* README.Portability: Remove note on an Irix compatibility issue.

[official-gcc.git] / libgo / go / runtime / runtime2.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

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

// defined constants
const (
        // G status
        //
        // Beyond indicating the general state of a G, the G status
        // acts like a lock on the goroutine's stack (and hence its
        // ability to execute user code).
        //
        // If you add to this list, add to the list
        // of "okay during garbage collection" status
        // in mgcmark.go too.

        // _Gidle means this goroutine was just allocated and has not
        // yet been initialized.
        _Gidle = iota // 0

        // _Grunnable means this goroutine is on a run queue. It is
        // not currently executing user code. The stack is not owned.
        _Grunnable // 1

        // _Grunning means this goroutine may execute user code. The
        // stack is owned by this goroutine. It is not on a run queue.
        // It is assigned an M and a P.
        _Grunning // 2

        // _Gsyscall means this goroutine is executing a system call.
        // It is not executing user code. The stack is owned by this
        // goroutine. It is not on a run queue. It is assigned an M.
        _Gsyscall // 3

        // _Gwaiting means this goroutine is blocked in the runtime.
        // It is not executing user code. It is not on a run queue,
        // but should be recorded somewhere (e.g., a channel wait
        // queue) so it can be ready()d when necessary. The stack is
        // not owned *except* that a channel operation may read or
        // write parts of the stack under the appropriate channel
        // lock. Otherwise, it is not safe to access the stack after a
        // goroutine enters _Gwaiting (e.g., it may get moved).
        _Gwaiting // 4

        // _Gmoribund_unused is currently unused, but hardcoded in gdb
        // scripts.
        _Gmoribund_unused // 5

        // _Gdead means this goroutine is currently unused. It may be
        // just exited, on a free list, or just being initialized. It
        // is not executing user code. It may or may not have a stack
        // allocated. The G and its stack (if any) are owned by the M
        // that is exiting the G or that obtained the G from the free
        // list.
        _Gdead // 6

        // _Genqueue_unused is currently unused.
        _Genqueue_unused // 7

        // _Gcopystack means this goroutine's stack is being moved. It
        // is not executing user code and is not on a run queue. The
        // stack is owned by the goroutine that put it in _Gcopystack.
        _Gcopystack // 8

        // _Gscan combined with one of the above states other than
        // _Grunning indicates that GC is scanning the stack. The
        // goroutine is not executing user code and the stack is owned
        // by the goroutine that set the _Gscan bit.
        //
        // _Gscanrunning is different: it is used to briefly block
        // state transitions while GC signals the G to scan its own
        // stack. This is otherwise like _Grunning.
        //
        // atomicstatus&~Gscan gives the state the goroutine will
        // return to when the scan completes.
        _Gscan         = 0x1000
        _Gscanrunnable = _Gscan + _Grunnable // 0x1001
        _Gscanrunning  = _Gscan + _Grunning  // 0x1002
        _Gscansyscall  = _Gscan + _Gsyscall  // 0x1003
        _Gscanwaiting  = _Gscan + _Gwaiting  // 0x1004
)

const (
        // P status
        _Pidle    = iota
        _Prunning // Only this P is allowed to change from _Prunning.
        _Psyscall
        _Pgcstop
        _Pdead
)

// Mutual exclusion locks.  In the uncontended case,
// as fast as spin locks (just a few user-level instructions),
// but on the contention path they sleep in the kernel.
// A zeroed Mutex is unlocked (no need to initialize each lock).
type mutex struct {
        // Futex-based impl treats it as uint32 key,
        // while sema-based impl as M* waitm.
        // Used to be a union, but unions break precise GC.
        key uintptr
}

