task.c

// This file is a part of Julia. License is MIT: https://julialang.org/license

/*
  task.c
  lightweight processes (symmetric coroutines)
*/

// need this to get the real definition of ucontext_t,
// if we're going to use the ucontext_t implementation there
//#if defined(__APPLE__) && defined(JL_HAVE_UCONTEXT)
//#pragma push_macro("_XOPEN_SOURCE")
//#define _XOPEN_SOURCE
//#include <ucontext.h>
//#pragma pop_macro("_XOPEN_SOURCE")
//#endif

// this is needed for !COPY_STACKS to work on linux
#ifdef _FORTIFY_SOURCE
// disable __longjmp_chk validation so that we can jump between stacks
// (which would normally be invalid to do with setjmp / longjmp)
#pragma push_macro("_FORTIFY_SOURCE")
#undef _FORTIFY_SOURCE
#include <setjmp.h>
#pragma pop_macro("_FORTIFY_SOURCE")
#endif

#include "platform.h"

#include <stdlib.h>
#include <string.h>
#include <signal.h>
#include <unistd.h>
#include <errno.h>
#include <inttypes.h>
#include "julia.h"
#include "julia_internal.h"
#include "threading.h"
#include "julia_assert.h"
#include "support/hashing.h"

#ifdef __cplusplus
extern "C" {
#endif

#if defined(_COMPILER_ASAN_ENABLED_)
static inline void sanitizer_start_switch_fiber(const void* bottom, size_t size) {
    __sanitizer_start_switch_fiber(NULL, bottom, size);
}
static inline void sanitizer_finish_switch_fiber(void) {
    __sanitizer_finish_switch_fiber(NULL, NULL, NULL);
}
#else
static inline void sanitizer_start_switch_fiber(const void* bottom, size_t size) {}
static inline void sanitizer_finish_switch_fiber(void) {}
#endif

#if defined(_COMPILER_TSAN_ENABLED_)
static inline void tsan_destroy_ctx(jl_ptls_t ptls, void *state) {
    if (state != &ptls->root_task->state) {
        __tsan_destroy_fiber(ctx->state);
    }
    ctx->state = NULL;
}
static inline void tsan_switch_to_ctx(void *state)  {
    __tsan_switch_to_fiber(state, 0);
}
#endif

// empirically, jl_finish_task needs about 64k stack space to infer/run
// and additionally, gc-stack reserves 64k for the guard pages
#if defined(MINSIGSTKSZ)
#define MINSTKSZ (MINSIGSTKSZ > 131072 ? MINSIGSTKSZ : 131072)
#else
#define MINSTKSZ 131072
#endif

#define ROOT_TASK_STACK_ADJUSTMENT 3000000

#ifdef JL_HAVE_ASYNCIFY
// Switching logic is implemented in JavaScript
#define STATIC_OR_JS JL_DLLEXPORT
#else
#define STATIC_OR_JS static
#endif

extern size_t jl_page_size;
static char *jl_alloc_fiber(jl_ucontext_t *t, size_t *ssize, jl_task_t *owner) JL_NOTSAFEPOINT;
STATIC_OR_JS void jl_set_fiber(jl_ucontext_t *t);
STATIC_OR_JS void jl_swap_fiber(jl_ucontext_t *lastt, jl_ucontext_t *t);
STATIC_OR_JS void jl_start_fiber_swap(jl_ucontext_t *savet, jl_ucontext_t *t);
STATIC_OR_JS void jl_start_fiber_set(jl_ucontext_t *t);

#ifdef ALWAYS_COPY_STACKS
# ifndef COPY_STACKS
# error "ALWAYS_COPY_STACKS requires COPY_STACKS"
# endif
static int always_copy_stacks = 1;
#else
static int always_copy_stacks = 0;
#endif

#ifdef COPY_STACKS
static void memcpy_a16(uint64_t *to, uint64_t *from, size_t nb)
{
    memcpy((char*)jl_assume_aligned(to, 16), (char*)jl_assume_aligned(from, 16), nb);
    //uint64_t *end = (uint64_t*)((char*)from + nb);
    //while (from < end)
    //    *(to++) = *(from++);
}

static void NOINLINE save_stack(jl_ptls_t ptls, jl_task_t *lastt, jl_task_t **pt)
{
    char *frame_addr = (char*)((uintptr_t)jl_get_frame_addr() & ~15);
    char *stackbase = (char*)ptls->stackbase;
    assert(stackbase > frame_addr);
    size_t nb = stackbase - frame_addr;
    void *buf;
    if (lastt->bufsz < nb) {
        buf = (void*)jl_gc_alloc_buf(ptls, nb);
        lastt->stkbuf = buf;
        lastt->bufsz = nb;
    }
    else {
        buf = lastt->stkbuf;
    }
    *pt = NULL; // clear the gc-root for the target task before copying the stack for saving
    lastt->copy_stack = nb;
    lastt->sticky = 1;
    memcpy_a16((uint64_t*)buf, (uint64_t*)frame_addr, nb);
    // this task's stack could have been modified after
    // it was marked by an incremental collection
    // move the barrier back instead of walking it again here
    jl_gc_wb_back(lastt);
}

static void NOINLINE JL_NORETURN restore_stack(jl_task_t *t, jl_ptls_t ptls, char *p)
{
    size_t nb = t->copy_stack;
    char *_x = (char*)ptls->stackbase - nb;
    if (!p) {
        // switch to a stackframe that's beyond the bounds of the last switch
        p = _x;
        if ((char*)&_x > _x) {
            p = (char*)alloca((char*)&_x - _x);
        }
        restore_stack(t, ptls, p); // pass p to ensure the compiler can't tailcall this or avoid the alloca
    }
    void *_y = t->stkbuf;
    assert(_x != NULL && _y != NULL);
    memcpy_a16((uint64_t*)_x, (uint64_t*)_y, nb); // destroys all but the current stackframe

    sanitizer_start_switch_fiber(t->stkbuf, t->bufsz);
#if defined(_OS_WINDOWS_)
    jl_setcontext(&t->ctx);
#else
    jl_longjmp(t->copy_stack_ctx.uc_mcontext, 1);
#endif
    abort(); // unreachable
}

static void restore_stack2(jl_task_t *t, jl_ptls_t ptls, jl_task_t *lastt)
{
    assert(t->copy_stack && !lastt->copy_stack);
    size_t nb = t->copy_stack;
    char *_x = (char*)ptls->stackbase - nb;
    void *_y = t->stkbuf;
    assert(_x != NULL && _y != NULL);
    memcpy_a16((uint64_t*)_x, (uint64_t*)_y, nb); // destroys all but the current stackframe
#if defined(JL_HAVE_UNW_CONTEXT)
    volatile int returns = 0;
    int r = unw_getcontext(&lastt->ctx);
    if (++returns == 2) // r is garbage after the first return
        return;
    if (r != 0 || returns != 1)
        abort();
#elif defined(JL_HAVE_ASM) || defined(JL_HAVE_SIGALTSTACK) || defined(_OS_WINDOWS_)
    if (jl_setjmp(lastt->copy_stack_ctx.uc_mcontext, 0))
        return;
#else
#error COPY_STACKS is incompatible with this platform
#endif
    sanitizer_start_switch_fiber(t->stkbuf, t->bufsz);
#if defined(_OS_WINDOWS_)
    jl_setcontext(&t->ctx);
#else
    jl_longjmp(t->copy_stack_ctx.uc_mcontext, 1);
#endif
}
#endif

