abi.RF_EXHAUSTED = 3 // iterator function call, e.g. f(func(x t){...}), returned so the sequence is "exhausted".

abi.RF_MISSING_PANIC = 4 // used to report errors.

The value of #stateK transitions
(1) before calling the iterator function,

	var #stateN = abi.RF_READY

(2) after the iterator function call returns,

	if #stateN == abi.RF_PANIC {

// we only need to pass one.
func ifaceeq(tab *uintptr, x, y unsafe.Pointer) (ret bool)
func efaceeq(typ *uintptr, x, y unsafe.Pointer) (ret bool)

// panic for various rangefunc iterator errors
func panicrangestate(state int)

// defer in range over func
func deferrangefunc() interface{}

func rand32() uint32

// *byte is really *runtime.Type

		nfor.Post = ir.NewBlockStmt(base.Pos, []ir.Node{nfor.Post, as})

	case k == types.TMAP:
		// order.stmt allocated the iterator for us.
		// we only use a once, so no copy needed.
		ha := a

		hit := nrange.Prealloc
		th := hit.Type()
		// depends on layout of iterator struct.
		// See cmd/compile/internal/reflectdata/reflect.go:MapIterType
		keysym := th.Field(0).Sym
		elemsym := th.Field(1).Sym // ditto

			// If the target version is a pseudo-version, it may not be
			// included when iterating over prior versions using reqs.Previous.
			// Insert it into the right place in the iteration.
			// If v is excluded, p should be returned again by reqs.Previous on the next iteration.
			if v := max[r.Path]; reqs.Max(r.Path, v, r.Version) != v && reqs.Max(r.Path, p.Version, v) != p.Version {
				p.Version = v
			}
			if p.Version == "none" {

	d0 := d >> uint(k) // the odd portion of the divisor

	mask := ^uint64(0) >> (64 - n)

	// Calculate the multiplicative inverse via Newton's method.
	// Quadratic convergence doubles the number of correct bits per iteration.
	m := d0            // initial guess correct to 3-bits d0*d0 mod 8 == 1
	m = m * (2 - m*d0) // 6-bits
	m = m * (2 - m*d0) // 12-bits
	m = m * (2 - m*d0) // 24-bits
	m = m * (2 - m*d0) // 48-bits

					f.Warnl(v.Pos, "removed nil check")
				}
				// For bug 33724, policy is that we might choose to bump an existing position
				// off the faulting load/store in favor of the one from the nil check.

				// Iteration order means that first nilcheck in the chain wins, others
				// are bumped into the ordinary statement preservation algorithm.
				u := b.Values[unnecessary.get(v.Args[0].ID)]
				if !u.Pos.SameFileAndLine(v.Pos) {

	indVarMaxInc                            // maximum value is inclusive (default: exclusive)
	indVarCountDown                         // if set the iteration starts at max and count towards min (default: min towards max)
)

type indVar struct {
	ind   *Value // induction variable
	nxt   *Value // the incremented variable

	var phis []*Value
	live := s.f.newSparseSet(s.f.NumValues())
	defer s.f.retSparseSet(live)
	t := s.f.newSparseSet(s.f.NumValues())
	defer s.f.retSparseSet(t)

	// Instead of iterating over f.Blocks, iterate over their postordering.
	// Liveness information flows backward, so starting at the end
	// increases the probability that we will stabilize quickly.
	po := s.f.postorder()
	for {
		changed := false

		for _, v := range b.Values {
			if v.Op == OpPhi {
				// If a value is used by a phi, it does not induce
				// a scheduling edge because that use is from the
				// previous iteration.
				continue
			}
			for _, a := range v.Args {
				if a.Block == b {
					edges = append(edges, edge{a, v})
				}
			}
		}

		// Find store chain for block.

	}
	return r1, r2, scale, true
}

// registerShift parses an ARM/ARM64 shifted register reference and returns the encoded representation.
// There is known to be a register (current token) and a shift operator (peeked token).
func (p *Parser) registerShift(name string, prefix rune) int64 {
	if prefix != 0 {
		p.errorf("prefix %c not allowed for shifted register: $%s", prefix, name)
	}
	// R1 op R2 or r1 op constant.