// sleep and wakeup on one-time events.
// before any calls to notesleep or notewakeup,
// must call noteclear to initialize the Note.
// then, exactly one thread can call notesleep
// and exactly one thread can call notewakeup (once).
// once notewakeup has been called, the notesleep
// will return.  future notesleep will return immediately.
// subsequent noteclear must be called only after
// previous notesleep has returned, e.g. it's disallowed
// to call noteclear straight after notewakeup.
//
// notetsleep is like notesleep but wakes up after
// a given number of nanoseconds even if the event
// has not yet happened.  if a goroutine uses notetsleep to
// wake up early, it must wait to call noteclear until it
// can be sure that no other goroutine is calling
// notewakeup.
//
// notesleep/notetsleep are generally called on g0,
// notetsleepg is similar to notetsleep but is called on user g.
type note struct {
        // Futex-based impl treats it as uint32 key,
        // while sema-based impl as M* waitm.
        // Used to be a union, but unions break precise GC.
        key uintptr
}

type funcval struct {
        fn uintptr
        // variable-size, fn-specific data here
}

// The representation of a non-empty interface.
// See comment in iface.go for more details on this struct.
type iface struct {
        tab  unsafe.Pointer
        data unsafe.Pointer
}

// The representation of an empty interface.
// See comment in iface.go for more details on this struct.
type eface struct {
        _type *_type
        data  unsafe.Pointer
}

func efaceOf(ep *interface{}) *eface {
        return (*eface)(unsafe.Pointer(ep))
}

// The guintptr, muintptr, and puintptr are all used to bypass write barriers.
// It is particularly important to avoid write barriers when the current P has
// been released, because the GC thinks the world is stopped, and an
// unexpected write barrier would not be synchronized with the GC,
// which can lead to a half-executed write barrier that has marked the object
// but not queued it. If the GC skips the object and completes before the
// queuing can occur, it will incorrectly free the object.
//
// We tried using special assignment functions invoked only when not
// holding a running P, but then some updates to a particular memory
// word went through write barriers and some did not. This breaks the
// write barrier shadow checking mode, and it is also scary: better to have
// a word that is completely ignored by the GC than to have one for which
// only a few updates are ignored.
//
// Gs, Ms, and Ps are always reachable via true pointers in the
// allgs, allm, and allp lists or (during allocation before they reach those lists)
// from stack variables.

// A guintptr holds a goroutine pointer, but typed as a uintptr
// to bypass write barriers. It is used in the Gobuf goroutine state
// and in scheduling lists that are manipulated without a P.
//
// The Gobuf.g goroutine pointer is almost always updated by assembly code.
// In one of the few places it is updated by Go code - func save - it must be
// treated as a uintptr to avoid a write barrier being emitted at a bad time.
// Instead of figuring out how to emit the write barriers missing in the
// assembly manipulation, we change the type of the field to uintptr,
// so that it does not require write barriers at all.
//
// Goroutine structs are published in the allg list and never freed.
// That will keep the goroutine structs from being collected.
// There is never a time that Gobuf.g's contain the only references
// to a goroutine: the publishing of the goroutine in allg comes first.
// Goroutine pointers are also kept in non-GC-visible places like TLS,
// so I can't see them ever moving. If we did want to start moving data
// in the GC, we'd need to allocate the goroutine structs from an
// alternate arena. Using guintptr doesn't make that problem any worse.
type guintptr uintptr

//go:nosplit
func (gp guintptr) ptr() *g { return (*g)(unsafe.Pointer(gp)) }

//go:nosplit
func (gp *guintptr) set(g *g) { *gp = guintptr(unsafe.Pointer(g)) }

//go:nosplit
func (gp *guintptr) cas(old, new guintptr) bool {
        return atomic.Casuintptr((*uintptr)(unsafe.Pointer(gp)), uintptr(old), uintptr(new))
}

