src/runtime/iface.go - go - Git at Google

 // 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.

 package runtime

 import (
 	"runtime/internal/atomic"
 	"runtime/internal/sys"
 	"unsafe"
 )

 const itabInitSize = 512

 var (
 	itabLock      mutex                               // lock for accessing itab table
 	itabTable     = &itabTableInit                    // pointer to current table
 	itabTableInit = itabTableType{size: itabInitSize} // starter table
 )

 //Note: change the formula in the mallocgc call in itabAdd if you change these fields.
 type itabTableType struct {
 	size    uintptr             // length of entries array. Always a power of 2.
 	count   uintptr             // current number of filled entries.
 	entries [itabInitSize]*itab // really [size] large
 }

 func itabHashFunc(inter *interfacetype, typ *_type) uintptr {
 	// compiler has provided some good hash codes for us.
 	return uintptr(inter.typ.hash ^ typ.hash)
 }

 func getitab(inter *interfacetype, typ *_type, canfail bool) *itab {
 	if len(inter.mhdr) == 0 {
 		throw("internal error - misuse of itab")
 	}

 	// easy case
 	if typ.tflag&tflagUncommon == 0 {
 		if canfail {
 			return nil
 		}
 		name := inter.typ.nameOff(inter.mhdr[0].name)
 		panic(&TypeAssertionError{"", typ.string(), inter.typ.string(), name.name()})
 	}

 	var m *itab

 	// First, look in the existing table to see if we can find the itab we need.
 	// This is by far the most common case, so do it without locks.
 	// Use atomic to ensure we see any previous writes done by the thread
 	// that updates the itabTable field (with atomic.Storep in itabAdd).
 	t := (*itabTableType)(atomic.Loadp(unsafe.Pointer(&itabTable)))
 	if m = t.find(inter, typ); m != nil {
 		goto finish
 	}

 	// Not found.  Grab the lock and try again.
 	lock(&itabLock)
 	if m = itabTable.find(inter, typ); m != nil {
 		unlock(&itabLock)
 		goto finish
 	}

 	// Entry doesn't exist yet. Make a new entry & add it.
 	m = (*itab)(persistentalloc(unsafe.Sizeof(itab{})+uintptr(len(inter.mhdr)-1)*sys.PtrSize, 0, &memstats.other_sys))
 	m.inter = inter
 	m._type = typ
 	m.init()
 	itabAdd(m)
 	unlock(&itabLock)
 finish:
 	if m.fun[0] != 0 {
 		return m
 	}
 	if canfail {
 		return nil
 	}
 	// this can only happen if the conversion
 	// was already done once using the , ok form
 	// and we have a cached negative result.
 	// The cached result doesn't record which
 	// interface function was missing, so initialize
 	// the itab again to get the missing function name.
 	panic(&TypeAssertionError{concreteString: typ.string(), assertedString: inter.typ.string(), missingMethod: m.init()})
 }

 // find finds the given interface/type pair in t.
 // Returns nil if the given interface/type pair isn't present.
 func (t *itabTableType) find(inter *interfacetype, typ *_type) *itab {
 	// Implemented using quadratic probing.
 	// Probe sequence is h(i) = h0 + i*(i+1)/2 mod 2^k.
 	// We're guaranteed to hit all table entries using this probe sequence.
 	mask := t.size - 1
 	h := itabHashFunc(inter, typ) & mask
 	for i := uintptr(1); ; i++ {
 		p := (**itab)(add(unsafe.Pointer(&t.entries), h*sys.PtrSize))
 		// Use atomic read here so if we see m != nil, we also see
 		// the initializations of the fields of m.
 		// m := *p
 		m := (*itab)(atomic.Loadp(unsafe.Pointer(p)))
 		if m == nil {
 			return nil
 		}
 		if m.inter == inter && m._type == typ {
 			return m
 		}
 		h += i
 		h &= mask
 	}
 }

 // itabAdd adds the given itab to the itab hash table.
 // itabLock must be held.
 func itabAdd(m *itab) {
 	t := itabTable
 	if t.count >= 3*(t.size/4) { // 75% load factor
 		// Grow hash table.
 		// t2 = new(itabTableType) + some additional entries
 		// We lie and tell malloc we want pointer-free memory because
 		// all the pointed-to values are not in the heap.
 		t2 := (*itabTableType)(mallocgc((2+2*t.size)*sys.PtrSize, nil, true))
 		t2.size = t.size * 2

 		// Copy over entries.
 		// Note: while copying, other threads may look for an itab and
 		// fail to find it. That's ok, they will then try to get the itab lock
 		// and as a consequence wait until this copying is complete.
 		iterate_itabs(t2.add)
 		if t2.count != t.count {
 			throw("mismatched count during itab table copy")
 		}
 		// Publish new hash table. Use an atomic write: see comment in getitab.
 		atomicstorep(unsafe.Pointer(&itabTable), unsafe.Pointer(t2))
 		// Adopt the new table as our own.
 		t = itabTable
 		// Note: the old table can be GC'ed here.
 	}
 	t.add(m)
 }

 // add adds the given itab to itab table t.
 // itabLock must be held.
 func (t *itabTableType) add(m *itab) {
 	// See comment in find about the probe sequence.
 	// Insert new itab in the first empty spot in the probe sequence.
 	mask := t.size - 1
 	h := itabHashFunc(m.inter, m._type) & mask
 	for i := uintptr(1); ; i++ {
 		p := (**itab)(add(unsafe.Pointer(&t.entries), h*sys.PtrSize))
 		m2 := *p
 		if m2 == m {
 			// A given itab may be used in more than one module
 			// and thanks to the way global symbol resolution works, the
 			// pointed-to itab may already have been inserted into the
 			// global 'hash'.
 			return
 		}
 		if m2 == nil {
 			// Use atomic write here so if a reader sees m, it also
 			// sees the correctly initialized fields of m.
 			// NoWB is ok because m is not in heap memory.
 			// *p = m
 			atomic.StorepNoWB(unsafe.Pointer(p), unsafe.Pointer(m))
 			t.count++
 			return
 		}
 		h += i
 		h &= mask
 	}
 }

