
-
-func ext۰syscall۰Stat(fr *frame, args []value) value {
-	// func Stat(name string, stat *Stat_t) (err error)
-	name := args[0].(string)
-	stat := (*args[1].(*value)).(structure)
-
-	var st syscall.Stat_t
-	err := syscall.Stat(name, &st)
-	fillStat(&st, stat)
-	return wrapError(err)
-}
-
-func ext۰syscall۰Symlink(fr *frame, args []value) value {
-	path := args[0].(string)
-	link := args[1].(string)
-	return wrapError(syscall.Symlink(path, link))
-}
-
-func ext۰syscall۰Unlink(fr *frame, args []value) value {
-	return wrapError(syscall.Unlink(args[0].(string)))
-}
-
-func ext۰syscall۰UtimesNano(fr *frame, args []value) value {
-	path := args[0].(string)
-	var ts [2]syscall.Timespec
-	err := syscall.UtimesNano(path, ts[:])
-	// TODO(adonovan): copy the Timespecs into args[1]
-	return wrapError(err)
-}
-
-func ext۰syscall۰Write(fr *frame, args []value) value {
-	// func Write(fd int, p []byte) (n int, err error)
-	n, err := write(args[0].(int), valueToBytes(args[1]))
-	return tuple{n, wrapError(err)}
-}
diff --git a/go/ssa/interp/interp.go b/go/ssa/interp/interp.go
index c0929609..76b7ca3a 100644
--- a/go/ssa/interp/interp.go
+++ b/go/ssa/interp/interp.go
@@ -630,30 +630,6 @@ func setGlobal(i *interpreter, pkg *ssa.Package, name string, v value) {
 	panic("no global variable: " + pkg.Pkg.Path() + "." + name)
 }
 
-var environ []value
-
-func init() {
-	for _, s := range os.Environ() {
-		environ = append(environ, s)
-	}
-	environ = append(environ, "GOSSAINTERP=1")
-	environ = append(environ, "GOARCH="+runtime.GOARCH)
-}
-
-// deleteBodies delete the bodies of all standalone functions except the
-// specified ones.  A missing intrinsic leads to a clear runtime error.
-func deleteBodies(pkg *ssa.Package, except ...string) {
-	keep := make(map[string]bool)
-	for _, e := range except {
-		keep[e] = true
-	}
-	for _, mem := range pkg.Members {
-		if fn, ok := mem.(*ssa.Function); ok && !keep[fn.Name()] {
-			fn.Blocks = nil
-		}
-	}
-}
-
 // 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.  sizes is the
@@ -665,11 +641,6 @@ func deleteBodies(pkg *ssa.Package, except ...string) {
 // The SSA program must include the "runtime" package.
 //
 func Interpret(mainpkg *ssa.Package, mode Mode, sizes types.Sizes, filename string, args []string) (exitCode int) {
-	if syswrite == nil {
-		fmt.Fprintln(os.Stderr, "Interpret: unsupported platform.")
-		return 1
-	}
-
 	i := &interpreter{
 		prog:       mainpkg.Prog,
 		globals:    make(map[ssa.Value]*value),
@@ -699,20 +670,6 @@ func Interpret(mainpkg *ssa.Package, mode Mode, sizes types.Sizes, filename stri
 				i.globals[v] = &cell
 			}
 		}
-
-		// Ad-hoc initialization for magic system variables.
-		switch pkg.Pkg.Path() {
-		case "syscall":
-			setGlobal(i, pkg, "envs", environ)
-
-		case "reflect":
-			deleteBodies(pkg, "DeepEqual", "deepValueEqual")
-
-		case "runtime":
-			sz := sizes.Sizeof(pkg.Pkg.Scope().Lookup("MemStats").Type())
-			setGlobal(i, pkg, "sizeof_C_MStats", uintptr(sz))
-			deleteBodies(pkg, "GOROOT", "gogetenv")
-		}
 	}
 
 	// Top-level error handler.
diff --git a/go/ssa/interp/interp_test.go b/go/ssa/interp/interp_test.go
index 07bb53c2..c1e30ec1 100644
--- a/go/ssa/interp/interp_test.go
+++ b/go/ssa/interp/interp_test.go
@@ -2,18 +2,27 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build linux darwin
-
 package interp_test
 