// setGNoWB performs *gp = new without a write barrier.
// For times when it's impractical to use a guintptr.
//go:nosplit
//go:nowritebarrier
func setGNoWB(gp **g, new *g) {
        (*guintptr)(unsafe.Pointer(gp)).set(new)
}

type puintptr uintptr

//go:nosplit
func (pp puintptr) ptr() *p { return (*p)(unsafe.Pointer(pp)) }

//go:nosplit
func (pp *puintptr) set(p *p) { *pp = puintptr(unsafe.Pointer(p)) }

type muintptr uintptr

//go:nosplit
func (mp muintptr) ptr() *m { return (*m)(unsafe.Pointer(mp)) }

//go:nosplit
func (mp *muintptr) set(m *m) { *mp = muintptr(unsafe.Pointer(m)) }

// setMNoWB performs *mp = new without a write barrier.
// For times when it's impractical to use an muintptr.
//go:nosplit
//go:nowritebarrier
func setMNoWB(mp **m, new *m) {
        (*muintptr)(unsafe.Pointer(mp)).set(new)
}

// sudog represents a g in a wait list, such as for sending/receiving
// on a channel.
//
// sudog is necessary because the g ↔ synchronization object relation
// is many-to-many. A g can be on many wait lists, so there may be
// many sudogs for one g; and many gs may be waiting on the same
// synchronization object, so there may be many sudogs for one object.
//
// sudogs are allocated from a special pool. Use acquireSudog and
// releaseSudog to allocate and free them.
type sudog struct {
        // The following fields are protected by the hchan.lock of the
        // channel this sudog is blocking on. shrinkstack depends on
        // this.

        g          *g
        selectdone *uint32 // CAS to 1 to win select race (may point to stack)
        next       *sudog
        prev       *sudog
        elem       unsafe.Pointer // data element (may point to stack)

        // The following fields are never accessed concurrently.
        // waitlink is only accessed by g.

        acquiretime int64
        releasetime int64
        ticket      uint32
        waitlink    *sudog // g.waiting list
        c           *hchan // channel
}

type gcstats struct {
        // the struct must consist of only uint64's,
        // because it is casted to uint64[].
        nhandoff    uint64
        nhandoffcnt uint64
        nprocyield  uint64
        nosyield    uint64
        nsleep      uint64
}

/*
Not used by gccgo.

type libcall struct {
        fn   uintptr
        n    uintptr // number of parameters
        args uintptr // parameters
        r1   uintptr // return values
        r2   uintptr
        err  uintptr // error number
}

*/

/*
Not used by gccgo.

// describes how to handle callback
type wincallbackcontext struct {
        gobody       unsafe.Pointer // go function to call
        argsize      uintptr        // callback arguments size (in bytes)
        restorestack uintptr        // adjust stack on return by (in bytes) (386 only)
        cleanstack   bool
}
*/

/*
Not used by gccgo.

// Stack describes a Go execution stack.
// The bounds of the stack are exactly [lo, hi),
// with no implicit data structures on either side.
type stack struct {
        lo uintptr
        hi uintptr
}

// stkbar records the state of a G's stack barrier.
type stkbar struct {
        savedLRPtr uintptr // location overwritten by stack barrier PC
        savedLRVal uintptr // value overwritten at savedLRPtr
}
*/

type g struct {
        // Stack parameters.
        // stack describes the actual stack memory: [stack.lo, stack.hi).
        // stackguard0 is the stack pointer compared in the Go stack growth prologue.
        // It is stack.lo+StackGuard normally, but can be StackPreempt to trigger a preemption.
        // stackguard1 is the stack pointer compared in the C stack growth prologue.
        // It is stack.lo+StackGuard on g0 and gsignal stacks.
        // It is ~0 on other goroutine stacks, to trigger a call to morestackc (and crash).
        // Not for gccgo: stack       stack   // offset known to runtime/cgo
        // Not for gccgo: stackguard0 uintptr // offset known to liblink
        // Not for gccgo: stackguard1 uintptr // offset known to liblink

