iface.go

Documentation: runtime

  // 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{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
  	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{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.
  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) {
  	// 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
  	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) {
  	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) {
  	panic(&TypeAssertionError{nil, nil, want, ""})
  	// 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, val uint16) (e eface) {
  	var x unsafe.Pointer
  	if val == 0 {
  		x = unsafe.Pointer(&zeroVal[0])
  	} else {
  		x = mallocgc(2, t, false)
  		*(*uint16)(x) = val
  	}
  	e._type = t
  	e.data = x
  	return
  }
  
  func convT2E32(t *_type, val uint32) (e eface) {
  	var x unsafe.Pointer
  	if val == 0 {
  		x = unsafe.Pointer(&zeroVal[0])
  	} else {
  		x = mallocgc(4, t, false)
  		*(*uint32)(x) = val
  	}
  	e._type = t
  	e.data = x
  	return
  }
  
  func convT2E64(t *_type, val uint64) (e eface) {
  	var x unsafe.Pointer
  	if val == 0 {
  		x = unsafe.Pointer(&zeroVal[0])
  	} else {
  		x = mallocgc(8, t, false)
  		*(*uint64)(x) = val
  	}
  	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, val uint16) (i iface) {
  	t := tab._type
  	var x unsafe.Pointer
  	if val == 0 {
  		x = unsafe.Pointer(&zeroVal[0])
  	} else {
  		x = mallocgc(2, t, false)
  		*(*uint16)(x) = val
  	}
  	i.tab = tab
  	i.data = x
  	return
  }
  
  func convT2I32(tab *itab, val uint32) (i iface) {
  	t := tab._type
  	var x unsafe.Pointer
  	if val == 0 {
  		x = unsafe.Pointer(&zeroVal[0])
  	} else {
  		x = mallocgc(4, t, false)
  		*(*uint32)(x) = val
  	}
  	i.tab = tab
  	i.data = x
  	return
  }
  
  func convT2I64(tab *itab, val uint64) (i iface) {
  	t := tab._type
  	var x unsafe.Pointer
  	if val == 0 {
  		x = unsafe.Pointer(&zeroVal[0])
  	} else {
  		x = mallocgc(8, t, false)
  		*(*uint64)(x) = val
  	}
  	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{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
  	if t == nil {
  		// 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)
  }
  
  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,
  }
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