+// This test runs the SSA interpreter over sample Go programs.
+// Because the interpreter requires intrinsics for assembly
+// functions and many low-level runtime routines, it is inherently
+// not robust to evolutionary change in the standard library.
+// Therefore the test cases are restricted to programs that
+// use a fake standard library in testdata/src containing a tiny
+// subset of simple functions useful for writing assertions.
+//
+// We no longer attempt to interpret any real standard packages such as
+// fmt or testing, as it proved too fragile.
+
 import (
 	"bytes"
 	"fmt"
 	"go/build"
 	"go/types"
+	"log"
 	"os"
 	"path/filepath"
-	"runtime"
 	"strings"
 	"testing"
 	"time"
@@ -28,12 +37,10 @@ import (
 // filenames comprising the main package of a program.
 // They are ordered quickest-first, roughly.
 //
-// TODO(adonovan): integrate into the $GOROOT/test driver scripts,
-// golden file checking, etc.
+// If a test in this list fails spuriously, remove it.
 var gorootTestTests = []string{
 	"235.go",
 	"alias1.go",
-	"chancap.go",
 	"func5.go",
 	"func6.go",
 	"func7.go",
@@ -63,12 +70,10 @@ var gorootTestTests = []string{
 	"bigmap.go",
 	"func.go",
 	"reorder2.go",
-	"closure.go",
 	"gc.go",
 	"simassign.go",
 	"iota.go",
 	"nilptr2.go",
-	"goprint.go", // doesn't actually assert anything (cmpout)
 	"utf.go",
 	"method.go",
 	"char_lit.go",
@@ -86,52 +91,18 @@ var gorootTestTests = []string{
 	"convert.go",
 	"convT2X.go",
 	"switch.go",
-	"initialize.go",
 	"ddd.go",
 	"blank.go", // partly disabled
-	"map.go",
 	"closedchan.go",
 	"divide.go",
 	"rename.go",
-	"const3.go",
 	"nil.go",
-	"recover.go", // reflection parts disabled
 	"recover1.go",
 	"recover2.go",
 	"recover3.go",
 	"typeswitch1.go",
 	"floatcmp.go",
 	"crlf.go", // doesn't actually assert anything (runoutput)
-	// Slow tests follow.
-	"bom.go",                         // ~1.7s
-	"gc1.go",                         // ~1.7s
-	"cmplxdivide.go cmplxdivide1.go", // ~2.4s
-
-	// Working, but not worth enabling:
-	// "append.go",    // works, but slow (15s).
-	// "gc2.go",       // works, but slow, and cheats on the memory check.
-	// "sigchld.go",   // works, but only on POSIX.
-	// "peano.go",     // works only up to n=9, and slow even then.
-	// "stack.go",     // works, but too slow (~30s) by default.
-	// "solitaire.go", // works, but too slow (~30s).
-	// "const.go",     // works but for but one bug: constant folder doesn't consider representations.
-	// "init1.go",     // too slow (80s) and not that interesting. Cheats on ReadMemStats check too.
-	// "rotate.go rotate0.go", // emits source for a test
-	// "rotate.go rotate1.go", // emits source for a test
-	// "rotate.go rotate2.go", // emits source for a test
-	// "rotate.go rotate3.go", // emits source for a test
-	// "64bit.go",             // emits source for a test
-	// "run.go",               // test driver, not a test.
-
-	// Broken.  TODO(adonovan): fix.
-	// copy.go         // very slow; but with N=4 quickly crashes, slice index out of range.
-	// nilptr.go       // interp: V > uintptr not implemented. Slow test, lots of mem
-	// args.go         // works, but requires specific os.Args from the driver.
-	// index.go        // a template, not a real test.
-	// mallocfin.go    // SetFinalizer not implemented.
-
-	// TODO(adonovan): add tests from $GOROOT/test/* subtrees:
-	// bench chan bugs fixedbugs interface ken.
 }
 
 // These are files in go.tools/go/ssa/interp/testdata/.
@@ -150,31 +121,21 @@ var testdataTests = []string{
 	"recover.go",
 	"reflect.go",
 	"static.go",
-	"callstack.go",
 }
 
-type successPredicate func(exitcode int, output string) error
-
-func run(t *testing.T, dir, input string, success successPredicate) bool {
-	t.Skip("https://golang.org/issue/27292")
-	if runtime.GOOS == "darwin" {
-		t.Skip("skipping on darwin until https://golang.org/issue/23166 is fixed")
-	}
-	fmt.Printf("Input: %s\n", input)
+func run(t *testing.T, input string) bool {
+	t.Logf("Input: %s\n", input)
 
 	start := time.Now()
 