        _panic *_panic // innermost panic - offset known to liblink
        _defer *_defer // innermost defer
        m      *m      // current m; offset known to arm liblink
        // Not for gccgo: stackAlloc     uintptr // stack allocation is [stack.lo,stack.lo+stackAlloc)
        // Not for gccgo: sched          gobuf
        syscallsp uintptr // if status==Gsyscall, syscallsp = sched.sp to use during gc
        syscallpc uintptr // if status==Gsyscall, syscallpc = sched.pc to use during gc
        // Not for gccgo: stkbar         []stkbar       // stack barriers, from low to high (see top of mstkbar.go)
        // Not for gccgo: stkbarPos      uintptr        // index of lowest stack barrier not hit
        // Not for gccgo: stktopsp       uintptr        // expected sp at top of stack, to check in traceback
        param        unsafe.Pointer // passed parameter on wakeup
        atomicstatus uint32
        // Not for gccgo: stackLock      uint32 // sigprof/scang lock; TODO: fold in to atomicstatus
        goid           int64
        waitsince      int64  // approx time when the g become blocked
        waitreason     string // if status==Gwaiting
        schedlink      guintptr
        preempt        bool     // preemption signal, duplicates stackguard0 = stackpreempt
        paniconfault   bool     // panic (instead of crash) on unexpected fault address
        preemptscan    bool     // preempted g does scan for gc
        gcscandone     bool     // g has scanned stack; protected by _Gscan bit in status
        gcscanvalid    bool     // false at start of gc cycle, true if G has not run since last scan; transition from true to false by calling queueRescan and false to true by calling dequeueRescan
        throwsplit     bool     // must not split stack
        raceignore     int8     // ignore race detection events
        sysblocktraced bool     // StartTrace has emitted EvGoInSyscall about this goroutine
        sysexitticks   int64    // cputicks when syscall has returned (for tracing)
        traceseq       uint64   // trace event sequencer
        tracelastp     puintptr // last P emitted an event for this goroutine
        lockedm        *m
        sig            uint32
        writebuf       []byte
        sigcode0       uintptr
        sigcode1       uintptr
        sigpc          uintptr
        gopc           uintptr // pc of go statement that created this goroutine
        startpc        uintptr // pc of goroutine function
        // Not for gccgo: racectx        uintptr
        waiting *sudog // sudog structures this g is waiting on (that have a valid elem ptr); in lock order
        // Not for gccgo: cgoCtxt        []uintptr // cgo traceback context

        // Per-G GC state

        // gcRescan is this G's index in work.rescan.list. If this is
        // -1, this G is not on the rescan list.
        //
        // If gcphase != _GCoff and this G is visible to the garbage
        // collector, writes to this are protected by work.rescan.lock.
        gcRescan int32

        // gcAssistBytes is this G's GC assist credit in terms of
        // bytes allocated. If this is positive, then the G has credit
        // to allocate gcAssistBytes bytes without assisting. If this
        // is negative, then the G must correct this by performing
        // scan work. We track this in bytes to make it fast to update
        // and check for debt in the malloc hot path. The assist ratio
        // determines how this corresponds to scan work debt.
        gcAssistBytes int64

        // Remaining fields are specific to gccgo.

        exception unsafe.Pointer // current exception being thrown
        isforeign bool           // whether current exception is not from Go

        // Fields that hold stack and context information if status is Gsyscall
        gcstack       unsafe.Pointer
        gcstacksize   uintptr
        gcnextsegment unsafe.Pointer
        gcnextsp      unsafe.Pointer
        gcinitialsp   unsafe.Pointer
        gcregs        g_ucontext_t

        entry    unsafe.Pointer // goroutine entry point
        fromgogo bool           // whether entered from gogo function

        issystem     bool // do not output in stack dump
        isbackground bool // ignore in deadlock detector

        traceback *tracebackg // stack traceback buffer

        context      g_ucontext_t       // saved context for setcontext
        stackcontext [10]unsafe.Pointer // split-stack context
}

type m struct {
        g0 *g // goroutine with scheduling stack
        // Not for gccgo: morebuf gobuf  // gobuf arg to morestack
        // Not for gccgo: divmod  uint32 // div/mod denominator for arm - known to liblink