 // init fills in the m.fun array with all the code pointers for
 // the m.inter/m._type pair. If the type does not implement the interface,
 // it sets m.fun[0] to 0 and returns the name of an interface function that is missing.
 // It is ok to call this multiple times on the same m, even concurrently.
 func (m *itab) init() string {
 	inter := m.inter
 	typ := m._type
 	x := typ.uncommon()

 	// both inter and typ have method sorted by name,
 	// and interface names are unique,
 	// so can iterate over both in lock step;
 	// the loop is O(ni+nt) not O(ni*nt).
 	ni := len(inter.mhdr)
 	nt := int(x.mcount)
 	xmhdr := (*[1 << 16]method)(add(unsafe.Pointer(x), uintptr(x.moff)))[:nt:nt]
 	j := 0
 imethods:
 	for k := 0; k < ni; k++ {
 		i := &inter.mhdr[k]
 		itype := inter.typ.typeOff(i.ityp)
 		name := inter.typ.nameOff(i.name)
 		iname := name.name()
 		ipkg := name.pkgPath()
 		if ipkg == "" {
 			ipkg = inter.pkgpath.name()
 		}
 		for ; j < nt; j++ {
 			t := &xmhdr[j]
 			tname := typ.nameOff(t.name)
 			if typ.typeOff(t.mtyp) == itype && tname.name() == iname {
 				pkgPath := tname.pkgPath()
 				if pkgPath == "" {
 					pkgPath = typ.nameOff(x.pkgpath).name()
 				}
 				if tname.isExported() || pkgPath == ipkg {
 					if m != nil {
 						ifn := typ.textOff(t.ifn)
 						*(*unsafe.Pointer)(add(unsafe.Pointer(&m.fun[0]), uintptr(k)*sys.PtrSize)) = ifn
 					}
 					continue imethods
 				}
 			}
 		}
 		// didn't find method
 		m.fun[0] = 0
 		return iname
 	}
 	m.hash = typ.hash
 	return ""
 }

 func itabsinit() {
 	lock(&itabLock)
 	for _, md := range activeModules() {
 		for _, i := range md.itablinks {
 			itabAdd(i)
 		}
 	}
 	unlock(&itabLock)
 }

 // panicdottypeE is called when doing an e.(T) conversion and the conversion fails.
 // have = the dynamic type we have.
 // want = the static type we're trying to convert to.
 // iface = the static type we're converting from.
 func panicdottypeE(have, want, iface *_type) {
 	haveString := ""
 	if have != nil {
 		haveString = have.string()
 	}
 	panic(&TypeAssertionError{iface.string(), haveString, want.string(), ""})
 }

 // panicdottypeI is called when doing an i.(T) conversion and the conversion fails.
 // Same args as panicdottypeE, but "have" is the dynamic itab we have.
 func panicdottypeI(have *itab, want, iface *_type) {
 	var t *_type
 	if have != nil {
 		t = have._type
 	}
 	panicdottypeE(t, want, iface)
 }

 // panicnildottype is called when doing a i.(T) conversion and the interface i is nil.
 // want = the static type we're trying to convert to.
 func panicnildottype(want *_type) {
 	panic(&TypeAssertionError{"", "", want.string(), ""})
 	// TODO: Add the static type we're converting from as well.
 	// It might generate a better error message.
 	// Just to match other nil conversion errors, we don't for now.
 }

 // The conv and assert functions below do very similar things.
 // The convXXX functions are guaranteed by the compiler to succeed.
 // The assertXXX functions may fail (either panicking or returning false,
 // depending on whether they are 1-result or 2-result).
 // The convXXX functions succeed on a nil input, whereas the assertXXX
 // functions fail on a nil input.

 func convT2E(t *_type, elem unsafe.Pointer) (e eface) {
 	if raceenabled {
 		raceReadObjectPC(t, elem, getcallerpc(), funcPC(convT2E))
 	}
 	if msanenabled {
 		msanread(elem, t.size)
 	}
 	x := mallocgc(t.size, t, true)
 	// TODO: We allocate a zeroed object only to overwrite it with actual data.
 	// Figure out how to avoid zeroing. Also below in convT2Eslice, convT2I, convT2Islice.
 	typedmemmove(t, x, elem)
 	e._type = t
 	e.data = x
 	return
 }

 func convT2E16(t *_type, elem unsafe.Pointer) (e eface) {
 	if raceenabled {
 		raceReadObjectPC(t, elem, getcallerpc(), funcPC(convT2E16))
 	}
 	if msanenabled {
 		msanread(elem, t.size)
 	}
 	var x unsafe.Pointer
 	if *(*uint16)(elem) == 0 {
 		x = unsafe.Pointer(&zeroVal[0])
 	} else {
 		x = mallocgc(2, t, false)
 		*(*uint16)(x) = *(*uint16)(elem)
 	}
 	e._type = t
 	e.data = x
 	return
 }

 func convT2E32(t *_type, elem unsafe.Pointer) (e eface) {
 	if raceenabled {
 		raceReadObjectPC(t, elem, getcallerpc(), funcPC(convT2E32))
 	}
 	if msanenabled {
 		msanread(elem, t.size)
 	}
 	var x unsafe.Pointer
 	if *(*uint32)(elem) == 0 {
 		x = unsafe.Pointer(&zeroVal[0])
 	} else {
 		x = mallocgc(4, t, false)
 		*(*uint32)(x) = *(*uint32)(elem)
 	}
 	e._type = t
 	e.data = x
 	return
 }