-	var inputs []string
-	for _, i := range strings.Split(input, " ") {
-		if strings.HasSuffix(i, ".go") {
-			i = dir + i
-		}
-		inputs = append(inputs, i)
-	}
+	ctx := build.Default    // copy
+	ctx.GOROOT = "testdata" // fake goroot
+	ctx.GOOS = "linux"
+	ctx.GOARCH = "amd64"
 
-	var conf loader.Config
-	if _, err := conf.FromArgs(inputs, true); err != nil {
-		t.Errorf("FromArgs(%s) failed: %s", inputs, err)
+	conf := loader.Config{Build: &ctx}
+	if _, err := conf.FromArgs([]string{input}, true); err != nil {
+		t.Errorf("FromArgs(%s) failed: %s", input, err)
 		return false
 	}
 
@@ -193,61 +154,43 @@ func run(t *testing.T, dir, input string, success successPredicate) bool {
 		interp.CapturedOutput = nil
 	}()
 
-	hint = fmt.Sprintf("To dump SSA representation, run:\n%% go build golang.org/x/tools/cmd/ssadump && ./ssadump -test -build=CFP %s\n", strings.Join(inputs, " "))
+	hint = fmt.Sprintf("To dump SSA representation, run:\n%% go build golang.org/x/tools/cmd/ssadump && ./ssadump -test -build=CFP %s\n", input)
 
 	iprog, err := conf.Load()
 	if err != nil {
-		t.Errorf("conf.Load(%s) failed: %s", inputs, err)
+		t.Errorf("conf.Load(%s) failed: %s", input, err)
 		return false
 	}
 
 	prog := ssautil.CreateProgram(iprog, ssa.SanityCheckFunctions)
 	prog.Build()
 
-	// Find first main or test package among the initial packages.
-	var mainPkg *ssa.Package
-	for _, info := range iprog.InitialPackages() {
-		if info.Pkg.Path() == "runtime" {
-			continue // not an initial package
-		}
-		p := prog.Package(info.Pkg)
-		if p.Pkg.Name() == "main" && p.Func("main") != nil {
-			mainPkg = p
-			break
-		}
-
-		mainPkg = prog.CreateTestMainPackage(p)
-		if mainPkg != nil {
-			break
-		}
-	}
+	mainPkg := prog.Package(iprog.Created[0].Pkg)
 	if mainPkg == nil {
-		t.Fatalf("no main or test packages among initial packages: %s", inputs)
+		t.Fatalf("not a main package: %s", input)
 	}
 
-	var out bytes.Buffer
-	interp.CapturedOutput = &out
+	interp.CapturedOutput = new(bytes.Buffer)
 
-	hint = fmt.Sprintf("To trace execution, run:\n%% go build golang.org/x/tools/cmd/ssadump && ./ssadump -build=C -test -run --interp=T %s\n", strings.Join(inputs, " "))
-	exitCode := interp.Interpret(mainPkg, 0, &types.StdSizes{WordSize: 8, MaxAlign: 8}, inputs[0], []string{})
-
-	// The definition of success varies with each file.
-	if err := success(exitCode, out.String()); err != nil {
-		t.Errorf("interp.Interpret(%s) failed: %s", inputs, err)
-		return false
+	hint = fmt.Sprintf("To trace execution, run:\n%% go build golang.org/x/tools/cmd/ssadump && ./ssadump -build=C -test -run --interp=T %s\n", input)
+	exitCode := interp.Interpret(mainPkg, 0, &types.StdSizes{WordSize: 8, MaxAlign: 8}, input, []string{})
+	if exitCode != 0 {
+		t.Fatalf("interpreting %s: exit code was %d", input, exitCode)
+	}
+	// $GOROOT/test tests use this convention:
+	if strings.Contains(interp.CapturedOutput.String(), "BUG") {
+		t.Fatalf("interpreting %s: exited zero but output contained 'BUG'", input)
 	}
 
 	hint = "" // call off the hounds
 
 	if false {
-		fmt.Println(input, time.Since(start)) // test profiling
+		t.Log(input, time.Since(start)) // test profiling
 	}
 
 	return true
 }
 
-const slash = string(os.PathSeparator)
-
 func printFailures(failures []string) {
 	if failures != nil {
 		fmt.Println("The following tests failed:")
@@ -257,26 +200,16 @@ func printFailures(failures []string) {
 	}
 }
 