        // Fields not known to debuggers.
        procid  uint64 // for debuggers, but offset not hard-coded
        gsignal *g     // signal-handling g
        sigmask sigset // storage for saved signal mask
        // Not for gccgo: tls           [6]uintptr // thread-local storage (for x86 extern register)
        mstartfn    uintptr
        curg        *g       // current running goroutine
        caughtsig   guintptr // goroutine running during fatal signal
        p           puintptr // attached p for executing go code (nil if not executing go code)
        nextp       puintptr
        id          int32
        mallocing   int32
        throwing    int32
        preemptoff  string // if != "", keep curg running on this m
        locks       int32
        softfloat   int32
        dying       int32
        profilehz   int32
        helpgc      int32
        spinning    bool // m is out of work and is actively looking for work
        blocked     bool // m is blocked on a note
        inwb        bool // m is executing a write barrier
        newSigstack bool // minit on C thread called sigaltstack
        printlock   int8
        fastrand    uint32
        ncgocall    uint64 // number of cgo calls in total
        ncgo        int32  // number of cgo calls currently in progress
        // Not for gccgo: cgoCallersUse uint32      // if non-zero, cgoCallers in use temporarily
        // Not for gccgo: cgoCallers    *cgoCallers // cgo traceback if crashing in cgo call
        park        note
        alllink     *m // on allm
        schedlink   muintptr
        mcache      *mcache
        lockedg     *g
        createstack [32]location // stack that created this thread.
        // Not for gccgo: freglo        [16]uint32  // d[i] lsb and f[i]
        // Not for gccgo: freghi        [16]uint32  // d[i] msb and f[i+16]
        // Not for gccgo: fflag         uint32      // floating point compare flags
        locked        uint32  // tracking for lockosthread
        nextwaitm     uintptr // next m waiting for lock
        gcstats       gcstats
        needextram    bool
        traceback     uint8
        waitunlockf   unsafe.Pointer // todo go func(*g, unsafe.pointer) bool
        waitlock      unsafe.Pointer
        waittraceev   byte
        waittraceskip int
        startingtrace bool
        syscalltick   uint32
        // Not for gccgo: thread        uintptr // thread handle

        // these are here because they are too large to be on the stack
        // of low-level NOSPLIT functions.
        // Not for gccgo: libcall   libcall
        // Not for gccgo: libcallpc uintptr // for cpu profiler
        // Not for gccgo: libcallsp uintptr
        // Not for gccgo: libcallg  guintptr
        // Not for gccgo: syscall   libcall // stores syscall parameters on windows

        mos mOS

        // Remaining fields are specific to gccgo.

        gsignalstack     unsafe.Pointer // stack for gsignal
        gsignalstacksize uintptr

        dropextram bool // drop after call is done

        gcing int32
}

type p struct {
        lock mutex

        id          int32
        status      uint32 // one of pidle/prunning/...
        link        puintptr
        schedtick   uint32   // incremented on every scheduler call
        syscalltick uint32   // incremented on every system call
        m           muintptr // back-link to associated m (nil if idle)
        mcache      *mcache
        // Not for gccgo: racectx     uintptr

        // gccgo has only one size of defer.
        deferpool    []*_defer
        deferpoolbuf [32]*_defer

        // Cache of goroutine ids, amortizes accesses to runtime·sched.goidgen.
        goidcache    uint64
        goidcacheend uint64

        // Queue of runnable goroutines. Accessed without lock.
        runqhead uint32
        runqtail uint32
        runq     [256]guintptr
        // runnext, if non-nil, is a runnable G that was ready'd by
        // the current G and should be run next instead of what's in
        // runq if there's time remaining in the running G's time
        // slice. It will inherit the time left in the current time
        // slice. If a set of goroutines is locked in a
        // communicate-and-wait pattern, this schedules that set as a
        // unit and eliminates the (potentially large) scheduling
        // latency that otherwise arises from adding the ready'd
        // goroutines to the end of the run queue.
        runnext guintptr

