[platform/upstream/gcc.git] / libgo / go / runtime / select.go

// Copyright 2009 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

// This file contains the implementation of Go select statements.

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

// For gccgo, use go:linkname to export compiler-called functions.
//
//go:linkname selectgo
//go:linkname block

const debugSelect = false

// scase.kind values.
// Known to compiler.
// Changes here must also be made in src/cmd/compile/internal/gc/select.go's walkselectcases.
const (
        caseNil = iota
        caseRecv
        caseSend
        caseDefault
)

// Select case descriptor.
// Known to compiler.
// Changes here must also be made in src/cmd/internal/gc/select.go's scasetype.
type scase struct {
        c    *hchan         // chan
        elem unsafe.Pointer // data element
        kind uint16
}

func sellock(scases []scase, lockorder []uint16) {
        var c *hchan
        for _, o := range lockorder {
                c0 := scases[o].c
                if c0 != c {
                        c = c0
                        lock(&c.lock)
                }
        }
}

func selunlock(scases []scase, lockorder []uint16) {
        // We must be very careful here to not touch sel after we have unlocked
        // the last lock, because sel can be freed right after the last unlock.
        // Consider the following situation.
        // First M calls runtime·park() in runtime·selectgo() passing the sel.
        // Once runtime·park() has unlocked the last lock, another M makes
        // the G that calls select runnable again and schedules it for execution.
        // When the G runs on another M, it locks all the locks and frees sel.
        // Now if the first M touches sel, it will access freed memory.
        for i := len(lockorder) - 1; i >= 0; i-- {
                c := scases[lockorder[i]].c
                if i > 0 && c == scases[lockorder[i-1]].c {
                        continue // will unlock it on the next iteration
                }
                unlock(&c.lock)
        }
}

func selparkcommit(gp *g, _ unsafe.Pointer) bool {
        // There are unlocked sudogs that point into gp's stack. Stack
        // copying must lock the channels of those sudogs.
        // Set activeStackChans here instead of before we try parking
        // because we could self-deadlock in stack growth on a
        // channel lock.
        gp.activeStackChans = true
        // Mark that it's safe for stack shrinking to occur now,
        // because any thread acquiring this G's stack for shrinking
        // is guaranteed to observe activeStackChans after this store.
        atomic.Store8(&gp.parkingOnChan, 0)
        // Make sure we unlock after setting activeStackChans and
        // unsetting parkingOnChan. The moment we unlock any of the
        // channel locks we risk gp getting readied by a channel operation
        // and so gp could continue running before everything before the
        // unlock is visible (even to gp itself).

        // This must not access gp's stack (see gopark). In
        // particular, it must not access the *hselect. That's okay,
        // because by the time this is called, gp.waiting has all
        // channels in lock order.
        var lastc *hchan
        for sg := gp.waiting; sg != nil; sg = sg.waitlink {
                if sg.c != lastc && lastc != nil {
                        // As soon as we unlock the channel, fields in
                        // any sudog with that channel may change,
                        // including c and waitlink. Since multiple
                        // sudogs may have the same channel, we unlock
                        // only after we've passed the last instance
                        // of a channel.
                        unlock(&lastc.lock)
                }
                lastc = sg.c
        }
        if lastc != nil {
                unlock(&lastc.lock)
        }
        return true
}

func block() {
        gopark(nil, nil, waitReasonSelectNoCases, traceEvGoStop, 1) // forever
}

// selectgo implements the select statement.
//
// cas0 points to an array of type [ncases]scase, and order0 points to
// an array of type [2*ncases]uint16 where ncases must be <= 65536.
// Both reside on the goroutine's stack (regardless of any escaping in
// selectgo).
//
// selectgo returns the index of the chosen scase, which matches the
// ordinal position of its respective select{recv,send,default} call.
// Also, if the chosen scase was a receive operation, it reports whether
// a value was received.
func selectgo(cas0 *scase, order0 *uint16, ncases int) (int, bool) {
        if debugSelect {
                print("select: cas0=", cas0, "\n")
        }

