且听风吟

package main

func main() {
    println("Hello Go")
}

$ readelf -h ./hello
ELF Header:
...
  Entry point address: 0x454020
...
$ go tool nm ./hello | grep 454020
  454020 T _rt0_amd64_linux

TEXT _rt0_amd64_linux(SB),NOSPLIT,$-8
    JMP _rt0_amd64(SB)

TEXT _rt0_amd64(SB),NOSPLIT,$-8
    MOVQ    0(SP), DI   // argc
    JMP runtime·rt0_go(SB)

TEXT runtime·rt0_go(SB),NOSPLIT|NOFRAME|TOPFRAME,$0
    // ...

    // Set up the stack space for g0
    MOVQ    $runtime·g0(SB), DI
    LEAQ    (-64*1024)(SP), BX
    MOVQ    BX, g_stackguard0(DI)
    MOVQ    BX, g_stackguard1(DI)
    MOVQ    BX, (g_stack+stack_lo)(DI)
    MOVQ    SP, (g_stack+stack_hi)(DI)

    // Use arch_prctl(ARCH_SET_FS) to set the TLS base address for m0 thread to m0.tls
    LEAQ    runtime·m0+m_tls(SB), DI
    CALL    runtime·settls(SB)

    // Perform basic correctness and safety checks
    // Including: size of basic types, size of platform pointer types, CAS correctness checks
    CALL    runtime·check(SB)

    MOVL    24(SP), AX        // copy argc
    MOVL    AX, 0(SP)
    MOVQ    32(SP), AX        // copy argv
    MOVQ    AX, 8(SP)
    CALL    runtime·args(SB)
    CALL    runtime·osinit(SB)
    CALL    runtime·schedinit(SB)

    // Pass the address of the runtime.main function to newproc
    // This adds runtime.main to the run queue of a p
    MOVQ    $runtime·mainPC(SB), AX        // entry
    PUSHQ    AX
    CALL    runtime·newproc(SB)
    POPQ    AX

    // The main thread executes the schedule dispatch loop
    // runtime.main will be scheduled for execution
    // runtime.main will internally call main.main 
    CALL    runtime·mstart(SB)

    // mstart should not return, so an error is raised here
    CALL    runtime·abort(SB)
    RET

TEXT runtime·mstart(SB),NOSPLIT|TOPFRAME|NOFRAME,$0
    CALL    runtime·mstart0(SB)
    RET // not reached

// runtime/runtime1.go
func args(c int32, v **byte) {
    argc = c
    argv = v
    sysargs(c, v)
}

var vdsoSymbolKeys = []vdsoSymbolKey{
    {"__vdso_gettimeofday", 0x315ca59, 0xb01bca00, &vdsoGettimeofdaySym},
    {"__vdso_clock_gettime", 0xd35ec75, 0x6e43a318, &vdsoClockgettimeSym},
}

// runtime/os_linux.go
func osinit() {
    ncpu = getproccount()
    physHugePageSize = getHugePageSize()
    if iscgo {
        // ... handle cgo signal related
    }
    osArchInit()
}

// runtime/proc.go
func schedinit() {
    gp := getg()
    sched.maxmcount = 10000
    moduledataverify()      // module data verification
    stackinit()             // coroutine stack memory pool initialization
    mallocinit()            // memory allocator initialization
    alginit()               // AES algorithm hardware support initialization
    fastrandinit()          // initialize random seed, using the previous startupRandomData
    mcommoninit(gp.m, -1)   // initialize thread increment ID signal handling coroutine and fast random seed
    modulesinit()           // read each module to initialize GC scanning global variable sizes
    typelinksinit()         // read each module to collect type link information
    itabsinit()             // initialize itab table based on typelink
    stkobjinit()            // stack initialization for GC related

    sigsave(&gp.m.sigmask)  // save thread signal mask

    goargs()                // set global variable argslice
    goenvs()                // set global variable envs
    parsedebugvars()        // read GODEBUG environment variable settings trace
    gcinit()                // GC initialization

    // Initialize allp based on the number of physical processors or GOMAXPROCS
    procs := ncpu
    if n, ok := atoi32(gogetenv("GOMAXPROCS")); ok && n > 0 {
        procs = n
    }
    if procresize(procs) != nil {
        throw("unknown runnable goroutine during bootstrap")
    }
}

// ir.OGETG -> ssa.OpGetG -> ssa.OpAMD64LoweredGetG -> MOVQ (TLS), r

TEXT runtime·gogo(SB), NOSPLIT, $0-8
    // ...
    JMP    gogo<>(SB)

TEXT gogo<>(SB), NOSPLIT, $0
    get_tls(CX)
    MOVQ    DX, g(CX)

// runtime/proc.go
func procresize(nprocs int32) *p {
    // The first half mainly modifies:
    // 1. allp []*p
    // 2. idlepMask, idle P mask where each bit indicates whether the corresponding index in allp is idle
    // 3. timerpMask, mask for P that may have timers, where each bit indicates whether the corresponding index in allp has a timer

    // When expanding the number of procs, initialize the newly created P
    // At the start of the program, allp is empty, here it will create and initialize all P
    for i := old; i < nprocs; i++ {
        pp := allp[i]
        if pp == nil {
            pp = new(p)
        }
        pp.init(i)
        atomicstorep(unsafe.Pointer(&allp[i]), unsafe.Pointer(pp))
    }

    // When subsequent P shrink, excess P will be destroyed
    // So here we first ensure that the current P is not the one to be destroyed
    // If it is, replace it with allp[0]
    gp := getg()
    if gp.m.p != 0 && gp.m.p.ptr().id < nprocs {
        // Continue using the current P
        gp.m.p.ptr().status = _Prunning
        gp.m.p.ptr().mcache.prepareForSweep()
    } else {
        if gp.m.p != 0 {
            gp.m.p.ptr().m = 0
        }
        gp.m.p = 0
        pp := allp[0]
        pp.m = 0
        pp.status = _Pidle
        acquirep(pp)
    }

