stack overflow (morestack).

For example, deferproc might call malloc, which does one of the
above checks (without allocating a full frame), which might trigger
a call to morestack.  This sequence needs to fit in the bottom
section of the stack.  On amd64, morestack's frame is 40 bytes, and
deferproc's frame is 56 bytes.  That fits well within the
StackGuard - StackSmall bytes at the bottom.

	var morestacksym *obj.LSym
	if c.cursym.CFunc() {
		morestacksym = c.ctxt.Lookup("runtime.morestackc")
	} else if !c.cursym.Func().Text.From.Sym.NeedCtxt() {
		morestacksym = c.ctxt.Lookup("runtime.morestack_noctxt")
	} else {
		morestacksym = c.ctxt.Lookup("runtime.morestack")
	}

	if NeedTOCpointer(c.ctxt) {
		// In PPC64 PIC code, R2 is used as TOC pointer derived from R12

	// stackguard1 is the stack pointer compared in the //go:systemstack 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).
	stack       stack   // offset known to runtime/cgo
	stackguard0 uintptr // offset known to liblink
	stackguard1 uintptr // offset known to liblink

	// should never return
	JAL	runtime·abort(SB)
	UNDEF

/*
 * support for morestack
 */

// Called during function prolog when more stack is needed.
// Caller has already loaded:
// loong64: R31: LR
//
// The traceback routines see morestack on a g0 as being
// the top of a stack (for example, morestack calling newstack
// calling the scheduler calling newm calling gc), so we must

	// should never return
	CALL	runtime·abort(SB)
	UNDEF

/*
 * support for morestack
 */

// Called during function prolog when more stack is needed.
// Caller has already loaded:
// R1: framesize, R2: argsize, R3: LR
//
// The traceback routines see morestack on a g0 as being
// the top of a stack (for example, morestack calling newstack
// calling the scheduler calling newm calling gc), so we must

	//   execution will continue, but it may not be the
	//   instruction following a CALL. This may be from
	//   cooperative preemption, in which case this is the
	//   instruction after the call to morestack. Or this may be
	//   from a signal or an un-started goroutine, in which case
	//   PC could be any instruction, including the first
	//   instruction in a function. Conventionally, we use pc-1

	// Move the stack repeatedly to make sure we're not doubling
	// it each time.
	for i := 0; i < 100; i++ {
		runtime.GCTestMoveStackOnNextCall()
		moveStack1(false)
	}
}

//go:noinline
func moveStack1(x bool) {
	// Make sure this function doesn't get auto-nosplit.
	if x {
		println("x")
	}
}

func TestGCTestIsReachable(t *testing.T) {
	var all, half []unsafe.Pointer

	// should never return
	CALL	runtime·abort(SB)
	UNDEF

/*
 * support for morestack
 */

// Called during function prolog when more stack is needed.
// R3 prolog's LR
// using NOFRAME means do not save LR on stack.
//
// The traceback routines see morestack on a g0 as being
// the top of a stack (for example, morestack calling newstack
// calling the scheduler calling newm calling gc), so we must

	MOVL	0(AX), AX
	CALL	AX

	// should never return
	CALL	runtime·abort(SB)
	UNDEF

/*
 * support for morestack
 */

// Called during function prolog when more stack is needed.
//
// The traceback routines see morestack on a g0 as being
// the top of a stack (for example, morestack calling newstack
// calling the scheduler calling newm calling gc), so we must

		// here we'll see a seemingly self-recursive call, which is in
		// fact not.
		// This is normally ok, as we use the return address saved on
		// stack for unwinding, not the LR value. But if this is a
		// call to morestack, we haven't created the frame, and we'll
		// use the LR for unwinding, which will be bad.
		return false, 0
	}
	up, startpc := pcdatavalue2(f, abi.PCDATA_UnsafePoint, pc)
	if up == abi.UnsafePointUnsafe {