        // NOTE: In order to maintain a lean stack size, the number of scases
        // is capped at 65536.
        cas1 := (*[1 << 16]scase)(unsafe.Pointer(cas0))
        order1 := (*[1 << 17]uint16)(unsafe.Pointer(order0))

        scases := cas1[:ncases:ncases]
        pollorder := order1[:ncases:ncases]
        lockorder := order1[ncases:][:ncases:ncases]

        var t0 int64
        if blockprofilerate > 0 {
                t0 = cputicks()
        }

        // The compiler rewrites selects that statically have
        // only 0 or 1 cases plus default into simpler constructs.
        // The only way we can end up with such small sel.ncase
        // values here is for a larger select in which most channels
        // have been nilled out. The general code handles those
        // cases correctly, and they are rare enough not to bother
        // optimizing (and needing to test).

        // needed for gccgo, which doesn't zero pollorder
        if ncases > 0 {
                pollorder[0] = 0
        }

        // generate permuted order
        dfli := -1
        norder := 0
        for i := range scases {
                cas := &scases[i]

                // Omit cases without channels from the poll and lock orders.
                if cas.c == nil {
                        if cas.kind == caseDefault {
                                dfli = i
                        }
                        cas.elem = nil // allow GC
                        continue
                }

                j := fastrandn(uint32(norder + 1))
                pollorder[norder] = pollorder[j]
                pollorder[j] = uint16(i)
                norder++
        }
        pollorder = pollorder[:norder]
        lockorder = lockorder[:norder]

        // sort the cases by Hchan address to get the locking order.
        // simple heap sort, to guarantee n log n time and constant stack footprint.
        for i := range lockorder {
                j := i
                // Start with the pollorder to permute cases on the same channel.
                c := scases[pollorder[i]].c
                for j > 0 && scases[lockorder[(j-1)/2]].c.sortkey() < c.sortkey() {
                        k := (j - 1) / 2
                        lockorder[j] = lockorder[k]
                        j = k
                }
                lockorder[j] = pollorder[i]
        }
        for i := len(lockorder) - 1; i >= 0; i-- {
                o := lockorder[i]
                c := scases[o].c
                lockorder[i] = lockorder[0]
                j := 0
                for {
                        k := j*2 + 1
                        if k >= i {
                                break
                        }
                        if k+1 < i && scases[lockorder[k]].c.sortkey() < scases[lockorder[k+1]].c.sortkey() {
                                k++
                        }
                        if c.sortkey() < scases[lockorder[k]].c.sortkey() {
                                lockorder[j] = lockorder[k]
                                j = k
                                continue
                        }
                        break
                }
                lockorder[j] = o
        }

        if debugSelect {
                for i := 0; i+1 < len(lockorder); i++ {
                        if scases[lockorder[i]].c.sortkey() > scases[lockorder[i+1]].c.sortkey() {
                                print("i=", i, " x=", lockorder[i], " y=", lockorder[i+1], "\n")
                                throw("select: broken sort")
                        }
                }
        }

        // lock all the channels involved in the select
        sellock(scases, lockorder)

        var (
                gp     *g
                sg     *sudog
                c      *hchan
                k      *scase
                sglist *sudog
                sgnext *sudog
                qp     unsafe.Pointer
                nextp  **sudog
        )

loop:
        // pass 1 - look for something already waiting
        var casi int
        var cas *scase
        var caseReleaseTime int64 = -1
        var recvOK bool
        for _, casei := range pollorder {
                casi = int(casei)
                cas = &scases[casi]
                c = cas.c

                switch cas.kind {
                case caseRecv:
                        sg = c.sendq.dequeue()
                        if sg != nil {
                                goto recv
                        }
                        if c.qcount > 0 {
                                goto bufrecv
                        }
                        if c.closed != 0 {
                                goto rclose
                        }

                case caseSend:
                        if c.closed != 0 {
                                goto sclose
                        }
                        sg = c.recvq.dequeue()
                        if sg != nil {
                                goto send
                        }
                        if c.qcount < c.dataqsiz {
                                goto bufsend
                        }
                }
        }

        if dfli >= 0 {
                selunlock(scases, lockorder)
                casi = dfli
                goto retc
        }