 func convT2E64(t *_type, elem unsafe.Pointer) (e eface) {
 	if raceenabled {
 		raceReadObjectPC(t, elem, getcallerpc(), funcPC(convT2E64))
 	}
 	if msanenabled {
 		msanread(elem, t.size)
 	}
 	var x unsafe.Pointer
 	if *(*uint64)(elem) == 0 {
 		x = unsafe.Pointer(&zeroVal[0])
 	} else {
 		x = mallocgc(8, t, false)
 		*(*uint64)(x) = *(*uint64)(elem)
 	}
 	e._type = t
 	e.data = x
 	return
 }

 func convT2Estring(t *_type, elem unsafe.Pointer) (e eface) {
 	if raceenabled {
 		raceReadObjectPC(t, elem, getcallerpc(), funcPC(convT2Estring))
 	}
 	if msanenabled {
 		msanread(elem, t.size)
 	}
 	var x unsafe.Pointer
 	if *(*string)(elem) == "" {
 		x = unsafe.Pointer(&zeroVal[0])
 	} else {
 		x = mallocgc(t.size, t, true)
 		*(*string)(x) = *(*string)(elem)
 	}
 	e._type = t
 	e.data = x
 	return
 }

 func convT2Eslice(t *_type, elem unsafe.Pointer) (e eface) {
 	if raceenabled {
 		raceReadObjectPC(t, elem, getcallerpc(), funcPC(convT2Eslice))
 	}
 	if msanenabled {
 		msanread(elem, t.size)
 	}
 	var x unsafe.Pointer
 	if v := *(*slice)(elem); uintptr(v.array) == 0 {
 		x = unsafe.Pointer(&zeroVal[0])
 	} else {
 		x = mallocgc(t.size, t, true)
 		*(*slice)(x) = *(*slice)(elem)
 	}
 	e._type = t
 	e.data = x
 	return
 }

 func convT2Enoptr(t *_type, elem unsafe.Pointer) (e eface) {
 	if raceenabled {
 		raceReadObjectPC(t, elem, getcallerpc(), funcPC(convT2Enoptr))
 	}
 	if msanenabled {
 		msanread(elem, t.size)
 	}
 	x := mallocgc(t.size, t, false)
 	memmove(x, elem, t.size)
 	e._type = t
 	e.data = x
 	return
 }

 func convT2I(tab *itab, elem unsafe.Pointer) (i iface) {
 	t := tab._type
 	if raceenabled {
 		raceReadObjectPC(t, elem, getcallerpc(), funcPC(convT2I))
 	}
 	if msanenabled {
 		msanread(elem, t.size)
 	}
 	x := mallocgc(t.size, t, true)
 	typedmemmove(t, x, elem)
 	i.tab = tab
 	i.data = x
 	return
 }

 func convT2I16(tab *itab, elem unsafe.Pointer) (i iface) {
 	t := tab._type
 	if raceenabled {
 		raceReadObjectPC(t, elem, getcallerpc(), funcPC(convT2I16))
 	}
 	if msanenabled {
 		msanread(elem, t.size)
 	}
 	var x unsafe.Pointer
 	if *(*uint16)(elem) == 0 {
 		x = unsafe.Pointer(&zeroVal[0])
 	} else {
 		x = mallocgc(2, t, false)
 		*(*uint16)(x) = *(*uint16)(elem)
 	}
 	i.tab = tab
 	i.data = x
 	return
 }

 func convT2I32(tab *itab, elem unsafe.Pointer) (i iface) {
 	t := tab._type
 	if raceenabled {
 		raceReadObjectPC(t, elem, getcallerpc(), funcPC(convT2I32))
 	}
 	if msanenabled {
 		msanread(elem, t.size)
 	}
 	var x unsafe.Pointer
 	if *(*uint32)(elem) == 0 {
 		x = unsafe.Pointer(&zeroVal[0])
 	} else {
 		x = mallocgc(4, t, false)
 		*(*uint32)(x) = *(*uint32)(elem)
 	}
 	i.tab = tab
 	i.data = x
 	return
 }

 func convT2I64(tab *itab, elem unsafe.Pointer) (i iface) {
 	t := tab._type
 	if raceenabled {
 		raceReadObjectPC(t, elem, getcallerpc(), funcPC(convT2I64))
 	}
 	if msanenabled {
 		msanread(elem, t.size)
 	}
 	var x unsafe.Pointer
 	if *(*uint64)(elem) == 0 {
 		x = unsafe.Pointer(&zeroVal[0])
 	} else {
 		x = mallocgc(8, t, false)
 		*(*uint64)(x) = *(*uint64)(elem)
 	}
 	i.tab = tab
 	i.data = x
 	return
 }

 func convT2Istring(tab *itab, elem unsafe.Pointer) (i iface) {
 	t := tab._type
 	if raceenabled {
 		raceReadObjectPC(t, elem, getcallerpc(), funcPC(convT2Istring))
 	}
 	if msanenabled {
 		msanread(elem, t.size)
 	}
 	var x unsafe.Pointer
 	if *(*string)(elem) == "" {
 		x = unsafe.Pointer(&zeroVal[0])
 	} else {
 		x = mallocgc(t.size, t, true)
 		*(*string)(x) = *(*string)(elem)
 	}
 	i.tab = tab
 	i.data = x
 	return
 }

 func convT2Islice(tab *itab, elem unsafe.Pointer) (i iface) {
 	t := tab._type
 	if raceenabled {
 		raceReadObjectPC(t, elem, getcallerpc(), funcPC(convT2Islice))
 	}
 	if msanenabled {
 		msanread(elem, t.size)
 	}
 	var x unsafe.Pointer
 	if v := *(*slice)(elem); uintptr(v.array) == 0 {
 		x = unsafe.Pointer(&zeroVal[0])
 	} else {
 		x = mallocgc(t.size, t, true)
 		*(*slice)(x) = *(*slice)(elem)
 	}
 	i.tab = tab
 	i.data = x
 	return
 }

