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runtime: new itab lookup table

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Keep itabs in a growable hash table.
Use a simple open-addressable hash table, quadratic probing, power
of two sized.
Synchronization gets a bit more tricky. The common read path now
has two atomic reads, one to get the table pointer and one to read
the entry out of the table.

I set the max load factor to 75%, kind of arbitrarily. There's a
space-speed tradeoff here, and I'm not sure where we should land.

Because we use open addressing the itab.link field is no longer needed.
I'll remove it in a separate CL.

Fixes #20505

Change-Id: Ifb3d9a337512d6cf968c1fceb1eeaf89559afebf
Reviewed-on: https://go-review.googlesource.com/44472
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
This commit is contained in:
Keith Randall 2017-05-31 08:45:10 -07:00
parent e098e5142d
commit 3d1699ea78
5 changed files with 174 additions and 97 deletions
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11
src/cmd/compile/internal/gc/reflect.go
View File

@ -1457,22 +1457,23 @@ func dumptabs() {
// type itab struct {
// inter *interfacetype
// _type *_type
// link *itab
// _ uintptr TODO: remove
// hash uint32
// bad bool
// inhash bool
// unused [2]byte
// _ [3]byte
// fun [1]uintptr // variable sized
// }
o := dsymptr(i.lsym, 0, dtypesym(i.itype).Linksym(), 0)
o = dsymptr(i.lsym, o, dtypesym(i.t).Linksym(), 0)
o += Widthptr // skip link field
o = duintptr(i.lsym, o, 0) // unused
o = duint32(i.lsym, o, typehash(i.t)) // copy of type hash
o += 4 // skip bad/inhash/unused fields
o += 1 // bad is false
o += 3 // skip unused fields
o += len(imethods(i.itype)) * Widthptr // skip fun method pointers
// at runtime the itab will contain pointers to types, other itabs and
// method functions. None are allocated on heap, so we can use obj.NOPTR.
ggloblsym(i.lsym, int32(o), int16(obj.DUPOK|obj.NOPTR))
// TODO: mark readonly after we pre-add the function pointers
ilink := itablinkpkg.Lookup(i.t.ShortString() + "," + i.itype.ShortString()).Linksym()
dsymptr(ilink, 0, i.lsym, 0)

13
src/reflect/value.go
View File

@ -180,12 +180,13 @@ type emptyInterface struct {
type nonEmptyInterface struct {
// see ../runtime/iface.go:/Itab
itab *struct {
ityp *rtype // static interface type
typ *rtype // dynamic concrete type
link unsafe.Pointer
bad int32
unused int32
fun [100000]unsafe.Pointer // method table
ityp *rtype // static interface type
typ *rtype // dynamic concrete type
_ uintptr
hash uint32 // copy of typ.hash
bad bool
_ [3]byte
fun [100000]unsafe.Pointer // method table
}
word unsafe.Pointer
}

224
src/runtime/iface.go
View File

@ -10,21 +10,24 @@ import (
"unsafe"
)
const (
hashSize = 1009
)
const itabInitSize = 512
var (
ifaceLock mutex // lock for accessing hash
hash [hashSize]*itab
itabLock mutex // lock for accessing itab table
itabTable = &itabTableInit // pointer to current table
itabTableInit = itabTableType{size: itabInitSize} // starter table
)
func itabhash(inter *interfacetype, typ *_type) uint32 {
//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.
h := inter.typ.hash
h += 17 * typ.hash
// TODO(rsc): h += 23 * x.mhash ?
return h % hashSize
return uintptr(inter.typ.hash ^ typ.hash)
}
func getitab(inter *interfacetype, typ *_type, canfail bool) *itab {
@ -41,50 +44,141 @@ func getitab(inter *interfacetype, typ *_type, canfail bool) *itab {
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 {
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
}
}
// 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 addItab).
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
additab(m, true, canfail)
unlock(&ifaceLock)
if m.bad {
m.init()
itabAdd(m)
unlock(&itabLock)
finish:
if !m.bad {
return m
}
if canfail {
return nil
}
return m
// 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()})
}
func additab(m *itab, locked, canfail bool) {
// itabFind 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. Use an atomic write: see comment in getitab.
// 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
atomicstorep(unsafe.Pointer(&itabTable), unsafe.Pointer(t2))
// 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.
for i := uintptr(0); i < t.size; i++ {
if m2 := *(**itab)(add(unsafe.Pointer(&t.entries), i*sys.PtrSize)); m2 != nil {
itabAdd(m2)
}
}
if itabTable.count != t.count {
throw("mismatched count during itab table copy")
}
// Adopt the new table as our own.
t = itabTable
// Note: the old table can be GC'ed here.
}
// See comment in itabFind 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
}
}
// Adds m to the set of initial itabs.
// itabLock must be held.
func itabAddStartup(m *itab) {
m.init() // TODO: remove after CL 44341
itabAdd(m)
}
// 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()
@ -97,6 +191,7 @@ func additab(m *itab, locked, canfail bool) {
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)
@ -119,45 +214,25 @@ func additab(m *itab, locked, canfail bool) {
ifn := typ.textOff(t.ifn)
*(*unsafe.Pointer)(add(unsafe.Pointer(&m.fun[0]), uintptr(k)*sys.PtrSize)) = ifn
}
goto nextimethod
continue imethods
}
}
}
// didn't find method
if !canfail {
if locked {
unlock(&ifaceLock)
}
panic(&TypeAssertionError{"", typ.string(), inter.typ.string(), iname})
}
m.bad = true
break
nextimethod:
return iname
}
if !locked {
throw("invalid itab locking")
}
h := itabhash(inter, typ)
m.link = hash[h]
m.inhash = true
atomicstorep(unsafe.Pointer(&hash[h]), unsafe.Pointer(m))
return ""
}
func itabsinit() {
lock(&ifaceLock)
lock(&itabLock)
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 {
additab(i, true, false)
}
itabAddStartup(i)
}
}
unlock(&ifaceLock)
unlock(&itabLock)
}
// panicdottypeE is called when doing an e.(T) conversion and the conversion fails.
@ -533,9 +608,12 @@ func reflect_ifaceE2I(inter *interfacetype, e eface, dst *iface) {
}
func iterate_itabs(fn func(*itab)) {
for _, h := range &hash {
for ; h != nil; h = h.link {
fn(h)
// Note: only runs during stop the world, so no 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)
}
}
}

8
src/runtime/plugin.go
View File

@ -54,13 +54,11 @@ func plugin_lastmoduleinit() (path string, syms map[string]interface{}, mismatch
pluginftabverify(md)
moduledataverify1(md)
lock(&ifaceLock)
lock(&itabLock)
for _, i := range md.itablinks {
if !i.inhash {
additab(i, true, false)
}
itabAddStartup(i)
}
unlock(&ifaceLock)
unlock(&itabLock)
// Build a map of symbol names to symbols. Here in the runtime
// we fill out the first word of the interface, the type. We

15
src/runtime/runtime2.go
View File

@ -624,14 +624,13 @@ type _func struct {
// Needs to be in sync with
// ../cmd/compile/internal/gc/reflect.go:/^func.dumptypestructs.
type itab struct {
inter *interfacetype
_type *_type
link *itab
hash uint32 // copy of _type.hash. Used for type switches.
bad bool // type does not implement interface
inhash bool // has this itab been added to hash?
unused [2]byte
fun [1]uintptr // variable sized
inter *interfacetype
_type *_type
_ uintptr
hash uint32 // copy of _type.hash. Used for type switches.
bad bool // type does not implement interface
_ [3]byte
fun [1]uintptr // variable sized
}
// Lock-free stack node.