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 (
  	hashSize = 1009
  )
  
  var (
  	ifaceLock mutex // lock for accessing hash
  	hash      [hashSize]*itab
  )
  
  func itabhash(inter *interfacetype, typ *_type) uint32 {
  	// compiler has provided some good hash codes for us.
  	h := inter.typ.hash
  	h += 17 * typ.hash
  	// TODO(rsc): h += 23 * x.mhash ?
  	return h % hashSize
  }
  
  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()})
  	}
  
  	h := itabhash(inter, typ)
  
  	// look twice - once without lock, once with.
  	// common case will be no lock contention.
  	var m *itab
  	var locked int
  	for locked = 0; locked < 2; locked++ {
  		if locked != 0 {
  			lock(&ifaceLock)
  		}
  		for m = (*itab)(atomic.Loadp(unsafe.Pointer(&hash[h]))); m != nil; m = m.link {
  			if m.inter == inter && m._type == typ {
  				if m.bad != 0 {
  					if !canfail {
  						// 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 try
  						// adding the itab again, which will throw an error.
  						additab(m, locked != 0, false)
  					}
  					m = nil
  				}
  				if locked != 0 {
  					unlock(&ifaceLock)
  				}
  				return m
  			}
  		}
  	}
  
  	m = (*itab)(persistentalloc(unsafe.Sizeof(itab{})+uintptr(len(inter.mhdr)-1)*sys.PtrSize, 0, &memstats.other_sys))
  	m.inter = inter
  	m._type = typ
  	additab(m, true, canfail)
  	unlock(&ifaceLock)
  	if m.bad != 0 {
  		return nil
  	}
  	return m
  }
  
  func additab(m *itab, locked, canfail bool) {
  	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
  	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
  					}
  					goto nextimethod
  				}
  			}
  		}
  		// didn't find method
  		if !canfail {
  			if locked {
  				unlock(&ifaceLock)
  			}
  			panic(&TypeAssertionError{"", typ.string(), inter.typ.string(), iname})
  		}
  		m.bad = 1
  		break
  	nextimethod:
  	}
  	if !locked {
  		throw("invalid itab locking")
  	}
  	h := itabhash(inter, typ)
  	m.link = hash[h]
  	m.inhash = 1
  	atomicstorep(unsafe.Pointer(&hash[h]), unsafe.Pointer(m))
  }
  
  func itabsinit() {
  	lock(&ifaceLock)
  	for _, md := range activeModules() {
  		for _, i := range md.itablinks {
  			// itablinks is a slice of pointers to the itabs used in this
  			// module. 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'.
  			if i.inhash == 0 {
  				additab(i, true, false)
  			}
  		}
  	}
  	unlock(&ifaceLock)
  }
  
  // panicdottype is called when doing an i.(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 panicdottype(have, want, iface *_type) {
  	haveString := ""
  	if have != nil {
  		haveString = have.string()
  	}
  	panic(&TypeAssertionError{iface.string(), haveString, want.string(), ""})
  }
  
  // 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(unsafe.Pointer(&t)), funcPC(convT2E))
  	}
  	if msanenabled {
  		msanread(elem, t.size)
  	}
  	if isDirectIface(t) {
  		// This case is implemented directly by the compiler.
  		throw("direct convT2E")
  	}
  	x := newobject(t)
  	// TODO: We allocate a zeroed object only to overwrite it with
  	// actual data. Figure out how to avoid zeroing. Also below in convT2I.
  	typedmemmove(t, x, elem)
  	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(unsafe.Pointer(&tab)), funcPC(convT2I))
  	}
  	if msanenabled {
  		msanread(elem, t.size)
  	}
  	if isDirectIface(t) {
  		// This case is implemented directly by the compiler.
  		throw("direct convT2I")
  	}
  	x := newobject(t)
  	typedmemmove(t, x, elem)
  	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)) {
  	for _, h := range &hash {
  		for ; h != nil; h = h.link {
  			fn(h)
  		}
  	}
  }
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