/* Rooted by the base module */
static jl_function_t *task_done_hook_func JL_GLOBALLY_ROOTED = NULL;

void JL_NORETURN jl_finish_task(jl_task_t *t)
{
    jl_task_t *ct = jl_current_task;
    JL_SIGATOMIC_BEGIN();
    if (t->_isexception)
        jl_atomic_store_release(&t->_state, JL_TASK_STATE_FAILED);
    else
        jl_atomic_store_release(&t->_state, JL_TASK_STATE_DONE);
    if (t->copy_stack) // early free of stkbuf
        t->stkbuf = NULL;
    // ensure that state is cleared
    ct->ptls->in_finalizer = 0;
    ct->ptls->in_pure_callback = 0;
    ct->world_age = jl_world_counter;
    // let the runtime know this task is dead and find a new task to run
    jl_function_t *done = jl_atomic_load_relaxed(&task_done_hook_func);
    if (done == NULL) {
        done = (jl_function_t*)jl_get_global(jl_base_module, jl_symbol("task_done_hook"));
        if (done != NULL)
            jl_atomic_store_release(&task_done_hook_func, done);
    }
    if (done != NULL) {
        jl_value_t *args[2] = {done, (jl_value_t*)t};
        JL_TRY {
            jl_apply(args, 2);
        }
        JL_CATCH {
            jl_no_exc_handler(jl_current_exception());
        }
    }
    gc_debug_critical_error();
    abort();
}

JL_DLLEXPORT void *jl_task_stack_buffer(jl_task_t *task, size_t *size, int *ptid)
{
    size_t off = 0;
#ifndef _OS_WINDOWS_
    if (jl_all_tls_states[0]->root_task == task) {
        // See jl_init_root_task(). The root task of the main thread
        // has its buffer enlarged by an artificial 3000000 bytes, but
        // that means that the start of the buffer usually points to
        // inaccessible memory. We need to correct for this.
        off = ROOT_TASK_STACK_ADJUSTMENT;
    }
#endif
    jl_ptls_t ptls2 = task->ptls;
    *ptid = -1;
    if (ptls2) {
        *ptid = task->tid;
#ifdef COPY_STACKS
        if (task->copy_stack) {
            *size = ptls2->stacksize;
            return (char *)ptls2->stackbase - *size;
        }
#endif
    }
    *size = task->bufsz - off;
    return (void *)((char *)task->stkbuf + off);
}

JL_DLLEXPORT void jl_active_task_stack(jl_task_t *task,
                                       char **active_start, char **active_end,
                                       char **total_start, char **total_end)
{
    if (!task->started) {
        *total_start = *active_start = 0;
        *total_end = *active_end = 0;
        return;
    }

    jl_ptls_t ptls2 = task->ptls;
    if (task->copy_stack && ptls2) {
        *total_start = *active_start = (char*)ptls2->stackbase - ptls2->stacksize;
        *total_end = *active_end = (char*)ptls2->stackbase;
    }
    else if (task->stkbuf) {
        *total_start = *active_start = (char*)task->stkbuf;
#ifndef _OS_WINDOWS_
        if (jl_all_tls_states[0]->root_task == task) {
            // See jl_init_root_task(). The root task of the main thread
            // has its buffer enlarged by an artificial 3000000 bytes, but
            // that means that the start of the buffer usually points to
            // inaccessible memory. We need to correct for this.
            *active_start += ROOT_TASK_STACK_ADJUSTMENT;
            *total_start += ROOT_TASK_STACK_ADJUSTMENT;
        }
#endif

        *total_end = *active_end = (char*)task->stkbuf + task->bufsz;
#ifdef COPY_STACKS
        // save_stack stores the stack of an inactive task in stkbuf, and the
        // actual number of used bytes in copy_stack.
        if (task->copy_stack > 1)
            *active_end = (char*)task->stkbuf + task->copy_stack;
#endif
    }
    else {
        // no stack allocated yet
        *total_start = *active_start = 0;
        *total_end = *active_end = 0;
        return;
    }

    if (task == jl_current_task) {
        // scan up to current `sp` for current thread and task
        *active_start = (char*)jl_get_frame_addr();
    }
}

// Marked noinline so we can consistently skip the associated frame.
// `skip` is number of additional frames to skip.
NOINLINE static void record_backtrace(jl_ptls_t ptls, int skip) JL_NOTSAFEPOINT
{
    // storing bt_size in ptls ensures roots in bt_data will be found
    ptls->bt_size = rec_backtrace(ptls->bt_data, JL_MAX_BT_SIZE, skip + 1);
}

JL_DLLEXPORT void jl_set_next_task(jl_task_t *task) JL_NOTSAFEPOINT
{
    jl_current_task->ptls->next_task = task;
}

JL_DLLEXPORT jl_task_t *jl_get_next_task(void) JL_NOTSAFEPOINT
{
    jl_task_t *ct = jl_current_task;
    if (ct->ptls->next_task)
        return ct->ptls->next_task;
    return ct;
}

#ifdef _COMPILER_TSAN_ENABLED_
const char tsan_state_corruption[] = "TSAN state corrupted. Exiting HARD!\n";
#endif

static void ctx_switch(jl_task_t *lastt)
{
    jl_ptls_t ptls = lastt->ptls;
    jl_task_t **pt = &ptls->next_task;
    jl_task_t *t = *pt;
    assert(t != lastt);
    // none of these locks should be held across a task switch
    assert(ptls->locks.len == 0);