 func convT2Inoptr(tab *itab, elem unsafe.Pointer) (i iface) {
 	t := tab._type
 	if raceenabled {
 		raceReadObjectPC(t, elem, getcallerpc(), funcPC(convT2Inoptr))
 	}
 	if msanenabled {
 		msanread(elem, t.size)
 	}
 	x := mallocgc(t.size, t, false)
 	memmove(x, elem, t.size)
 	i.tab = tab
 	i.data = x
 	return
 }

 func convI2I(inter *interfacetype, i iface) (r iface) {
 	tab := i.tab
 	if tab == nil {
 		return
 	}
 	if tab.inter == inter {
 		r.tab = tab
 		r.data = i.data
 		return
 	}
 	r.tab = getitab(inter, tab._type, false)
 	r.data = i.data
 	return
 }

 func assertI2I(inter *interfacetype, i iface) (r iface) {
 	tab := i.tab
 	if tab == nil {
 		// explicit conversions require non-nil interface value.
 		panic(&TypeAssertionError{"", "", inter.typ.string(), ""})
 	}
 	if tab.inter == inter {
 		r.tab = tab
 		r.data = i.data
 		return
 	}
 	r.tab = getitab(inter, tab._type, false)
 	r.data = i.data
 	return
 }

 func assertI2I2(inter *interfacetype, i iface) (r iface, b bool) {
 	tab := i.tab
 	if tab == nil {
 		return
 	}
 	if tab.inter != inter {
 		tab = getitab(inter, tab._type, true)
 		if tab == nil {
 			return
 		}
 	}
 	r.tab = tab
 	r.data = i.data
 	b = true
 	return
 }

 func assertE2I(inter *interfacetype, e eface) (r iface) {
 	t := e._type
 	if t == nil {
 		// explicit conversions require non-nil interface value.
 		panic(&TypeAssertionError{"", "", inter.typ.string(), ""})
 	}
 	r.tab = getitab(inter, t, false)
 	r.data = e.data
 	return
 }

 func assertE2I2(inter *interfacetype, e eface) (r iface, b bool) {
 	t := e._type
 	if t == nil {
 		return
 	}
 	tab := getitab(inter, t, true)
 	if tab == nil {
 		return
 	}
 	r.tab = tab
 	r.data = e.data
 	b = true
 	return
 }

 //go:linkname reflect_ifaceE2I reflect.ifaceE2I
 func reflect_ifaceE2I(inter *interfacetype, e eface, dst *iface) {
 	*dst = assertE2I(inter, e)
 }

 func iterate_itabs(fn func(*itab)) {
 	// Note: only runs during stop the world or with itabLock held,
 	// so no other locks/atomics needed.
 	t := itabTable
 	for i := uintptr(0); i < t.size; i++ {
 		m := *(**itab)(add(unsafe.Pointer(&t.entries), i*sys.PtrSize))
 		if m != nil {
 			fn(m)
 		}
 	}
 }

 // staticbytes is used to avoid convT2E for byte-sized values.
 var staticbytes = [...]byte{
 	0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07,
 	0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0f,
 	0x10, 0x11, 0x12, 0x13, 0x14, 0x15, 0x16, 0x17,
 	0x18, 0x19, 0x1a, 0x1b, 0x1c, 0x1d, 0x1e, 0x1f,
 	0x20, 0x21, 0x22, 0x23, 0x24, 0x25, 0x26, 0x27,
 	0x28, 0x29, 0x2a, 0x2b, 0x2c, 0x2d, 0x2e, 0x2f,
 	0x30, 0x31, 0x32, 0x33, 0x34, 0x35, 0x36, 0x37,
 	0x38, 0x39, 0x3a, 0x3b, 0x3c, 0x3d, 0x3e, 0x3f,
 	0x40, 0x41, 0x42, 0x43, 0x44, 0x45, 0x46, 0x47,
 	0x48, 0x49, 0x4a, 0x4b, 0x4c, 0x4d, 0x4e, 0x4f,
 	0x50, 0x51, 0x52, 0x53, 0x54, 0x55, 0x56, 0x57,
 	0x58, 0x59, 0x5a, 0x5b, 0x5c, 0x5d, 0x5e, 0x5f,
 	0x60, 0x61, 0x62, 0x63, 0x64, 0x65, 0x66, 0x67,
 	0x68, 0x69, 0x6a, 0x6b, 0x6c, 0x6d, 0x6e, 0x6f,
 	0x70, 0x71, 0x72, 0x73, 0x74, 0x75, 0x76, 0x77,
 	0x78, 0x79, 0x7a, 0x7b, 0x7c, 0x7d, 0x7e, 0x7f,
 	0x80, 0x81, 0x82, 0x83, 0x84, 0x85, 0x86, 0x87,
 	0x88, 0x89, 0x8a, 0x8b, 0x8c, 0x8d, 0x8e, 0x8f,
 	0x90, 0x91, 0x92, 0x93, 0x94, 0x95, 0x96, 0x97,
 	0x98, 0x99, 0x9a, 0x9b, 0x9c, 0x9d, 0x9e, 0x9f,
 	0xa0, 0xa1, 0xa2, 0xa3, 0xa4, 0xa5, 0xa6, 0xa7,
 	0xa8, 0xa9, 0xaa, 0xab, 0xac, 0xad, 0xae, 0xaf,
 	0xb0, 0xb1, 0xb2, 0xb3, 0xb4, 0xb5, 0xb6, 0xb7,
 	0xb8, 0xb9, 0xba, 0xbb, 0xbc, 0xbd, 0xbe, 0xbf,
 	0xc0, 0xc1, 0xc2, 0xc3, 0xc4, 0xc5, 0xc6, 0xc7,
 	0xc8, 0xc9, 0xca, 0xcb, 0xcc, 0xcd, 0xce, 0xcf,
 	0xd0, 0xd1, 0xd2, 0xd3, 0xd4, 0xd5, 0xd6, 0xd7,
 	0xd8, 0xd9, 0xda, 0xdb, 0xdc, 0xdd, 0xde, 0xdf,
 	0xe0, 0xe1, 0xe2, 0xe3, 0xe4, 0xe5, 0xe6, 0xe7,
 	0xe8, 0xe9, 0xea, 0xeb, 0xec, 0xed, 0xee, 0xef,
 	0xf0, 0xf1, 0xf2, 0xf3, 0xf4, 0xf5, 0xf6, 0xf7,
 	0xf8, 0xf9, 0xfa, 0xfb, 0xfc, 0xfd, 0xfe, 0xff,
 }
	// 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.