-func success(exitcode int, output string) error {
-	if exitcode != 0 {
-		return fmt.Errorf("exit code was %d", exitcode)
-	}
-	if strings.Contains(output, "BUG") {
-		return fmt.Errorf("exited zero but output contained 'BUG'")
-	}
-	return nil
-}
-
 // TestTestdataFiles runs the interpreter on testdata/*.go.
 func TestTestdataFiles(t *testing.T) {
+	cwd, err := os.Getwd()
+	if err != nil {
+		log.Fatal(err)
+	}
+
 	var failures []string
-	start := time.Now()
 	for _, input := range testdataTests {
-		if testing.Short() && time.Since(start) > 30*time.Second {
-			printFailures(failures)
-			t.Skipf("timeout - aborting test")
-		}
-		if !run(t, "testdata"+slash, input, success) {
+		if !run(t, filepath.Join(cwd, "testdata", input)) {
 			failures = append(failures, input)
 		}
 	}
@@ -285,34 +218,12 @@ func TestTestdataFiles(t *testing.T) {
 
 // TestGorootTest runs the interpreter on $GOROOT/test/*.go.
 func TestGorootTest(t *testing.T) {
-	if testing.Short() {
-		t.Skip() // too slow (~30s)
-	}
-
 	var failures []string
 
 	for _, input := range gorootTestTests {
-		if !run(t, filepath.Join(build.Default.GOROOT, "test")+slash, input, success) {
+		if !run(t, filepath.Join(build.Default.GOROOT, "test", input)) {
 			failures = append(failures, input)
 		}
 	}
 	printFailures(failures)
 }
-
-// CreateTestMainPackage should return nil if there were no tests.
-func TestNullTestmainPackage(t *testing.T) {
-	var conf loader.Config
-	conf.CreateFromFilenames("", "testdata/b_test.go")
-	iprog, err := conf.Load()
-	if err != nil {
-		t.Fatalf("CreatePackages failed: %s", err)
-	}
-	prog := ssautil.CreateProgram(iprog, ssa.SanityCheckFunctions)
-	mainPkg := prog.Package(iprog.Created[0].Pkg)
-	if mainPkg.Func("main") != nil {
-		t.Fatalf("unexpected main function")
-	}
-	if prog.CreateTestMainPackage(mainPkg) != nil {
-		t.Fatalf("CreateTestMainPackage returned non-nil")
-	}
-}
diff --git a/go/ssa/interp/ops.go b/go/ssa/interp/ops.go
index 334d5b7b..ca004d71 100644
--- a/go/ssa/interp/ops.go
+++ b/go/ssa/interp/ops.go
@@ -10,6 +10,7 @@ import (
 	"go/constant"
 	"go/token"
 	"go/types"
+	"os"
 	"strings"
 	"sync"
 	"unsafe"
@@ -918,21 +919,18 @@ func typeAssert(i *interpreter, instr *ssa.TypeAssert, itf iface) value {
 var CapturedOutput *bytes.Buffer
 var capturedOutputMu sync.Mutex
 
-// write writes bytes b to the target program's file descriptor fd.
+// write writes bytes b to the target program's standard output.
 // The print/println built-ins and the write() system call funnel
 // through here so they can be captured by the test driver.
-func write(fd int, b []byte) (int, error) {
-	// TODO(adonovan): fix: on Windows, std{out,err} are not 1, 2.
-	if CapturedOutput != nil && (fd == 1 || fd == 2) {
+func print(b []byte) (int, error) {
+	if CapturedOutput != nil {
 		capturedOutputMu.Lock()
 		CapturedOutput.Write(b) // ignore errors
 		capturedOutputMu.Unlock()
 	}
-	return syswrite(fd, b)
+	return os.Stdout.Write(b)
 }
 
-var syswrite func(int, []byte) (int, error) // set on darwin/linux only
-
 // callBuiltin interprets a call to builtin fn with arguments args,
 // returning its result.
 func callBuiltin(caller *frame, callpos token.Pos, fn *ssa.Builtin, args []value) value {
@@ -987,7 +985,7 @@ func callBuiltin(caller *frame, callpos token.Pos, fn *ssa.Builtin, args []value
 		if ln {
 			buf.WriteRune('\n')
 		}
-		write(1, buf.Bytes())
+		print(buf.Bytes())
 		return nil
 
 	case "len":
diff --git a/go/ssa/interp/reflect.go b/go/ssa/interp/reflect.go
index 2b828996..0a4465b0 100644
--- a/go/ssa/interp/reflect.go
+++ b/go/ssa/interp/reflect.go
@@ -68,11 +68,6 @@ func makeReflectType(rt rtype) value {
 	return iface{rtypeType, rt}
 }
 