#ifdef _COMPILER_TSAN_ENABLED_
    if (lastt->ctx.tsan_state != __tsan_get_current_fiber()) {
        // Something went really wrong - don't even assume that we can
        // use assert/abort which involve lots of signal handling that
        // looks at the tsan state.
        write(STDERR_FILENO, tsan_state_corruption, sizeof(tsan_state_corruption) - 1);
        _exit(1);
    }
#endif

    int killed = lastt->_state != JL_TASK_STATE_RUNNABLE;
    if (!t->started && !t->copy_stack) {
        // may need to allocate the stack
        if (t->stkbuf == NULL) {
            t->stkbuf = jl_alloc_fiber(&t->ctx, &t->bufsz, t);
            if (t->stkbuf == NULL) {
#ifdef COPY_STACKS
                // fall back to stack copying if mmap fails
                t->copy_stack = 1;
                t->sticky = 1;
                t->bufsz = 0;
                if (always_copy_stacks)
                    memcpy(&t->copy_stack_ctx, &ptls->copy_stack_ctx, sizeof(t->copy_stack_ctx));
                else
                    memcpy(&t->ctx, &ptls->base_ctx, sizeof(t->ctx));
#else
                jl_throw(jl_memory_exception);
#endif
            }
        }
    }

    if (killed) {
        *pt = NULL; // can't fail after here: clear the gc-root for the target task now
        lastt->gcstack = NULL;
        if (!lastt->copy_stack && lastt->stkbuf) {
            // early free of stkbuf back to the pool
            jl_release_task_stack(ptls, lastt);
        }
    }
    else {
#ifdef COPY_STACKS
        if (lastt->copy_stack) { // save the old copy-stack
            save_stack(ptls, lastt, pt); // allocates (gc-safepoint, and can also fail)
            if (jl_setjmp(lastt->copy_stack_ctx.uc_mcontext, 0)) {
                sanitizer_finish_switch_fiber();
                // TODO: mutex unlock the thread we just switched from
                return;
            }
        }
        else
#endif
        *pt = NULL; // can't fail after here: clear the gc-root for the target task now
        lastt->ptls = NULL;
    }

    // set up global state for new task and clear global state for old task
    t->ptls = ptls;
    ptls->current_task = t;
    JL_GC_PROMISE_ROOTED(t);
    lastt->ptls = NULL;
#ifdef MIGRATE_TASKS
    ptls->previous_task = lastt;
#endif
    jl_set_pgcstack(&t->gcstack);

#if defined(_COMPILER_TSAN_ENABLED_)
    tsan_switch_to_ctx(&t->tsan_state);
    if (killed)
        tsan_destroy_ctx(ptls, &lastt->tsan_state);
#endif
    if (t->started) {
#ifdef COPY_STACKS
        if (t->copy_stack) {
            if (!killed && !lastt->copy_stack)
                restore_stack2(t, ptls, lastt);
            else if (lastt->copy_stack) {
                restore_stack(t, ptls, NULL); // (doesn't return)
            }
            else {
                restore_stack(t, ptls, (char*)1); // (doesn't return)
            }
        }
        else
#endif
        {
            sanitizer_start_switch_fiber(t->stkbuf, t->bufsz);
            if (killed) {
                jl_set_fiber(&t->ctx); // (doesn't return)
                abort(); // unreachable
            }
            else {
                if (lastt->copy_stack) {
                    // Resume at the jl_setjmp earlier in this function,
                    // don't do a full task swap
                    jl_set_fiber(&t->ctx); // (doesn't return)
                }
                else {
                    jl_swap_fiber(&lastt->ctx, &t->ctx);
                }
            }
        }
    }
    else {
        sanitizer_start_switch_fiber(t->stkbuf, t->bufsz);
        if (t->copy_stack && always_copy_stacks) {
#ifdef COPY_STACKS
#if defined(_OS_WINDOWS_)
            jl_setcontext(&t->ctx);
#else
            jl_longjmp(t->copy_stack_ctx.uc_mcontext, 1);
#endif
#endif
            abort(); // unreachable
        }
        else {
            if (killed) {
                jl_start_fiber_set(&t->ctx); // (doesn't return)
                abort();
            }
            else if (lastt->copy_stack) {
                // Resume at the jl_setjmp earlier in this function
                jl_start_fiber_set(&t->ctx); // (doesn't return)
                abort();
            }
            else {
                jl_start_fiber_swap(&lastt->ctx, &t->ctx);
            }
        }
    }
    sanitizer_finish_switch_fiber();
}

JL_DLLEXPORT void jl_switch(void)
{
    jl_task_t *ct = jl_current_task;
    jl_ptls_t ptls = ct->ptls;
    jl_task_t *t = ptls->next_task;
    if (t == ct) {
        return;
    }
    if (t->started && t->stkbuf == NULL)
        jl_error("attempt to switch to exited task");
    if (ptls->in_finalizer)
        jl_error("task switch not allowed from inside gc finalizer");
    if (ptls->in_pure_callback)
        jl_error("task switch not allowed from inside staged nor pure functions");
    if (!jl_set_task_tid(t, ptls->tid)) // manually yielding to a task
        jl_error("cannot switch to task running on another thread");

    // Store old values on the stack and reset
    sig_atomic_t defer_signal = ptls->defer_signal;
    int8_t gc_state = jl_gc_unsafe_enter(ptls);
    int finalizers_inhibited = ptls->finalizers_inhibited;
    ptls->finalizers_inhibited = 0;

#ifdef ENABLE_TIMINGS
    jl_timing_block_t *blk = ptls->timing_stack;
    if (blk)
        jl_timing_block_stop(blk);
    ptls->timing_stack = NULL;
#endif

    ctx_switch(ct);

#ifdef MIGRATE_TASKS
    ptls = ct->ptls;
    t = ptls->previous_task;
    ptls->previous_task = NULL;
    assert(t != ct);
    assert(t->tid == ptls->tid);
    if (!t->sticky && !t->copy_stack)
        jl_atomic_store_release(&t->tid, -1);
#else
    assert(ptls == ct->ptls);
#endif

    // Pop old values back off the stack
    assert(ct == jl_current_task &&
           0 != ct->ptls &&
           0 == ptls->finalizers_inhibited);
    ptls->finalizers_inhibited = finalizers_inhibited;

#ifdef ENABLE_TIMINGS
    assert(ptls->timing_stack == NULL);
    ptls->timing_stack = blk;
    if (blk)
        jl_timing_block_start(blk);
#else
    (void)ct;
#endif

    jl_gc_unsafe_leave(ptls, gc_state);
    sig_atomic_t other_defer_signal = ptls->defer_signal;
    ptls->defer_signal = defer_signal;
    if (other_defer_signal && !defer_signal)
        jl_sigint_safepoint(ptls);
}

JL_DLLEXPORT void jl_switchto(jl_task_t **pt)
{
    jl_set_next_task(*pt);
    jl_switch();
}

JL_DLLEXPORT JL_NORETURN void jl_no_exc_handler(jl_value_t *e)
{
    // NULL exception objects are used when rethrowing. we don't have a handler to process
    // the exception stack, so at least report the exception at the top of the stack.
    if (!e)
        e = jl_current_exception();

    jl_printf((JL_STREAM*)STDERR_FILENO, "fatal: error thrown and no exception handler available.\n");
    jl_static_show((JL_STREAM*)STDERR_FILENO, e);
    jl_printf((JL_STREAM*)STDERR_FILENO, "\n");
    jlbacktrace(); // written to STDERR_FILENO
    jl_exit(1);
}