	package runtime

	import (
	"runtime/internal/atomic"
	"runtime/internal/sys"
	"unsafe"
	)

	const itabInitSize = 512

	var (
	itabLock mutex // lock for accessing itab table
	itabTable = &itabTableInit // pointer to current table
	itabTableInit = itabTableType{size: itabInitSize} // starter table
	)

	//Note: change the formula in the mallocgc call in itabAdd if you change these fields.
	type itabTableType struct {
	size uintptr // length of entries array. Always a power of 2.
	count uintptr // current number of filled entries.
	entries [itabInitSize]*itab // really [size] large
	}

	func itabHashFunc(inter interfacetype, typ _type) uintptr {
	// compiler has provided some good hash codes for us.
	return uintptr(inter.typ.hash ^ typ.hash)
	}

	func getitab(inter interfacetype, typ _type, canfail bool) *itab {
	if len(inter.mhdr) == 0 {
	throw("internal error - misuse of itab")
	}

	// easy case
	if typ.tflag&tflagUncommon == 0 {
	if canfail {
	return nil
	}
	name := inter.typ.nameOff(inter.mhdr[0].name)
	panic(&TypeAssertionError{"", typ.string(), inter.typ.string(), name.name()})
	}

	var m *itab

	// First, look in the existing table to see if we can find the itab we need.
	// This is by far the most common case, so do it without locks.
	// Use atomic to ensure we see any previous writes done by the thread
	// that updates the itabTable field (with atomic.Storep in itabAdd).
	t := (*itabTableType)(atomic.Loadp(unsafe.Pointer(&itabTable)))
	if m = t.find(inter, typ); m != nil {
	goto finish
	}

	// Not found. Grab the lock and try again.
	lock(&itabLock)
	if m = itabTable.find(inter, typ); m != nil {
	unlock(&itabLock)
	goto finish
	}

	// Entry doesn't exist yet. Make a new entry & add it.
	m = (itab)(persistentalloc(unsafe.Sizeof(itab{})+uintptr(len(inter.mhdr)-1)sys.PtrSize, 0, &memstats.other_sys))
	m.inter = inter
	m._type = typ
	m.init()
	itabAdd(m)
	unlock(&itabLock)
	finish:
	if m.fun[0] != 0 {
	return m
	}
	if canfail {
	return nil
	}
	// this can only happen if the conversion
	// was already done once using the , ok form
	// and we have a cached negative result.
	// The cached result doesn't record which
	// interface function was missing, so initialize
	// the itab again to get the missing function name.
	panic(&TypeAssertionError{concreteString: typ.string(), assertedString: inter.typ.string(), missingMethod: m.init()})
	}

	// find finds the given interface/type pair in t.
	// Returns nil if the given interface/type pair isn't present.
	func (t itabTableType) find(inter interfacetype, typ _type) itab {
	// Implemented using quadratic probing.
	// Probe sequence is h(i) = h0 + i*(i+1)/2 mod 2^k.
	// We're guaranteed to hit all table entries using this probe sequence.
	mask := t.size - 1
	h := itabHashFunc(inter, typ) & mask
	for i := uintptr(1); ; i++ {
	p := (*itab)(add(unsafe.Pointer(&t.entries), hsys.PtrSize))
	// Use atomic read here so if we see m != nil, we also see
	// the initializations of the fields of m.
	// m := *p
	m := (*itab)(atomic.Loadp(unsafe.Pointer(p)))
	if m == nil {
	return nil
	}
	if m.inter == inter && m._type == typ {
	return m
	}
	h += i
	h &= mask
	}
	}

	// itabAdd adds the given itab to the itab hash table.
	// itabLock must be held.
	func itabAdd(m *itab) {
	t := itabTable
	if t.count >= 3*(t.size/4) { // 75% load factor
	// Grow hash table.
	// t2 = new(itabTableType) + some additional entries
	// We lie and tell malloc we want pointer-free memory because
	// all the pointed-to values are not in the heap.
	t2 := (itabTableType)(mallocgc((2+2t.size)*sys.PtrSize, nil, true))
	t2.size = t.size * 2

	// Copy over entries.
	// Note: while copying, other threads may look for an itab and
	// fail to find it. That's ok, they will then try to get the itab lock
	// and as a consequence wait until this copying is complete.
	iterate_itabs(t2.add)
	if t2.count != t.count {
	throw("mismatched count during itab table copy")
	}
	// Publish new hash table. Use an atomic write: see comment in getitab.
	atomicstorep(unsafe.Pointer(&itabTable), unsafe.Pointer(t2))
	// Adopt the new table as our own.
	t = itabTable
	// Note: the old table can be GC'ed here.
	}
	t.add(m)
	}