        // Available G's (status == Gdead)
        gfree    *g
        gfreecnt int32

        sudogcache []*sudog
        sudogbuf   [128]*sudog

        tracebuf traceBufPtr

        // Not for gccgo for now: palloc persistentAlloc // per-P to avoid mutex

        // Per-P GC state
        gcAssistTime     int64 // Nanoseconds in assistAlloc
        gcBgMarkWorker   guintptr
        gcMarkWorkerMode gcMarkWorkerMode

        // gcw is this P's GC work buffer cache. The work buffer is
        // filled by write barriers, drained by mutator assists, and
        // disposed on certain GC state transitions.
        // Not for gccgo for now: gcw gcWork

        runSafePointFn uint32 // if 1, run sched.safePointFn at next safe point

        pad [sys.CacheLineSize]byte
}

const (
        // The max value of GOMAXPROCS.
        // There are no fundamental restrictions on the value.
        _MaxGomaxprocs = 1 << 8
)

type schedt struct {
        // accessed atomically. keep at top to ensure alignment on 32-bit systems.
        goidgen  uint64
        lastpoll uint64

        lock mutex

        midle        muintptr // idle m's waiting for work
        nmidle       int32    // number of idle m's waiting for work
        nmidlelocked int32    // number of locked m's waiting for work
        mcount       int32    // number of m's that have been created
        maxmcount    int32    // maximum number of m's allowed (or die)

        ngsys uint32 // number of system goroutines; updated atomically

        pidle      puintptr // idle p's
        npidle     uint32
        nmspinning uint32 // See "Worker thread parking/unparking" comment in proc.go.

        // Global runnable queue.
        runqhead guintptr
        runqtail guintptr
        runqsize int32

        // Global cache of dead G's.
        gflock mutex
        gfree  *g
        ngfree int32

        // Central cache of sudog structs.
        sudoglock  mutex
        sudogcache *sudog

        // Central pool of available defer structs.
        deferlock mutex
        deferpool *_defer

        gcwaiting  uint32 // gc is waiting to run
        stopwait   int32
        stopnote   note
        sysmonwait uint32
        sysmonnote note

        // safepointFn should be called on each P at the next GC
        // safepoint if p.runSafePointFn is set.
        safePointFn   func(*p)
        safePointWait int32
        safePointNote note

        profilehz int32 // cpu profiling rate

        procresizetime int64 // nanotime() of last change to gomaxprocs
        totaltime      int64 // ∫gomaxprocs dt up to procresizetime
}

// The m.locked word holds two pieces of state counting active calls to LockOSThread/lockOSThread.
// The low bit (LockExternal) is a boolean reporting whether any LockOSThread call is active.
// External locks are not recursive; a second lock is silently ignored.
// The upper bits of m.locked record the nesting depth of calls to lockOSThread
// (counting up by LockInternal), popped by unlockOSThread (counting down by LockInternal).
// Internal locks can be recursive. For instance, a lock for cgo can occur while the main
// goroutine is holding the lock during the initialization phase.
const (
        _LockExternal = 1
        _LockInternal = 2
)

const (
        _SigNotify   = 1 << iota // let signal.Notify have signal, even if from kernel
        _SigKill                 // if signal.Notify doesn't take it, exit quietly
        _SigThrow                // if signal.Notify doesn't take it, exit loudly
        _SigPanic                // if the signal is from the kernel, panic
        _SigDefault              // if the signal isn't explicitly requested, don't monitor it
        _SigHandling             // our signal handler is registered
        _SigGoExit               // cause all runtime procs to exit (only used on Plan 9).
        _SigSetStack             // add SA_ONSTACK to libc handler
        _SigUnblock              // unblocked in minit
)

// Lock-free stack node.
// // Also known to export_test.go.
type lfnode struct {
        next    uint64
        pushcnt uintptr
}

type forcegcstate struct {
        lock mutex
        g    *g
        idle uint32
}

// startup_random_data holds random bytes initialized at startup. These come from
// the ELF AT_RANDOM auxiliary vector (vdso_linux_amd64.go or os_linux_386.go).
var startupRandomData []byte