// yield to exception handler
static void JL_NORETURN throw_internal(jl_task_t *ct, jl_value_t *exception JL_MAYBE_UNROOTED)
{
    assert(!jl_get_safe_restore());
    jl_ptls_t ptls = ct->ptls;
    ptls->io_wait = 0;
    // @time needs its compile timer disabled on error,
    // and cannot use a try-finally as it would break scope for assignments
    // Though if the user has requested to _always_ measure compile time, we don't need to
    // enable or disable it.
    if (!jl_always_measure_compile_time) {
        // We blindly disable compilation time tracking here, for all running Tasks, even
        // though it may cause some incorrect measurements. This is a known bug, and is
        // being tracked here: https://github.com/JuliaLang/julia/pull/39138
        jl_atomic_store_relaxed(&jl_measure_compile_time_enabled, 0);
    }
    JL_GC_PUSH1(&exception);
    jl_gc_unsafe_enter(ptls);
    if (exception) {
        // The temporary ptls->bt_data is rooted by special purpose code in the
        // GC. This exists only for the purpose of preserving bt_data until we
        // set ptls->bt_size=0 below.
        jl_push_excstack(&ct->excstack, exception,
                          ptls->bt_data, ptls->bt_size);
        ptls->bt_size = 0;
    }
    assert(ct->excstack && ct->excstack->top);
    jl_handler_t *eh = ct->eh;
    if (eh != NULL) {
#ifdef ENABLE_TIMINGS
        jl_timing_block_t *cur_block = ptls->timing_stack;
        while (cur_block && eh->timing_stack != cur_block) {
            cur_block = jl_pop_timing_block(cur_block);
        }
        assert(cur_block == eh->timing_stack);
#endif
        jl_longjmp(eh->eh_ctx, 1);
    }
    else {
        jl_no_exc_handler(exception);
    }
    assert(0);
}

// record backtrace and raise an error
JL_DLLEXPORT void jl_throw(jl_value_t *e JL_MAYBE_UNROOTED)
{
    assert(e != NULL);
    jl_jmp_buf *safe_restore = jl_get_safe_restore();
    if (safe_restore)
        jl_longjmp(*safe_restore, 1);
    jl_task_t *ct = jl_get_current_task();
    if (ct == NULL) // During startup
        jl_no_exc_handler(e);
    JL_GC_PROMISE_ROOTED(ct);
    record_backtrace(ct->ptls, 1);
    throw_internal(ct, e);
}

// rethrow with current excstack state
JL_DLLEXPORT void jl_rethrow(void)
{
    jl_task_t *ct = jl_current_task;
    jl_excstack_t *excstack = ct->excstack;
    if (!excstack || excstack->top == 0)
        jl_error("rethrow() not allowed outside a catch block");
    throw_internal(ct, NULL);
}

// Special case throw for errors detected inside signal handlers.  This is not
// (cannot be) called directly in the signal handler itself, but is returned to
// after the signal handler exits.
JL_DLLEXPORT void JL_NORETURN jl_sig_throw(void)
{
CFI_NORETURN
    jl_jmp_buf *safe_restore = jl_get_safe_restore();
    if (safe_restore)
        jl_longjmp(*safe_restore, 1);
    jl_task_t *ct = jl_current_task;
    jl_ptls_t ptls = ct->ptls;
    jl_value_t *e = ptls->sig_exception;
    ptls->sig_exception = NULL;
    throw_internal(ct, e);
}

JL_DLLEXPORT void jl_rethrow_other(jl_value_t *e JL_MAYBE_UNROOTED)
{
    // TODO: Should uses of `rethrow(exc)` be replaced with a normal throw, now
    // that exception stacks allow root cause analysis?
    jl_task_t *ct = jl_current_task;
    jl_excstack_t *excstack = ct->excstack;
    if (!excstack || excstack->top == 0)
        jl_error("rethrow(exc) not allowed outside a catch block");
    // overwrite exception on top of stack. see jl_excstack_exception
    jl_excstack_raw(excstack)[excstack->top-1].jlvalue = e;
    JL_GC_PROMISE_ROOTED(e);
    throw_internal(ct, NULL);
}

/* This is xoshiro256++ 1.0, used for tasklocal random number generation in Julia.
   This implementation is intended for embedders and internal use by the runtime, and is
   based on the reference implementation at https://prng.di.unimi.it

   Credits go to David Blackman and Sebastiano Vigna for coming up with this PRNG.
   They described xoshiro256++ in "Scrambled Linear Pseudorandom Number Generators",
   ACM Trans. Math. Softw., 2021.

   There is a pure Julia implementation in stdlib that tends to be faster when used from
   within Julia, due to inlining and more agressive architecture-specific optimizations.
*/
JL_DLLEXPORT uint64_t jl_tasklocal_genrandom(jl_task_t *task) JL_NOTSAFEPOINT
{
    uint64_t s0 = task->rngState0;
    uint64_t s1 = task->rngState1;
    uint64_t s2 = task->rngState2;
    uint64_t s3 = task->rngState3;

    uint64_t t = s0 << 17;
    uint64_t tmp = s0 + s3;
    uint64_t res = ((tmp << 23) | (tmp >> 41)) + s0;
    s2 ^= s0;
    s3 ^= s1;
    s1 ^= s2;
    s0 ^= s3;
    s2 ^= t;
    s3 = (s3 << 45) | (s3 >> 19);

    task->rngState0 = s0;
    task->rngState1 = s1;
    task->rngState2 = s2;
    task->rngState3 = s3;
    return res;
}

void rng_split(jl_task_t *from, jl_task_t *to) JL_NOTSAFEPOINT
{
    /* TODO: consider a less ad-hoc construction
       Ideally we could just use the output of the random stream to seed the initial
       state of the child. Out of an overabundance of caution we multiply with
       effectively random coefficients, to break possible self-interactions.

       It is not the goal to mix bits -- we work under the assumption that the
       source is well-seeded, and its output looks effectively random.
       However, xoshiro has never been studied in the mode where we seed the
       initial state with the output of another xoshiro instance.