-func ext۰reflect۰Init(fr *frame, args []value) value {
-	// Signature: func()
-	return nil
-}
-
 func ext۰reflect۰rtype۰Bits(fr *frame, args []value) value {
 	// Signature: func (t reflect.rtype) int
 	rt := args[0].(rtype).t
diff --git a/go/ssa/interp/testdata/a_test.go b/go/ssa/interp/testdata/a_test.go
deleted file mode 100644
index 844ec5cd..00000000
--- a/go/ssa/interp/testdata/a_test.go
+++ /dev/null
@@ -1,17 +0,0 @@
-package a
-
-import "testing"
-
-func TestFoo(t *testing.T) {
-	t.Error("foo")
-}
-
-func TestBar(t *testing.T) {
-	t.Error("bar")
-}
-
-func BenchmarkWiz(b *testing.B) {
-	b.Error("wiz")
-}
-
-// Don't test Examples since that testing package needs pipe(2) for that.
diff --git a/go/ssa/interp/testdata/b_test.go b/go/ssa/interp/testdata/b_test.go
deleted file mode 100644
index 4a30e96a..00000000
--- a/go/ssa/interp/testdata/b_test.go
+++ /dev/null
@@ -1,11 +0,0 @@
-package b
-
-import "testing"
-
-func NotATest(t *testing.T) {
-	t.Error("foo")
-}
-
-func NotABenchmark(b *testing.B) {
-	b.Error("wiz")
-}
diff --git a/go/ssa/interp/testdata/boundmeth.go b/go/ssa/interp/testdata/boundmeth.go
index 255cc607..69937f9d 100644
--- a/go/ssa/interp/testdata/boundmeth.go
+++ b/go/ssa/interp/testdata/boundmeth.go
@@ -2,7 +2,10 @@
 
 package main
 
-import "fmt"
+import (
+	"errors"
+	"fmt"
+)
 
 func assert(b bool) {
 	if !b {
@@ -105,7 +108,7 @@ func regress1(x error) func() string {
 // Regression test for b/7269:
 // taking the value of an interface method performs a nil check.
 func nilInterfaceMethodValue() {
-	err := fmt.Errorf("ok")
+	err := errors.New("ok")
 	f := err.Error
 	if got := f(); got != "ok" {
 		panic(got)
@@ -119,7 +122,7 @@ func nilInterfaceMethodValue() {
 	defer func() {
 		r := fmt.Sprint(recover())
 		// runtime panic string varies across toolchains
-		if r != "runtime error: interface conversion: interface is nil, not error" &&
+		if r != "interface conversion: interface is nil, not error" &&
 			r != "runtime error: invalid memory address or nil pointer dereference" {
 			panic("want runtime panic from nil interface method value, got " + r)
 		}
diff --git a/go/ssa/interp/testdata/c_test.go b/go/ssa/interp/testdata/c_test.go
deleted file mode 100644
index ad80b910..00000000
--- a/go/ssa/interp/testdata/c_test.go
+++ /dev/null
@@ -1,17 +0,0 @@
-package c_test
-
-import (
-	"os"
-	"testing"
-)
-
-func TestC(t *testing.T) {
-	println("TestC")
-}
-
-func TestMain(m *testing.M) {
-	println("TestMain start")
-	code := m.Run()
-	println("TestMain end")
-	os.Exit(code)
-}
diff --git a/go/ssa/interp/testdata/callstack.go b/go/ssa/interp/testdata/callstack.go
deleted file mode 100644
index 56f3b281..00000000
--- a/go/ssa/interp/testdata/callstack.go
+++ /dev/null
@@ -1,52 +0,0 @@
-package main
-
-import (
-	"fmt"
-	"path"
-	"runtime"
-	"strings"
-)
-
-var stack string
-
-func f() {
-	pc := make([]uintptr, 6)
-	pc = pc[:runtime.Callers(1, pc)]
-	for _, f := range pc {
-		Func := runtime.FuncForPC(f)
-		name := Func.Name()
-		if strings.Contains(name, "$") || strings.Contains(name, ".func") {
-			name = "func" // anon funcs vary across toolchains
-		}
-		file, line := Func.FileLine(0)
-		stack += fmt.Sprintf("%s at %s:%d\n", name, path.Base(file), line)
-	}
-}
-
-func g() { f() }
-func h() { g() }
-func i() { func() { h() }() }
-
-// Hack: the 'func' and the call to Caller are on the same line,
-// to paper over differences between toolchains.
-// (The interpreter's location info isn't yet complete.)
-func runtimeCaller0() (uintptr, string, int, bool) { return runtime.Caller(0) }
-
-func main() {
-	i()
-	if stack != `main.f at callstack.go:12
-main.g at callstack.go:26
-main.h at callstack.go:27
-func at callstack.go:28
-main.i at callstack.go:28
-main.main at callstack.go:35
-` {
-		panic("unexpected stack: " + stack)
-	}
-
-	pc, file, line, _ := runtimeCaller0()
-	got := fmt.Sprintf("%s @ %s:%d", runtime.FuncForPC(pc).Name(), path.Base(file), line)
-	if got != "main.runtimeCaller0 @ callstack.go:33" {
-		panic("runtime.Caller: " + got)
-	}
-}
diff --git a/go/ssa/interp/testdata/coverage.go b/go/ssa/interp/testdata/coverage.go
index 37ef95c6..00c7165c 100644
--- a/go/ssa/interp/testdata/coverage.go
+++ b/go/ssa/interp/testdata/coverage.go
@@ -9,6 +9,7 @@ package main
 import (
 	"fmt"
 	"reflect"
+	"strings"
 )
 