// extendRandom extends the random numbers in r[:n] to the whole slice r.
// Treats n<0 as n==0.
func extendRandom(r []byte, n int) {
        if n < 0 {
                n = 0
        }
        for n < len(r) {
                // Extend random bits using hash function & time seed
                w := n
                if w > 16 {
                        w = 16
                }
                h := memhash(unsafe.Pointer(&r[n-w]), uintptr(nanotime()), uintptr(w))
                for i := 0; i < sys.PtrSize && n < len(r); i++ {
                        r[n] = byte(h)
                        n++
                        h >>= 8
                }
        }
}

// deferred subroutine calls
// This is the gccgo version.
type _defer struct {
        // The next entry in the stack.
        link *_defer

        // The stack variable for the function which called this defer
        // statement.  This is set to true if we are returning from
        // that function, false if we are panicing through it.
        frame *bool

        // The value of the panic stack when this function is
        // deferred.  This function can not recover this value from
        // the panic stack.  This can happen if a deferred function
        // has a defer statement itself.
        _panic *_panic

        // The function to call.
        pfn uintptr

        // The argument to pass to the function.
        arg unsafe.Pointer

        // The return address that a recover thunk matches against.
        // This is set by __go_set_defer_retaddr which is called by
        // the thunks created by defer statements.
        retaddr uintptr

        // Set to true if a function created by reflect.MakeFunc is
        // permitted to recover.  The return address of such a
        // function function will be somewhere in libffi, so __retaddr
        // is not useful.
        makefunccanrecover bool

        // Set to true if this defer stack entry is not part of the
        // defer pool.
        special bool
}

// panics
// This is the gccgo version.
type _panic struct {
        // The next entry in the stack.
        link *_panic

        // The value associated with this panic.
        arg interface{}

        // Whether this panic has been recovered.
        recovered bool

        // Whether this panic was pushed on the stack because of an
        // exception thrown in some other language.
        isforeign bool
}

const (
        _TraceRuntimeFrames = 1 << iota // include frames for internal runtime functions.
        _TraceTrap                      // the initial PC, SP are from a trap, not a return PC from a call
        _TraceJumpStack                 // if traceback is on a systemstack, resume trace at g that called into it
)

// The maximum number of frames we print for a traceback
const _TracebackMaxFrames = 100

var (
        //      emptystring string

        allglen    uintptr
        allm       *m
        allp       [_MaxGomaxprocs + 1]*p
        gomaxprocs int32
        panicking  uint32
        ncpu       int32
        forcegc    forcegcstate
        sched      schedt
        newprocs   int32

        // Information about what cpu features are available.
        // Set on startup in asm_{x86,amd64}.s.
        cpuid_ecx   uint32
        support_aes bool

        // cpuid_edx         uint32
        // cpuid_ebx7        uint32
        // lfenceBeforeRdtsc bool
        // support_avx       bool
        // support_avx2      bool
        // support_bmi1      bool
        // support_bmi2      bool

//      goarm                uint8 // set by cmd/link on arm systems
//      framepointer_enabled bool  // set by cmd/link
)

// Set by the linker so the runtime can determine the buildmode.
var (
        islibrary bool // -buildmode=c-shared
        isarchive bool // -buildmode=c-archive
)

// Types that are only used by gccgo.

// g_ucontext_t is a Go version of the C ucontext_t type, used by getcontext.
// _sizeof_ucontext_t is defined by mkrsysinfo.sh from <ucontext.h>.
// On some systems getcontext and friends require a value that is
// aligned to a 16-byte boundary.  We implement this by increasing the
// required size and picking an appropriate offset when we use the
// array.
type g_ucontext_t [(_sizeof_ucontext_t + 15) / unsafe.Sizeof(unsafe.Pointer(nil))]unsafe.Pointer

// sigset is the Go version of the C type sigset_t.
// _sigset_t is defined by the Makefile from <signal.h>.
type sigset _sigset_t