	// add adds the given itab to itab table t.
	// itabLock must be held.
	func (t itabTableType) add(m itab) {
	// See comment in find about the probe sequence.
	// Insert new itab in the first empty spot in the probe sequence.
	mask := t.size - 1
	h := itabHashFunc(m.inter, m._type) & mask
	for i := uintptr(1); ; i++ {
	p := (*itab)(add(unsafe.Pointer(&t.entries), hsys.PtrSize))
	m2 := *p
	if m2 == m {
	// A given itab may be used in more than one module
	// and thanks to the way global symbol resolution works, the
	// pointed-to itab may already have been inserted into the
	// global 'hash'.
	return
	}
	if m2 == nil {
	// Use atomic write here so if a reader sees m, it also
	// sees the correctly initialized fields of m.
	// NoWB is ok because m is not in heap memory.
	// *p = m
	atomic.StorepNoWB(unsafe.Pointer(p), unsafe.Pointer(m))
	t.count++
	return
	}
	h += i
	h &= mask
	}
	}

	// init fills in the m.fun array with all the code pointers for
	// the m.inter/m._type pair. If the type does not implement the interface,
	// it sets m.fun[0] to 0 and returns the name of an interface function that is missing.
	// It is ok to call this multiple times on the same m, even concurrently.
	func (m *itab) init() string {
	inter := m.inter
	typ := m._type
	x := typ.uncommon()

	// both inter and typ have method sorted by name,
	// and interface names are unique,
	// so can iterate over both in lock step;
	// the loop is O(ni+nt) not O(ni*nt).
	ni := len(inter.mhdr)
	nt := int(x.mcount)
	xmhdr := (*[1 << 16]method)(add(unsafe.Pointer(x), uintptr(x.moff)))[:nt:nt]
	j := 0
	imethods:
	for k := 0; k < ni; k++ {
	i := &inter.mhdr[k]
	itype := inter.typ.typeOff(i.ityp)
	name := inter.typ.nameOff(i.name)
	iname := name.name()
	ipkg := name.pkgPath()
	if ipkg == "" {
	ipkg = inter.pkgpath.name()
	}
	for ; j < nt; j++ {
	t := &xmhdr[j]
	tname := typ.nameOff(t.name)
	if typ.typeOff(t.mtyp) == itype && tname.name() == iname {
	pkgPath := tname.pkgPath()
	if pkgPath == "" {
	pkgPath = typ.nameOff(x.pkgpath).name()
	}
	if tname.isExported() \|\| pkgPath == ipkg {
	if m != nil {
	ifn := typ.textOff(t.ifn)
	(unsafe.Pointer)(add(unsafe.Pointer(&m.fun[0]), uintptr(k)*sys.PtrSize)) = ifn
	}
	continue imethods
	}
	}
	}
	// didn't find method
	m.fun[0] = 0
	return iname
	}
	m.hash = typ.hash
	return ""
	}

	func itabsinit() {
	lock(&itabLock)
	for _, md := range activeModules() {
	for _, i := range md.itablinks {
	itabAdd(i)
	}
	}
	unlock(&itabLock)
	}

	// panicdottypeE is called when doing an e.(T) conversion and the conversion fails.
	// have = the dynamic type we have.
	// want = the static type we're trying to convert to.
	// iface = the static type we're converting from.
	func panicdottypeE(have, want, iface *_type) {
	haveString := ""
	if have != nil {
	haveString = have.string()
	}
	panic(&TypeAssertionError{iface.string(), haveString, want.string(), ""})
	}

	// panicdottypeI is called when doing an i.(T) conversion and the conversion fails.
	// Same args as panicdottypeE, but "have" is the dynamic itab we have.
	func panicdottypeI(have itab, want, iface _type) {
	var t *_type
	if have != nil {
	t = have._type
	}
	panicdottypeE(t, want, iface)
	}

	// panicnildottype is called when doing a i.(T) conversion and the interface i is nil.
	// want = the static type we're trying to convert to.
	func panicnildottype(want *_type) {
	panic(&TypeAssertionError{"", "", want.string(), ""})
	// TODO: Add the static type we're converting from as well.
	// It might generate a better error message.
	// Just to match other nil conversion errors, we don't for now.
	}

	// The conv and assert functions below do very similar things.
	// The convXXX functions are guaranteed by the compiler to succeed.
	// The assertXXX functions may fail (either panicking or returning false,
	// depending on whether they are 1-result or 2-result).
	// The convXXX functions succeed on a nil input, whereas the assertXXX
	// functions fail on a nil input.

	func convT2E(t *_type, elem unsafe.Pointer) (e eface) {
	if raceenabled {
	raceReadObjectPC(t, elem, getcallerpc(), funcPC(convT2E))
	}
	if msanenabled {
	msanread(elem, t.size)
	}
	x := mallocgc(t.size, t, true)
	// TODO: We allocate a zeroed object only to overwrite it with actual data.
	// Figure out how to avoid zeroing. Also below in convT2Eslice, convT2I, convT2Islice.
	typedmemmove(t, x, elem)
	e._type = t
	e.data = x
	return
	}

	func convT2E16(t *_type, elem unsafe.Pointer) (e eface) {
	if raceenabled {
	raceReadObjectPC(t, elem, getcallerpc(), funcPC(convT2E16))
	}
	if msanenabled {
	msanread(elem, t.size)
	}
	var x unsafe.Pointer
	if (uint16)(elem) == 0 {
	x = unsafe.Pointer(&zeroVal[0])
	} else {
	x = mallocgc(2, t, false)
	(uint16)(x) = (uint16)(elem)
	}
	e._type = t
	e.data = x
	return
	}

	func convT2E32(t *_type, elem unsafe.Pointer) (e eface) {
	if raceenabled {
	raceReadObjectPC(t, elem, getcallerpc(), funcPC(convT2E32))
	}
	if msanenabled {
	msanread(elem, t.size)
	}
	var x unsafe.Pointer
	if (uint32)(elem) == 0 {
	x = unsafe.Pointer(&zeroVal[0])
	} else {
	x = mallocgc(4, t, false)
	(uint32)(x) = (uint32)(elem)
	}
	e._type = t
	e.data = x
	return
	}