 func init() {
@@ -171,7 +172,7 @@ func main() {
 
 	// fmt.
 	const message = "Hello, World!"
-	if fmt.Sprintf("%s, %s!", "Hello", "World") != message {
+	if fmt.Sprint("Hello", ", ", "World", "!") != message {
 		panic("oops")
 	}
 
@@ -425,7 +426,7 @@ func init() {
 	// But two slices cannot be compared, even if one is nil.
 	defer func() {
 		r := fmt.Sprint(recover())
-		if r != "runtime error: comparing uncomparable type []string" {
+		if !(strings.Contains(r, "compar") && strings.Contains(r, "[]string")) {
 			panic("want panic from slice comparison, got " + r)
 		}
 	}()
@@ -508,7 +509,7 @@ func init() {
 		r := fmt.Sprint(recover())
 		// Exact error varies by toolchain:
 		if r != "runtime error: value method (main.T).f called using nil *main.T pointer" &&
-			r != "value method main.T.f called using nil *T pointer" {
+			r != "value method (main.T).f called using nil *main.T pointer" {
 			panic("want panic from call with nil receiver, got " + r)
 		}
 	}()
diff --git a/go/ssa/interp/testdata/range.go b/go/ssa/interp/testdata/range.go
index da8a421e..a1063366 100644
--- a/go/ssa/interp/testdata/range.go
+++ b/go/ssa/interp/testdata/range.go
@@ -23,7 +23,7 @@ func init() {
 	}
 	s := ""
 	for _, r := range runes {
-		s = fmt.Sprintf("%s%c", s, r)
+		s += string(r)
 	}
 	if s != "Hello, 世界" {
 		panic(s)
diff --git a/go/ssa/interp/testdata/src/errors/errors.go b/go/ssa/interp/testdata/src/errors/errors.go
new file mode 100644
index 00000000..2377563c
--- /dev/null
+++ b/go/ssa/interp/testdata/src/errors/errors.go
@@ -0,0 +1,7 @@
+package errors
+
+func New(text string) error { return errorString{text} }
+
+type errorString struct{ s string }
+
+func (e errorString) Error() string { return e.s }
diff --git a/go/ssa/interp/testdata/src/fmt/fmt.go b/go/ssa/interp/testdata/src/fmt/fmt.go
new file mode 100644
index 00000000..2185eb70
--- /dev/null
+++ b/go/ssa/interp/testdata/src/fmt/fmt.go
@@ -0,0 +1,26 @@
+package fmt
+
+func Sprint(args ...interface{}) string
+
+func Print(args ...interface{}) {
+	for i, arg := range args {
+		if i > 0 {
+			print(" ")
+		}
+		print(Sprint(arg))
+	}
+}
+
+func Println(args ...interface{}) {
+	Print(args...)
+	println()
+}
+
+// formatting is too complex to fake
+
+func Printf(args ...interface{}) string {
+	panic("Printf is not supported")
+}
+func Sprintf(format string, args ...interface{}) string {
+	panic("Sprintf is not supported")
+}
diff --git a/go/ssa/interp/testdata/src/math/math.go b/go/ssa/interp/testdata/src/math/math.go
new file mode 100644
index 00000000..f51e5f57
--- /dev/null
+++ b/go/ssa/interp/testdata/src/math/math.go
@@ -0,0 +1,13 @@
+package math
+
+func NaN() float64
+
+func Inf(int) float64
+
+func IsNaN(float64) bool
+
+func Float64bits(float64) uint64
+