        // pass 2 - enqueue on all chans
        gp = getg()
        if gp.waiting != nil {
                throw("gp.waiting != nil")
        }
        nextp = &gp.waiting
        for _, casei := range lockorder {
                casi = int(casei)
                cas = &scases[casi]
                c = cas.c
                sg := acquireSudog()
                sg.g = gp
                sg.isSelect = true
                // No stack splits between assigning elem and enqueuing
                // sg on gp.waiting where copystack can find it.
                sg.elem = cas.elem
                sg.releasetime = 0
                if t0 != 0 {
                        sg.releasetime = -1
                }
                sg.c = c
                // Construct waiting list in lock order.
                *nextp = sg
                nextp = &sg.waitlink

                switch cas.kind {
                case caseRecv:
                        c.recvq.enqueue(sg)

                case caseSend:
                        c.sendq.enqueue(sg)
                }
        }

        // wait for someone to wake us up
        gp.param = nil
        // Signal to anyone trying to shrink our stack that we're about
        // to park on a channel. The window between when this G's status
        // changes and when we set gp.activeStackChans is not safe for
        // stack shrinking.
        atomic.Store8(&gp.parkingOnChan, 1)
        gopark(selparkcommit, nil, waitReasonSelect, traceEvGoBlockSelect, 1)
        gp.activeStackChans = false

        sellock(scases, lockorder)

        gp.selectDone = 0
        sg = (*sudog)(gp.param)
        gp.param = nil

        // pass 3 - dequeue from unsuccessful chans
        // otherwise they stack up on quiet channels
        // record the successful case, if any.
        // We singly-linked up the SudoGs in lock order.
        casi = -1
        cas = nil
        sglist = gp.waiting
        // Clear all elem before unlinking from gp.waiting.
        for sg1 := gp.waiting; sg1 != nil; sg1 = sg1.waitlink {
                sg1.isSelect = false
                sg1.elem = nil
                sg1.c = nil
        }
        gp.waiting = nil

        for _, casei := range lockorder {
                k = &scases[casei]
                if sg == sglist {
                        // sg has already been dequeued by the G that woke us up.
                        casi = int(casei)
                        cas = k
                        if sglist.releasetime > 0 {
                                caseReleaseTime = sglist.releasetime
                        }
                } else {
                        c = k.c
                        if k.kind == caseSend {
                                c.sendq.dequeueSudoG(sglist)
                        } else {
                                c.recvq.dequeueSudoG(sglist)
                        }
                }
                sgnext = sglist.waitlink
                sglist.waitlink = nil
                releaseSudog(sglist)
                sglist = sgnext
        }

        if cas == nil {
                // We can wake up with gp.param == nil (so cas == nil)
                // when a channel involved in the select has been closed.
                // It is easiest to loop and re-run the operation;
                // we'll see that it's now closed.
                // Maybe some day we can signal the close explicitly,
                // but we'd have to distinguish close-on-reader from close-on-writer.
                // It's easiest not to duplicate the code and just recheck above.
                // We know that something closed, and things never un-close,
                // so we won't block again.
                goto loop
        }

        c = cas.c

        if debugSelect {
                print("wait-return: cas0=", cas0, " c=", c, " cas=", cas, " kind=", cas.kind, "\n")
        }

        if cas.kind == caseRecv {
                recvOK = true
        }

        selunlock(scases, lockorder)
        goto retc

bufrecv:
        // can receive from buffer
        recvOK = true
        qp = chanbuf(c, c.recvx)
        if cas.elem != nil {
                typedmemmove(c.elemtype, cas.elem, qp)
        }
        typedmemclr(c.elemtype, qp)
        c.recvx++
        if c.recvx == c.dataqsiz {
                c.recvx = 0
        }
        c.qcount--
        selunlock(scases, lockorder)
        goto retc