       Constants have nothing up their sleeve:
       0x02011ce34bce797f == hash(UInt(1))|0x01
       0x5a94851fb48a6e05 == hash(UInt(2))|0x01
       0x3688cf5d48899fa7 == hash(UInt(3))|0x01
       0x867b4bb4c42e5661 == hash(UInt(4))|0x01
    */
    to->rngState0 = 0x02011ce34bce797f * jl_tasklocal_genrandom(from);
    to->rngState1 = 0x5a94851fb48a6e05 * jl_tasklocal_genrandom(from);
    to->rngState2 = 0x3688cf5d48899fa7 * jl_tasklocal_genrandom(from);
    to->rngState3 = 0x867b4bb4c42e5661 * jl_tasklocal_genrandom(from);
}

JL_DLLEXPORT jl_task_t *jl_new_task(jl_function_t *start, jl_value_t *completion_future, size_t ssize)
{
    jl_task_t *ct = jl_current_task;
    jl_task_t *t = (jl_task_t*)jl_gc_alloc(ct->ptls, sizeof(jl_task_t), jl_task_type);
    t->copy_stack = 0;
    if (ssize == 0) {
        // stack size unspecified; use default
        if (always_copy_stacks) {
            t->copy_stack = 1;
            t->bufsz = 0;
        }
        else {
            t->bufsz = JL_STACK_SIZE;
        }
        t->stkbuf = NULL;
    }
    else {
        // user requested dedicated stack of a certain size
        if (ssize < MINSTKSZ)
            ssize = MINSTKSZ;
        t->bufsz = ssize;
        t->stkbuf = jl_alloc_fiber(&t->ctx, &t->bufsz, t);
        if (t->stkbuf == NULL)
            jl_throw(jl_memory_exception);
    }
    t->next = jl_nothing;
    t->queue = jl_nothing;
    t->tls = jl_nothing;
    t->_state = JL_TASK_STATE_RUNNABLE;
    t->start = start;
    t->result = jl_nothing;
    t->donenotify = completion_future;
    t->_isexception = 0;
    // Inherit logger state from parent task
    t->logstate = ct->logstate;
    // Fork task-local random state from parent
    rng_split(ct, t);
    // there is no active exception handler available on this stack yet
    t->eh = NULL;
    t->sticky = 1;
    t->gcstack = NULL;
    t->excstack = NULL;
    t->started = 0;
    t->prio = -1;
    t->tid = t->copy_stack ? ct->tid : -1; // copy_stacks are always pinned since they can't be moved
    t->ptls = NULL;
    t->world_age = 0;

#ifdef COPY_STACKS
    if (!t->copy_stack) {
#if defined(JL_DEBUG_BUILD)
        memset(&t->ctx, 0, sizeof(t->ctx));
#endif
    }
    else {
        if (always_copy_stacks)
            memcpy(&t->copy_stack_ctx, &ct->ptls->copy_stack_ctx, sizeof(t->copy_stack_ctx));
        else
            memcpy(&t->ctx, &ct->ptls->base_ctx, sizeof(t->ctx));
    }
#endif
#ifdef _COMPILER_TSAN_ENABLED_
    t->tsan_state = __tsan_create_fiber(0);
#endif
    return t;
}

// a version of jl_current_task safe for unmanaged threads
JL_DLLEXPORT jl_task_t *jl_get_current_task(void)
{
    jl_gcframe_t **pgcstack = jl_get_pgcstack();
    return pgcstack == NULL ? NULL : container_of(pgcstack, jl_task_t, gcstack);
}

#ifdef JL_HAVE_ASYNCIFY
JL_DLLEXPORT jl_ucontext_t *task_ctx_ptr(jl_task_t *t)
{
    return &t->ctx;
}

JL_DLLEXPORT jl_value_t *jl_get_root_task(void)
{
    jl_task_t *ct = jl_current_task;
    return (jl_value_t*)ct->ptls->root_task;
}

JL_DLLEXPORT void jl_task_wait()
{
    static jl_function_t *wait_func = NULL;
    if (!wait_func) {
        wait_func = (jl_function_t*)jl_get_global(jl_base_module, jl_symbol("wait"));
    }
    jl_task_t *ct = jl_current_task;
    size_t last_age = ct->world_age;
    ct->world_age = jl_get_world_counter();
    jl_apply(&wait_func, 1);
    ct->world_age = last_age;
}

JL_DLLEXPORT void jl_schedule_task(jl_task_t *task)
{
    static jl_function_t *sched_func = NULL;
    if (!sched_func) {
        sched_func = (jl_function_t*)jl_get_global(jl_base_module, jl_symbol("schedule"));
    }
    jl_task_t *ct = jl_current_task;
    size_t last_age = ct->world_age;
    ct->world_age = jl_get_world_counter();
    jl_value_t *args[] = {(jl_value_t*)sched_func, (jl_value_t*)task};
    jl_apply(args, 2);
    ct->world_age = last_age;
}
#endif

// Do one-time initializations for task system
void jl_init_tasks(void) JL_GC_DISABLED
{
    char *acs = getenv("JULIA_COPY_STACKS");
    if (acs) {
        if (!strcmp(acs, "1") || !strcmp(acs, "yes"))
            always_copy_stacks = 1;
        else if (!strcmp(acs, "0") || !strcmp(acs, "no"))
            always_copy_stacks = 0;
        else {
            jl_safe_printf("invalid JULIA_COPY_STACKS value: %s\n", acs);
            exit(1);
        }
    }
#ifndef COPY_STACKS
    if (always_copy_stacks) {
        jl_safe_printf("Julia built without COPY_STACKS support");
        exit(1);
    }
#endif
}

STATIC_OR_JS void NOINLINE JL_NORETURN start_task(void)
{
CFI_NORETURN
    // this runs the first time we switch to a task
    sanitizer_finish_switch_fiber();
#ifdef __clang_analyzer__
    jl_task_t *ct = jl_get_current_task();
    JL_GC_PROMISE_ROOTED(ct);
#else
    jl_task_t *ct = jl_current_task;
#endif
    jl_ptls_t ptls = ct->ptls;
    jl_value_t *res;
    assert(ptls->finalizers_inhibited == 0);

#ifdef MIGRATE_TASKS
    jl_task_t *pt = ptls->previous_task;
    ptls->previous_task = NULL;
    if (!pt->sticky && !pt->copy_stack)
        jl_atomic_store_release(&pt->tid, -1);
#endif

    ct->started = 1;
    if (ct->_isexception) {
        record_backtrace(ptls, 0);
        jl_push_excstack(&ct->excstack, ct->result,
                         ptls->bt_data, ptls->bt_size);
        res = ct->result;
    }
    else {
        JL_TRY {
            if (ptls->defer_signal) {
                ptls->defer_signal = 0;
                jl_sigint_safepoint(ptls);
            }
            JL_TIMING(ROOT);
            ct->world_age = jl_world_counter;
            res = jl_apply(&ct->start, 1);
        }
        JL_CATCH {
            res = jl_current_exception();
            ct->_isexception = 1;
            goto skip_pop_exception;
        }
skip_pop_exception:;
    }
    ct->result = res;
    jl_gc_wb(ct, ct->result);
    jl_finish_task(ct);
    gc_debug_critical_error();
    abort();
}