+func Signbit(x float64) bool {
+	return Float64bits(x)&(1<<63) != 0
+}
diff --git a/go/ssa/interp/testdata/src/os/os.go b/go/ssa/interp/testdata/src/os/os.go
new file mode 100644
index 00000000..555ef549
--- /dev/null
+++ b/go/ssa/interp/testdata/src/os/os.go
@@ -0,0 +1,5 @@
+package os
+
+func Getenv(string) string
+
+func Exit(int)
diff --git a/go/ssa/interp/testdata/src/reflect/reflect.go b/go/ssa/interp/testdata/src/reflect/reflect.go
new file mode 100644
index 00000000..f6c4e279
--- /dev/null
+++ b/go/ssa/interp/testdata/src/reflect/reflect.go
@@ -0,0 +1,16 @@
+package reflect
+
+type Type interface {
+	String() string
+}
+
+type Value struct {
+}
+
+func (Value) String() string
+
+func SliceOf(Type) Type
+
+func TypeOf(interface{}) Type
+
+func ValueOf(interface{}) Value
diff --git a/go/ssa/interp/testdata/src/runtime/runtime.go b/go/ssa/interp/testdata/src/runtime/runtime.go
new file mode 100644
index 00000000..c60c7fc2
--- /dev/null
+++ b/go/ssa/interp/testdata/src/runtime/runtime.go
@@ -0,0 +1,22 @@
+package runtime
+
+// An errorString represents a runtime error described by a single string.
+type errorString string
+
+func (e errorString) RuntimeError() {}
+
+func (e errorString) Error() string {
+	return "runtime error: " + string(e)
+}
+
+func Breakpoint()
+
+type Error interface {
+	error
+	RuntimeError()
+}
+
+const GOOS = "linux"
+const GOARCH = "amd64"
+
+func GC()
diff --git a/go/ssa/interp/testdata/src/strings/strings.go b/go/ssa/interp/testdata/src/strings/strings.go
new file mode 100644
index 00000000..dd86dcf4
--- /dev/null
+++ b/go/ssa/interp/testdata/src/strings/strings.go
@@ -0,0 +1,9 @@
+package strings
+
+func Replace(s, old, new string, n int) string
+
+func Index(haystack, needle string) int
+
+func Contains(haystack, needle string) bool {
+	return Index(haystack, needle) >= 0
+}
diff --git a/go/ssa/interp/testdata/src/time/time.go b/go/ssa/interp/testdata/src/time/time.go
new file mode 100644
index 00000000..66aa9ab1
--- /dev/null
+++ b/go/ssa/interp/testdata/src/time/time.go
@@ -0,0 +1,5 @@
+package time
+
+type Duration int64
+
+func Sleep(Duration)
diff --git a/go/ssa/interp/testdata/src/unicode/utf8/utf8.go b/go/ssa/interp/testdata/src/unicode/utf8/utf8.go
new file mode 100644
index 00000000..0e44f7cb
--- /dev/null
+++ b/go/ssa/interp/testdata/src/unicode/utf8/utf8.go
@@ -0,0 +1,9 @@
+package utf8
+
+func DecodeRuneInString(string) (rune, int)
+
+func DecodeRune(b []byte) (rune, int) {
+	return DecodeRuneInString(string(b))
+}
+
+const RuneError = '\uFFFD'
diff --git a/go/ssa/interp/testdata/src/unsafe/unsafe.go b/go/ssa/interp/testdata/src/unsafe/unsafe.go
new file mode 100644
index 00000000..2d727856
--- /dev/null
+++ b/go/ssa/interp/testdata/src/unsafe/unsafe.go
@@ -0,0 +1 @@
+package unsafe