    // When the number of procs shrinks, clean up old P
    for i := nprocs; i < old; i++ {
        pp := allp[i]
        pp.destroy()
    }

    // Return the runnable P linked list
    var runnablePs *p
    for i := nprocs - 1; i >= 0; i-- {
        pp := allp[i]
        if gp.m.p.ptr() == pp {
            continue
        }
        pp.status = _Pidle
        if runqempty(pp) {
            // If there are no runnable g on the local queue of p, place it on the idle p mask
            pidleput(pp, now)
        } else {
            // Find an idle m for p, here m may be empty
            pp.m.set(mget())
            pp.link.set(runnablePs)
            runnablePs = pp
        }
    }

    if old != nprocs {
        // If the number of procs changes, modify the proc capacity occupied by GC (default takes 25% of the proc count)
        gcCPULimiter.resetCapacity(now, nprocs)
    }
    return runnablePs
}

func newproc(fn *funcval) {
    gp := getg()
    pc := getcallerpc()
    systemstack(func() {
        newg := newproc1(fn, gp, pc)
        pp := getg().m.p.ptr()
        runqput(pp, newg, true)
        if mainStarted {
            // mainStarted is set in runtime.main
            wakep()
        }
    })
}

func newproc1(fn *funcval, callergp *g, callerpc uintptr) *g {
    mp := acquirem()
    pp := mp.p.ptr()
    newg := gfget(pp) // Here it first tries to take one from the freeg linked list of p
    if newg == nil {
        // If not found, create a new g using malg
        // The new g's stack has not been initialized, to avoid being scanned by GC, set the state to dead first
        // Add it to the global allg
        newg = malg(_StackMin)
        casgstatus(newg, _Gidle, _Gdead)
        allgadd(newg)
    }

    // Calculate the stack pointer position
    totalSize := uintptr(4*goarch.PtrSize + sys.MinFrameSize) // extra space in case of reads slightly beyond frame
    totalSize = alignUp(totalSize, sys.StackAlign)
    sp := newg.stack.hi - totalSize

    // Set up newg's scheduling data, stack pointer, function address, program counter, caller information, etc.
    memclrNoHeapPointers(unsafe.Pointer(&newg.sched), unsafe.Sizeof(newg.sched))
    newg.sched.sp = sp
    newg.stktopsp = sp
    newg.sched.pc = abi.FuncPCABI0(goexit) + sys.PCQuantum // +PCQuantum so that previous instruction is in the same function
    newg.sched.g = guintptr(unsafe.Pointer(newg))
    gostartcallfn(&newg.sched, fn)
    newg.parentGoid = callergp.goid
    newg.gopc = callerpc
    newg.ancestors = saveAncestors(callergp)
    newg.startpc = fn.fn

    // Change the state to runnable and add the stack to the GC stack scan
    casgstatus(newg, _Gdead, _Grunnable)
    gcController.addScannableStack(pp, int64(newg.stack.hi-newg.stack.lo))

    newg.goid = pp.goidcache
    pp.goidcache++

    releasem(mp)

    return newg
}

func mstart0() {
    gp := getg()

    // Initialize g0's stackguard for stack overflow and stack expansion checks
    gp.stackguard0 = gp.stack.lo + _StackGuard
    gp.stackguard1 = gp.stackguard0
    mstart1()

    mexit(osStack)
}

func mstart1() {
    gp := getg()

    // Set up m.g0.sched as a label returning to just
    // after the mstart1 call in mstart0 above, for use by goexit0 and mcall.
    // We're never coming back to mstart1 after we call schedule,
    // so other calls can reuse the current frame.
    // And goexit0 does a gogo that needs to return from mstart1
    // and let mstart0 exit the thread.
    gp.sched.g = guintptr(unsafe.Pointer(gp))
    gp.sched.pc = getcallerpc()
    gp.sched.sp = getcallersp()

    // Initialize the thread's signal handling coroutine stack and signal mask
    minit()
    if gp.m == &m0 {
        // Set the thread's signal handling function sighandler
        mstartm0()
    }

    // Some internal threads like sysmon start directly here
    if fn := gp.m.mstartfn; fn != nil {
        fn()
    }

    if gp.m != &m0 {
        acquirep(gp.m.nextp.ptr())
        gp.m.nextp = 0
    }

    // Execute the scheduling loop, never returns
    // Here there is currently only one p and one g
    // All will jump to runtime.main
    schedule()
}

func main() {
    mp := getg().m

    // Set the maximum stack to 1G
    if goarch.PtrSize == 8 {
        maxstacksize = 1000000000
    } else {
        maxstacksize = 250000000
    }

    // Coroutines created by newproc can be found or created by wakep
    mainStarted = true

    // Execute the init function under runtime and initialize global variables
    doInit(&runtime_inittask)

    // Enable GC
    gcenable()

    // Execute user-level init functions and initialize global variables
    doInit(&main_inittask)

    // -buildmode=c-archive or c-shared does not execute main
    if isarchive || islibrary {
        return
    }

    // Execute the user-level main.main function
    fn := main_main
    fn()

    // Before the main coroutine exits, if there are currently other coroutines handling panic-defer
    // It needs to wait for other coroutines to finish first (such as printing panic information, etc.)
    if runningPanicDefers.Load() != 0 {
        for c := 0; c < 1000; c++ {
            if runningPanicDefers.Load() == 0 {
                break
            }
            Gosched()
        }
    }

    // Execute registered hooks before the process exits
    // For example, output code coverage data in compile -cover mode
    runExitHooks(0)
    exit(0)
}