#if defined(JL_HAVE_UCONTEXT)
#ifdef _OS_WINDOWS_
#define setcontext jl_setcontext
#define swapcontext jl_swapcontext
#define makecontext jl_makecontext
#endif
static char *jl_alloc_fiber(jl_ucontext_t *t, size_t *ssize, jl_task_t *owner) JL_NOTSAFEPOINT
{
#ifndef _OS_WINDOWS_
    int r = getcontext(t);
    if (r != 0)
        jl_error("getcontext failed");
#endif
    void *stk = jl_malloc_stack(ssize, owner);
    if (stk == NULL)
        return NULL;
    t->uc_stack.ss_sp = stk;
    t->uc_stack.ss_size = *ssize;
#ifdef _OS_WINDOWS_
    makecontext(t, &start_task);
#else
    t->uc_link = NULL;
    makecontext(t, &start_task, 0);
#endif
    return (char*)stk;
}
static void jl_start_fiber_set(jl_ucontext_t *t)
{
    setcontext(t);
}
static void jl_start_fiber_swap(jl_ucontext_t *lastt, jl_ucontext_t *t)
{
    assert(lastt);
    swapcontext(lastt, t);
}
static void jl_swap_fiber(jl_ucontext_t *lastt, jl_ucontext_t *t)
{
    swapcontext(lastt, t);
}
static void jl_set_fiber(jl_ucontext_t *t)
{
    setcontext(t);
}
#endif

#if defined(JL_HAVE_UNW_CONTEXT) || defined(JL_HAVE_ASM)
static char *jl_alloc_fiber(jl_ucontext_t *t, size_t *ssize, jl_task_t *owner)
{
    char *stkbuf = (char*)jl_malloc_stack(ssize, owner);
    if (stkbuf == NULL)
        return NULL;
#ifndef __clang_analyzer__
    ((char**)t)[0] = stkbuf; // stash the stack pointer somewhere for start_fiber
    ((size_t*)t)[1] = *ssize; // stash the stack size somewhere for start_fiber
#endif
    return stkbuf;
}
#endif

#if defined(JL_HAVE_UNW_CONTEXT)
static inline void jl_unw_swapcontext(unw_context_t *old, unw_cursor_t *c)
{
    volatile int returns = 0;
    int r = unw_getcontext(old);
    if (++returns == 2) // r is garbage after the first return
        return;
    if (r != 0 || returns != 1)
        abort();
    unw_resume(c);
}
static void jl_swap_fiber(jl_ucontext_t *lastt, jl_ucontext_t *t)
{
    unw_cursor_t c;
    int r = unw_init_local(&c, t);
    if (r < 0)
        abort();
    jl_unw_swapcontext(lastt, &c);
}
static void jl_set_fiber(unw_context_t *t)
{
    unw_cursor_t c;
    int r = unw_init_local(&c, t);
    if (r < 0)
        abort();
    unw_resume(&c);
}
#elif defined(JL_HAVE_ASM)
static void jl_swap_fiber(jl_ucontext_t *lastt, jl_ucontext_t *t)
{
    if (jl_setjmp(lastt->uc_mcontext, 0))
        return;
    jl_set_fiber(t); // doesn't return
}
static void jl_set_fiber(jl_ucontext_t *t)
{
    jl_longjmp(t->uc_mcontext, 1);
}
#endif

#if defined(JL_HAVE_UNW_CONTEXT) && !defined(JL_HAVE_ASM)
#if defined(_CPU_X86_) || defined(_CPU_X86_64_)
#define PUSH_RET(ctx, stk) \
    do { \
        stk -= sizeof(uintptr_t); \
        *(uintptr_t*)stk = 0; /* push null RIP/EIP onto the stack */ \
    } while (0)
#elif defined(_CPU_ARM_)
#define PUSH_RET(ctx, stk) \
    if (unw_set_reg(ctx, UNW_ARM_R14, 0)) /* put NULL into the LR */ \
        abort();
#else
#error please define how to simulate a CALL on this platform
#endif
static void jl_start_fiber_set(unw_context_t *t)
{
    unw_cursor_t c;
    char *stk = ((char**)t)[0];
    size_t ssize = ((size_t*)t)[1];
    uintptr_t fn = (uintptr_t)&start_task;
    stk += ssize;
    int r = unw_getcontext(t);
    if (r)
        abort();
    if (unw_init_local(&c, t))
        abort();
    PUSH_RET(&c, stk);
#if defined __linux__
#error savannah nongnu libunwind is not capable of setting UNW_REG_SP, as required
#endif
    if (unw_set_reg(&c, UNW_REG_SP, (uintptr_t)stk))
        abort();
    if (unw_set_reg(&c, UNW_REG_IP, fn))
        abort();
    unw_resume(&c); // (doesn't return)
}
static void jl_start_fiber_swap(unw_context_t *lastt, unw_context_t *t)
{
    assert(lastt);
    unw_cursor_t c;
    char *stk = ((char**)t)[0];
    size_t ssize = ((size_t*)t)[1];
    uintptr_t fn = (uintptr_t)&start_task;
    stk += ssize;
    volatile int returns = 0;
    int r = unw_getcontext(lastt);
    if (++returns == 2) // r is garbage after the first return
        return;
    if (r != 0 || returns != 1)
        abort();
    int r = unw_getcontext(t);
    if (r != 0)
        abort();
    if (unw_init_local(&c, t))
        abort();
    PUSH_RET(&c, stk);
    if (unw_set_reg(&c, UNW_REG_SP, (uintptr_t)stk))
        abort();
    if (unw_set_reg(&c, UNW_REG_IP, fn))
        abort();
    jl_unw_swapcontext(lastt, &c);
}
#endif