	func convT2E64(t *_type, elem unsafe.Pointer) (e eface) {
	if raceenabled {
	raceReadObjectPC(t, elem, getcallerpc(), funcPC(convT2E64))
	}
	if msanenabled {
	msanread(elem, t.size)
	}
	var x unsafe.Pointer
	if (uint64)(elem) == 0 {
	x = unsafe.Pointer(&zeroVal[0])
	} else {
	x = mallocgc(8, t, false)
	(uint64)(x) = (uint64)(elem)
	}
	e._type = t
	e.data = x
	return
	}

	func convT2Estring(t *_type, elem unsafe.Pointer) (e eface) {
	if raceenabled {
	raceReadObjectPC(t, elem, getcallerpc(), funcPC(convT2Estring))
	}
	if msanenabled {
	msanread(elem, t.size)
	}
	var x unsafe.Pointer
	if (string)(elem) == "" {
	x = unsafe.Pointer(&zeroVal[0])
	} else {
	x = mallocgc(t.size, t, true)
	(string)(x) = (string)(elem)
	}
	e._type = t
	e.data = x
	return
	}

	func convT2Eslice(t *_type, elem unsafe.Pointer) (e eface) {
	if raceenabled {
	raceReadObjectPC(t, elem, getcallerpc(), funcPC(convT2Eslice))
	}
	if msanenabled {
	msanread(elem, t.size)
	}
	var x unsafe.Pointer
	if v := (slice)(elem); uintptr(v.array) == 0 {
	x = unsafe.Pointer(&zeroVal[0])
	} else {
	x = mallocgc(t.size, t, true)
	(slice)(x) = (slice)(elem)
	}
	e._type = t
	e.data = x
	return
	}

	func convT2Enoptr(t *_type, elem unsafe.Pointer) (e eface) {
	if raceenabled {
	raceReadObjectPC(t, elem, getcallerpc(), funcPC(convT2Enoptr))
	}
	if msanenabled {
	msanread(elem, t.size)
	}
	x := mallocgc(t.size, t, false)
	memmove(x, elem, t.size)
	e._type = t
	e.data = x
	return
	}

	func convT2I(tab *itab, elem unsafe.Pointer) (i iface) {
	t := tab._type
	if raceenabled {
	raceReadObjectPC(t, elem, getcallerpc(), funcPC(convT2I))
	}
	if msanenabled {
	msanread(elem, t.size)
	}
	x := mallocgc(t.size, t, true)
	typedmemmove(t, x, elem)
	i.tab = tab
	i.data = x
	return
	}

	func convT2I16(tab *itab, elem unsafe.Pointer) (i iface) {
	t := tab._type
	if raceenabled {
	raceReadObjectPC(t, elem, getcallerpc(), funcPC(convT2I16))
	}
	if msanenabled {
	msanread(elem, t.size)
	}
	var x unsafe.Pointer
	if (uint16)(elem) == 0 {
	x = unsafe.Pointer(&zeroVal[0])
	} else {
	x = mallocgc(2, t, false)
	(uint16)(x) = (uint16)(elem)
	}
	i.tab = tab
	i.data = x
	return
	}

	func convT2I32(tab *itab, elem unsafe.Pointer) (i iface) {
	t := tab._type
	if raceenabled {
	raceReadObjectPC(t, elem, getcallerpc(), funcPC(convT2I32))
	}
	if msanenabled {
	msanread(elem, t.size)
	}
	var x unsafe.Pointer
	if (uint32)(elem) == 0 {
	x = unsafe.Pointer(&zeroVal[0])
	} else {
	x = mallocgc(4, t, false)
	(uint32)(x) = (uint32)(elem)
	}
	i.tab = tab
	i.data = x
	return
	}

	func convT2I64(tab *itab, elem unsafe.Pointer) (i iface) {
	t := tab._type
	if raceenabled {
	raceReadObjectPC(t, elem, getcallerpc(), funcPC(convT2I64))
	}
	if msanenabled {
	msanread(elem, t.size)
	}
	var x unsafe.Pointer
	if (uint64)(elem) == 0 {
	x = unsafe.Pointer(&zeroVal[0])
	} else {
	x = mallocgc(8, t, false)
	(uint64)(x) = (uint64)(elem)
	}
	i.tab = tab
	i.data = x
	return
	}

	func convT2Istring(tab *itab, elem unsafe.Pointer) (i iface) {
	t := tab._type
	if raceenabled {
	raceReadObjectPC(t, elem, getcallerpc(), funcPC(convT2Istring))
	}
	if msanenabled {
	msanread(elem, t.size)
	}
	var x unsafe.Pointer
	if (string)(elem) == "" {
	x = unsafe.Pointer(&zeroVal[0])
	} else {
	x = mallocgc(t.size, t, true)
	(string)(x) = (string)(elem)
	}
	i.tab = tab
	i.data = x
	return
	}

	func convT2Islice(tab *itab, elem unsafe.Pointer) (i iface) {
	t := tab._type
	if raceenabled {
	raceReadObjectPC(t, elem, getcallerpc(), funcPC(convT2Islice))
	}
	if msanenabled {
	msanread(elem, t.size)
	}
	var x unsafe.Pointer
	if v := (slice)(elem); uintptr(v.array) == 0 {
	x = unsafe.Pointer(&zeroVal[0])
	} else {
	x = mallocgc(t.size, t, true)
	(slice)(x) = (slice)(elem)
	}
	i.tab = tab
	i.data = x
	return
	}

	func convT2Inoptr(tab *itab, elem unsafe.Pointer) (i iface) {
	t := tab._type
	if raceenabled {
	raceReadObjectPC(t, elem, getcallerpc(), funcPC(convT2Inoptr))
	}
	if msanenabled {
	msanread(elem, t.size)
	}
	x := mallocgc(t.size, t, false)
	memmove(x, elem, t.size)
	i.tab = tab
	i.data = x
	return
	}