bufsend:
        // can send to buffer
        typedmemmove(c.elemtype, chanbuf(c, c.sendx), cas.elem)
        c.sendx++
        if c.sendx == c.dataqsiz {
                c.sendx = 0
        }
        c.qcount++
        selunlock(scases, lockorder)
        goto retc

recv:
        // can receive from sleeping sender (sg)
        recv(c, sg, cas.elem, func() { selunlock(scases, lockorder) }, 2)
        if debugSelect {
                print("syncrecv: cas0=", cas0, " c=", c, "\n")
        }
        recvOK = true
        goto retc

rclose:
        // read at end of closed channel
        selunlock(scases, lockorder)
        recvOK = false
        if cas.elem != nil {
                typedmemclr(c.elemtype, cas.elem)
        }
        if raceenabled {
                raceacquire(c.raceaddr())
        }
        goto retc

send:
        // can send to a sleeping receiver (sg)
        send(c, sg, cas.elem, func() { selunlock(scases, lockorder) }, 2)
        if debugSelect {
                print("syncsend: cas0=", cas0, " c=", c, "\n")
        }
        goto retc

retc:
        if caseReleaseTime > 0 {
                blockevent(caseReleaseTime-t0, 1)
        }

        // Check preemption, since unlike gc we don't check on every call.
        // A test case for this one is BenchmarkPingPongHog in proc_test.go.
        if dfli >= 0 && getg().preempt {
                checkPreempt()
        }

        return casi, recvOK

sclose:
        // send on closed channel
        selunlock(scases, lockorder)
        panic(plainError("send on closed channel"))
}

func (c *hchan) sortkey() uintptr {
        return uintptr(unsafe.Pointer(c))
}

// A runtimeSelect is a single case passed to rselect.
// This must match ../reflect/value.go:/runtimeSelect
type runtimeSelect struct {
        dir selectDir
        typ unsafe.Pointer // channel type (not used here)
        ch  *hchan         // channel
        val unsafe.Pointer // ptr to data (SendDir) or ptr to receive buffer (RecvDir)
}

// These values must match ../reflect/value.go:/SelectDir.
type selectDir int

const (
        _             selectDir = iota
        selectSend              // case Chan <- Send
        selectRecv              // case <-Chan:
        selectDefault           // default
)

//go:linkname reflect_rselect reflect.rselect
func reflect_rselect(cases []runtimeSelect) (int, bool) {
        if len(cases) == 0 {
                block()
        }
        sel := make([]scase, len(cases))
        order := make([]uint16, 2*len(cases))
        for i := range cases {
                rc := &cases[i]
                switch rc.dir {
                case selectDefault:
                        sel[i] = scase{kind: caseDefault}
                case selectSend:
                        sel[i] = scase{kind: caseSend, c: rc.ch, elem: rc.val}
                case selectRecv:
                        sel[i] = scase{kind: caseRecv, c: rc.ch, elem: rc.val}
                }
        }

        return selectgo(&sel[0], &order[0], len(cases))
}

func (q *waitq) dequeueSudoG(sgp *sudog) {
        x := sgp.prev
        y := sgp.next
        if x != nil {
                if y != nil {
                        // middle of queue
                        x.next = y
                        y.prev = x
                        sgp.next = nil
                        sgp.prev = nil
                        return
                }
                // end of queue
                x.next = nil
                q.last = x
                sgp.prev = nil
                return
        }
        if y != nil {
                // start of queue
                y.prev = nil
                q.first = y
                sgp.next = nil
                return
        }

        // x==y==nil. Either sgp is the only element in the queue,
        // or it has already been removed. Use q.first to disambiguate.
        if q.first == sgp {
                q.first = nil
                q.last = nil
        }
}