#if defined(JL_HAVE_ASM)
static void jl_start_fiber_swap(jl_ucontext_t *lastt, jl_ucontext_t *t)
{
    assert(lastt);
#ifdef JL_HAVE_UNW_CONTEXT
    volatile int returns = 0;
    int r = unw_getcontext(lastt);
    if (++returns == 2) // r is garbage after the first return
        return;
    if (r != 0 || returns != 1)
        abort();
#else
    if (jl_setjmp(lastt->uc_mcontext, 0))
        return;
#endif
    jl_start_fiber_set(t); // doesn't return
}
static void jl_start_fiber_set(jl_ucontext_t *t)
{
    char *stk = ((char**)t)[0];
    size_t ssize = ((size_t*)t)[1];
    uintptr_t fn = (uintptr_t)&start_task;
    stk += ssize;
#ifdef _CPU_X86_64_
    asm volatile (
        " movq %0, %%rsp;\n"
        " movq %1, %%rax;\n"
        " xorq %%rbp, %%rbp;\n"
        " push %%rbp;\n" // instead of RSP
        " jmpq *%%rax;\n" // call `fn` with fake stack frame
        " ud2"
        : : "r"(stk), "r"(fn) : "memory" );
#elif defined(_CPU_X86_)
    asm volatile (
        " movl %0, %%esp;\n"
        " movl %1, %%eax;\n"
        " xorl %%ebp, %%ebp;\n"
        " push %%ebp;\n" // instead of ESP
        " jmpl *%%eax;\n" // call `fn` with fake stack frame
        " ud2"
        : : "r"(stk), "r"(fn) : "memory" );
#elif defined(_CPU_AARCH64_)
    asm volatile(
        " mov sp, %0;\n"
        " mov x29, xzr;\n" // Clear link register (x29) and frame pointer
        " mov x30, xzr;\n" // (x30) to terminate unwinder.
        " br %1;\n" // call `fn` with fake stack frame
        " brk #0x1" // abort
        : : "r" (stk), "r"(fn) : "memory" );
#elif defined(_CPU_ARM_)
    // A "i" constraint on `&start_task` works only on clang and not on GCC.
    asm(" mov sp, %0;\n"
        " mov lr, #0;\n" // Clear link register (lr) and frame pointer
        " mov fp, #0;\n" // (fp) to terminate unwinder.
        " bx %1;\n" // call `fn` with fake stack frame.  While `bx` can change
                    // the processor mode to thumb, this will never happen
                    // because all our addresses are word-aligned.
        " udf #0" // abort
        : : "r" (stk), "r"(fn) : "memory" );
#elif defined(_CPU_PPC64_)
    // N.B.: There is two iterations of the PPC64 ABI.
    // v2 is current and used here. Make sure you have the
    // correct version of the ABI reference when working on this code.
    asm volatile(
        // Move stack (-0x30 for initial stack frame) to stack pointer
        " addi 1, %0, -0x30;\n"
        // Build stack frame
        // Skip local variable save area
        " std 2, 0x28(1);\n" // Save TOC
        // Clear link editor/compiler words
        " std 0, 0x20(1);\n"
        " std 0, 0x18(1);\n"
        // Clear LR/CR save area
        " std 0, 0x10(1);\n"
        " std 0, 0x8(1);\n"
        " std 0, 0x0(1); \n" // Clear back link to terminate unwinder
        " mtlr 0; \n"        // Clear link register
        " mr 12, %1; \n"     // Set up target global entry point
        " mtctr 12; \n"      // Move jump target to counter register
        " bctr; \n"          // branch to counter (lr update disabled)
        " trap; \n"
        : : "r"(stk), "r"(fn) : "memory");
#else
#error JL_HAVE_ASM defined but not implemented for this CPU type
#endif
    __builtin_unreachable();
}
#endif

#if defined(JL_HAVE_SIGALTSTACK)
#if defined(_COMPILER_TSAN_ENABLED_)
#error TSAN support not currently implemented for this tasking model
#endif

static void start_basefiber(int sig)
{
    jl_ptls_t ptls = jl_current_task->ptls;
    if (jl_setjmp(ptls->base_ctx.uc_mcontext, 0))
        start_task(); // sanitizer_finish_switch_fiber is part of start_task
}
static char *jl_alloc_fiber(jl_ucontext_t *t, size_t *ssize, jl_task_t *owner)
{
    stack_t uc_stack, osigstk;
    struct sigaction sa, osa;
    sigset_t set, oset;
    void *stk = jl_malloc_stack(ssize, owner);
    if (stk == NULL)
        return NULL;
    // setup
    jl_ptls_t ptls = jl_current_task->ptls;
    jl_ucontext_t base_ctx;
    memcpy(&base_ctx, &ptls->base_ctx, sizeof(ptls->base_ctx));
    sigfillset(&set);
    if (sigprocmask(SIG_BLOCK, &set, &oset) != 0) {
       jl_free_stack(stk, *ssize);
       jl_error("sigprocmask failed");
    }
    uc_stack.ss_sp = stk;
    uc_stack.ss_size = *ssize;
    uc_stack.ss_flags = 0;
    if (sigaltstack(&uc_stack, &osigstk) != 0) {
       jl_free_stack(stk, *ssize);
       jl_error("sigaltstack failed");
    }
    memset(&sa, 0, sizeof(sa));
    sigemptyset(&sa.sa_mask);
    sa.sa_handler = start_basefiber;
    sa.sa_flags = SA_ONSTACK;
    if (sigaction(SIGUSR2, &sa, &osa) != 0) {
       jl_free_stack(stk, *ssize);
       jl_error("sigaction failed");
    }
    // emit signal
    pthread_kill(pthread_self(), SIGUSR2); // initializes jl_basectx
    sigdelset(&set, SIGUSR2);
    sigsuspend(&set);
    // cleanup
    if (sigaction(SIGUSR2, &osa, NULL) != 0) {
       jl_free_stack(stk, *ssize);
       jl_error("sigaction failed");
    }
    if (osigstk.ss_size < MINSTKSZ && (osigstk.ss_flags | SS_DISABLE))
       osigstk.ss_size = MINSTKSZ;
    if (sigaltstack(&osigstk, NULL) != 0) {
       jl_free_stack(stk, *ssize);
       jl_error("sigaltstack failed");
    }
    if (sigprocmask(SIG_SETMASK, &oset, NULL) != 0) {
       jl_free_stack(stk, *ssize);
       jl_error("sigprocmask failed");
    }
    if (&ptls->base_ctx != t) {
        memcpy(t, &ptls->base_ctx, sizeof(ptls->base_ctx));
        memcpy(&ptls->base_ctx, &base_ctx, sizeof(ptls->base_ctx)); // restore COPY_STACKS context
    }
    return (char*)stk;
}
static void jl_start_fiber_set(jl_ucontext_t *t) {
    jl_longjmp(t->uc_mcontext, 1); // (doesn't return)
}
static void jl_start_fiber_swap(jl_ucontext_t *lastt, jl_ucontext_t *t)
{
    assert(lastt);
    if (lastt && jl_setjmp(lastt->uc_mcontext, 0))
        return;
    jl_start_fiber_set(t);
}
static void jl_swap_fiber(jl_ucontext_t *lastt, jl_ucontext_t *t)
{
    if (jl_setjmp(lastt->uc_mcontext, 0))
        return;
    jl_start_fiber_set(t); // doesn't return
}
static void jl_set_fiber(jl_ucontext_t *t)
{
    jl_longjmp(t->uc_mcontext, 1);
}
#endif