	func convI2I(inter *interfacetype, i iface) (r iface) {
	tab := i.tab
	if tab == nil {
	return
	}
	if tab.inter == inter {
	r.tab = tab
	r.data = i.data
	return
	}
	r.tab = getitab(inter, tab._type, false)
	r.data = i.data
	return
	}

	func assertI2I(inter *interfacetype, i iface) (r iface) {
	tab := i.tab
	if tab == nil {
	// explicit conversions require non-nil interface value.
	panic(&TypeAssertionError{"", "", inter.typ.string(), ""})
	}
	if tab.inter == inter {
	r.tab = tab
	r.data = i.data
	return
	}
	r.tab = getitab(inter, tab._type, false)
	r.data = i.data
	return
	}

	func assertI2I2(inter *interfacetype, i iface) (r iface, b bool) {
	tab := i.tab
	if tab == nil {
	return
	}
	if tab.inter != inter {
	tab = getitab(inter, tab._type, true)
	if tab == nil {
	return
	}
	}
	r.tab = tab
	r.data = i.data
	b = true
	return
	}

	func assertE2I(inter *interfacetype, e eface) (r iface) {
	t := e._type
	if t == nil {
	// explicit conversions require non-nil interface value.
	panic(&TypeAssertionError{"", "", inter.typ.string(), ""})
	}
	r.tab = getitab(inter, t, false)
	r.data = e.data
	return
	}

	func assertE2I2(inter *interfacetype, e eface) (r iface, b bool) {
	t := e._type
	if t == nil {
	return
	}
	tab := getitab(inter, t, true)
	if tab == nil {
	return
	}
	r.tab = tab
	r.data = e.data
	b = true
	return
	}

	//go:linkname reflect_ifaceE2I reflect.ifaceE2I
	func reflect_ifaceE2I(inter interfacetype, e eface, dst iface) {
	*dst = assertE2I(inter, e)
	}

	func iterate_itabs(fn func(*itab)) {
	// Note: only runs during stop the world or with itabLock held,
	// so no other locks/atomics needed.
	t := itabTable
	for i := uintptr(0); i < t.size; i++ {
	m := (itab)(add(unsafe.Pointer(&t.entries), isys.PtrSize))
	if m != nil {
	fn(m)
	}
	}
	}

	// staticbytes is used to avoid convT2E for byte-sized values.
	var staticbytes = [...]byte{
	0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07,
	0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0f,
	0x10, 0x11, 0x12, 0x13, 0x14, 0x15, 0x16, 0x17,
	0x18, 0x19, 0x1a, 0x1b, 0x1c, 0x1d, 0x1e, 0x1f,
	0x20, 0x21, 0x22, 0x23, 0x24, 0x25, 0x26, 0x27,
	0x28, 0x29, 0x2a, 0x2b, 0x2c, 0x2d, 0x2e, 0x2f,
	0x30, 0x31, 0x32, 0x33, 0x34, 0x35, 0x36, 0x37,
	0x38, 0x39, 0x3a, 0x3b, 0x3c, 0x3d, 0x3e, 0x3f,
	0x40, 0x41, 0x42, 0x43, 0x44, 0x45, 0x46, 0x47,
	0x48, 0x49, 0x4a, 0x4b, 0x4c, 0x4d, 0x4e, 0x4f,
	0x50, 0x51, 0x52, 0x53, 0x54, 0x55, 0x56, 0x57,
	0x58, 0x59, 0x5a, 0x5b, 0x5c, 0x5d, 0x5e, 0x5f,
	0x60, 0x61, 0x62, 0x63, 0x64, 0x65, 0x66, 0x67,
	0x68, 0x69, 0x6a, 0x6b, 0x6c, 0x6d, 0x6e, 0x6f,
	0x70, 0x71, 0x72, 0x73, 0x74, 0x75, 0x76, 0x77,
	0x78, 0x79, 0x7a, 0x7b, 0x7c, 0x7d, 0x7e, 0x7f,
	0x80, 0x81, 0x82, 0x83, 0x84, 0x85, 0x86, 0x87,
	0x88, 0x89, 0x8a, 0x8b, 0x8c, 0x8d, 0x8e, 0x8f,
	0x90, 0x91, 0x92, 0x93, 0x94, 0x95, 0x96, 0x97,
	0x98, 0x99, 0x9a, 0x9b, 0x9c, 0x9d, 0x9e, 0x9f,
	0xa0, 0xa1, 0xa2, 0xa3, 0xa4, 0xa5, 0xa6, 0xa7,
	0xa8, 0xa9, 0xaa, 0xab, 0xac, 0xad, 0xae, 0xaf,
	0xb0, 0xb1, 0xb2, 0xb3, 0xb4, 0xb5, 0xb6, 0xb7,
	0xb8, 0xb9, 0xba, 0xbb, 0xbc, 0xbd, 0xbe, 0xbf,
	0xc0, 0xc1, 0xc2, 0xc3, 0xc4, 0xc5, 0xc6, 0xc7,
	0xc8, 0xc9, 0xca, 0xcb, 0xcc, 0xcd, 0xce, 0xcf,
	0xd0, 0xd1, 0xd2, 0xd3, 0xd4, 0xd5, 0xd6, 0xd7,
	0xd8, 0xd9, 0xda, 0xdb, 0xdc, 0xdd, 0xde, 0xdf,
	0xe0, 0xe1, 0xe2, 0xe3, 0xe4, 0xe5, 0xe6, 0xe7,
	0xe8, 0xe9, 0xea, 0xeb, 0xec, 0xed, 0xee, 0xef,
	0xf0, 0xf1, 0xf2, 0xf3, 0xf4, 0xf5, 0xf6, 0xf7,
	0xf8, 0xf9, 0xfa, 0xfb, 0xfc, 0xfd, 0xfe, 0xff,
	}