Source: iface.go in package runtime


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
}

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{nil, typ, &inter.typ, 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

The hash is used in type switches. However, compiler statically generates itab's for all interface/type pairs used in switches (which are added to itabTable in itabsinit). The dynamically-generated itab's never participate in type switches, and thus the hash is irrelevant. Note: m.hash is _not_ the hash used for the runtime itabTable hash table.

	m.hash = 0
	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{concrete: typ, asserted: &inter.typ, missingMethod: m.init()})
}

find finds the given interface/type pair in t. Returns nil if the given interface/type pair isn't present.

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++ {

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.

Bugs can lead to calling this while mallocing is set, typically because this is called while panicing. Crash reliably, rather than only when we need to grow the hash table.

	if getg().m.mallocing != 0 {
		throw("malloc deadlock")
	}

	t := itabTable

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.

Adopt the new table as our own.

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.

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

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
		}

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
	methods := (*[1 << 16]unsafe.Pointer)(unsafe.Pointer(&m.fun[0]))[:ni:ni]
	var fun0 unsafe.Pointer
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)
						if k == 0 {
							fun0 = ifn // we'll set m.fun[0] at the end
						} else {
							methods[k] = ifn
						}
					}
					continue imethods
				}
			}

didn't find method

		m.fun[0] = 0
		return iname
	}
	m.fun[0] = uintptr(fun0)
	return ""
}

func itabsinit() {
	lockInit(&itabLock, lockRankItab)
	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) {
	panic(&TypeAssertionError{iface, have, want, ""})
}

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) {

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 specialized convTx routines need a type descriptor to use when calling mallocgc. We don't need the type to be exact, just to have the correct size, alignment, and pointer-ness. However, when debugging, it'd be nice to have some indication in mallocgc where the types came from, so we use named types here. We then construct interface values of these types, and then extract the type word to use as needed.

type (
	uint16InterfacePtr uint16
	uint32InterfacePtr uint32
	uint64InterfacePtr uint64
	stringInterfacePtr string
	sliceInterfacePtr  []byte
)

var (
	uint16Eface interface{} = uint16InterfacePtr(0)
	uint32Eface interface{} = uint32InterfacePtr(0)
	uint64Eface interface{} = uint64InterfacePtr(0)
	stringEface interface{} = stringInterfacePtr("")
	sliceEface  interface{} = sliceInterfacePtr(nil)

	uint16Type *_type = efaceOf(&uint16Eface)._type
	uint32Type *_type = efaceOf(&uint32Eface)._type
	uint64Type *_type = efaceOf(&uint64Eface)._type
	stringType *_type = efaceOf(&stringEface)._type
	sliceType  *_type = efaceOf(&sliceEface)._type
)

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)
	}

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 convT16(val uint16) (x unsafe.Pointer) {
	if val < uint16(len(staticuint64s)) {
		x = unsafe.Pointer(&staticuint64s[val])
		if sys.BigEndian {
			x = add(x, 6)
		}
	} else {
		x = mallocgc(2, uint16Type, false)
		*(*uint16)(x) = val
	}
	return
}

func convT32(val uint32) (x unsafe.Pointer) {
	if val < uint32(len(staticuint64s)) {
		x = unsafe.Pointer(&staticuint64s[val])
		if sys.BigEndian {
			x = add(x, 4)
		}
	} else {
		x = mallocgc(4, uint32Type, false)
		*(*uint32)(x) = val
	}
	return
}

func convT64(val uint64) (x unsafe.Pointer) {
	if val < uint64(len(staticuint64s)) {
		x = unsafe.Pointer(&staticuint64s[val])
	} else {
		x = mallocgc(8, uint64Type, false)
		*(*uint64)(x) = val
	}
	return
}

func convTstring(val string) (x unsafe.Pointer) {
	if val == "" {
		x = unsafe.Pointer(&zeroVal[0])
	} else {
		x = mallocgc(unsafe.Sizeof(val), stringType, true)
		*(*string)(x) = val
	}
	return
}

Note: this must work for any element type, not just byte.

	if (*slice)(unsafe.Pointer(&val)).array == nil {
		x = unsafe.Pointer(&zeroVal[0])
	} else {
		x = mallocgc(unsafe.Sizeof(val), sliceType, true)
		*(*[]byte)(x) = val
	}
	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 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

explicit conversions require non-nil interface value.

		panic(&TypeAssertionError{nil, nil, &inter.typ, ""})
	}
	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

explicit conversions require non-nil interface value.

		panic(&TypeAssertionError{nil, nil, &inter.typ, ""})
	}
	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)
}

go:linkname reflectlite_ifaceE2I internal/reflectlite.ifaceE2I

func reflectlite_ifaceE2I(inter *interfacetype, e eface, dst *iface) {
	*dst = assertE2I(inter, e)
}

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)
		}
	}
}

staticuint64s is used to avoid allocating in convTx for small integer values.

var staticuint64s = [...]uint64{
	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,