#if defined(JL_HAVE_ASYNCIFY)
#if defined(_COMPILER_TSAN_ENABLED_)
#error TSAN support not currently implemented for this tasking model
#endif

static char *jl_alloc_fiber(jl_ucontext_t *t, size_t *ssize, jl_task_t *owner) JL_NOTSAFEPOINT
{
    void *stk = jl_malloc_stack(ssize, owner);
    if (stk == NULL)
        return NULL;
    t->stackbottom = stk;
    t->stacktop = ((char*)stk) + *ssize;
    return (char*)stk;
}
// jl_*_fiber implemented in js
#endif

// Initialize a root task using the given stack.
void jl_init_root_task(jl_ptls_t ptls, void *stack_lo, void *stack_hi)
{
    assert(ptls->root_task == NULL);
    // We need `gcstack` in `Task` to allocate Julia objects; *including* the `Task` type.
    // However, to allocate a `Task` via `jl_gc_alloc` as done in `jl_init_root_task`,
    // we need the `Task` type itself. We use stack-allocated "raw" `jl_task_t` struct to
    // workaround this chicken-and-egg problem. Note that this relies on GC to be turned
    // off as GC fails because we don't/can't allocate the type tag.
    struct {
        jl_value_t *type;
        jl_task_t value;
    } bootstrap_task = {0};
    jl_set_pgcstack(&bootstrap_task.value.gcstack);
    bootstrap_task.value.ptls = ptls;
    if (jl_nothing == NULL) // make a placeholder
        jl_nothing = jl_gc_permobj(0, jl_nothing_type);
    jl_task_t *ct = (jl_task_t*)jl_gc_alloc(ptls, sizeof(jl_task_t), jl_task_type);
    memset(ct, 0, sizeof(jl_task_t));
    void *stack = stack_lo;
    size_t ssize = (char*)stack_hi - (char*)stack_lo;
#ifndef _OS_WINDOWS_
    if (ptls->tid == 0) {
        stack = (void*)((char*)stack - ROOT_TASK_STACK_ADJUSTMENT); // offset our guess of the address of the bottom of stack to cover the guard pages too
        ssize += ROOT_TASK_STACK_ADJUSTMENT; // sizeof stack is known exactly, but not where we are in that stack
    }
#endif
    if (always_copy_stacks) {
        ct->copy_stack = 1;
        ct->stkbuf = NULL;
        ct->bufsz = 0;
    }
    else {
        ct->copy_stack = 0;
        ct->stkbuf = stack;
        ct->bufsz = ssize;
    }
    ct->started = 1;
    ct->next = jl_nothing;
    ct->queue = jl_nothing;
    ct->tls = jl_nothing;
    ct->_state = JL_TASK_STATE_RUNNABLE;
    ct->start = NULL;
    ct->result = jl_nothing;
    ct->donenotify = jl_nothing;
    ct->_isexception = 0;
    ct->logstate = jl_nothing;
    ct->eh = NULL;
    ct->gcstack = NULL;
    ct->excstack = NULL;
    ct->tid = ptls->tid;
    ct->sticky = 1;
    ct->ptls = ptls;
    ct->world_age = 1; // OK to run Julia code on this task
    ptls->root_task = ct;
    ptls->current_task = ct;
    JL_GC_PROMISE_ROOTED(ct);
    jl_set_pgcstack(&ct->gcstack);
    assert(jl_current_task == ct);

#ifdef _COMPILER_TSAN_ENABLED_
    ct->tsan_state = __tsan_get_current_fiber();
#endif

#ifdef COPY_STACKS
    // initialize the base_ctx from which all future copy_stacks will be copies
    if (always_copy_stacks) {
        // when this is set, we will attempt to corrupt the process stack to switch tasks,
        // although this is unreliable, and thus not recommended
        ptls->stackbase = stack_hi;
        ptls->stacksize = ssize;
#ifdef _OS_WINDOWS_
        ptls->copy_stack_ctx.uc_stack.ss_sp = stack_hi;
        ptls->copy_stack_ctx.uc_stack.ss_size = ssize;
#endif
        if (jl_setjmp(ptls->copy_stack_ctx.uc_mcontext, 0))
            start_task(); // sanitizer_finish_switch_fiber is part of start_task
        return;
    }
    ssize = JL_STACK_SIZE;
    char *stkbuf = jl_alloc_fiber(&ptls->base_ctx, &ssize, NULL);
    ptls->stackbase = stkbuf + ssize;
    ptls->stacksize = ssize;
#endif
}

JL_DLLEXPORT int jl_is_task_started(jl_task_t *t) JL_NOTSAFEPOINT
{
    return t->started;
}

JL_DLLEXPORT int16_t jl_get_task_tid(jl_task_t *t) JL_NOTSAFEPOINT
{
    return t->tid;
}


#ifdef _OS_WINDOWS_
#if defined(_CPU_X86_)
extern DWORD32 __readgsdword(int);
extern DWORD32 __readgs(void);
#endif
JL_DLLEXPORT void jl_gdb_dump_threadinfo(void)
{
#if defined(_CPU_X86_64_)
    DWORD64 gs0 = __readgsqword(0x0);
    DWORD64 gs8 = __readgsqword(0x8);
    DWORD64 gs16 = __readgsqword(0x10);
    jl_safe_printf("ThreadId: %u, Stack: %p -- %p to %p, SEH: %p\n",
                   (unsigned)GetCurrentThreadId(),
                   jl_get_frame_addr(),
                   (void*)gs8, (void*)gs16, (void*)gs0);
#elif defined(_CPU_X86_)
    DWORD32 fs0 = __readfsdword(0x0);
    DWORD32 fs4 = __readfsdword(0x4);
    DWORD32 fs8 = __readfsdword(0x8);
    jl_safe_printf("ThreadId: %u, Stack: %p -- %p to %p, SEH: %p\n",
                   (unsigned)GetCurrentThreadId(),
                   jl_get_frame_addr(),
                   (void*)fs4, (void*)fs8, (void*)fs0);
    if (__readgs()) { // WoW64 if GS is non-zero
        DWORD32 gs0 = __readgsdword(0x0);
        DWORD32 gs4 = __readgsdword(0x4);
        DWORD32 gs8 = __readgsdword(0x8);
        DWORD32 gs12 = __readgsdword(0xc);
        DWORD32 gs16 = __readgsdword(0x10);
        DWORD32 gs20 = __readgsdword(0x14);
        jl_safe_printf("Stack64: %p%p to %p%p, SEH64: %p%p\n",
                       (void*)gs12, (void*)gs8,
                       (void*)gs20, (void*)gs16,
                       (void*)gs4, (void*)gs0);
    }
#else
    jl_safe_printf("ThreadId: %u, Stack: %p\n",
                   (unsigned)GetCurrentThreadId(),
                   jl_get_frame_addr());
#endif
}
#endif


#ifdef __cplusplus
}
#endif