type Flags struct {
	FieldTrack        bool
	PreemptibleLoops  bool
	StaticLockRanking bool
	BoringCrypto      bool

	// Regabi is split into several sub-experiments that can be
	// enabled individually. Not all combinations work.
	// The "regabi" GOEXPERIMENT is an alias for all "working"
	// subexperiments.

	// RegabiWrappers enables ABI wrappers for calling between

	// accordingly.

	// Search for the PUSHQ BP instruction inside the prologue.
	var pushbp *obj.Prog
	for p := s.Func().Text; p != nil; p = p.Link {
		if p.As == APUSHQ && p.From.Type == obj.TYPE_REG && p.From.Reg == REG_BP {
			pushbp = p
			break
		}
		if p.Pos.Xlogue() == src.PosPrologueEnd {
			break
		}
	}
	if pushbp == nil {
		ctxt.Diag("missing frame pointer instruction: PUSHQ BP")
		return
	}

	REG_FCC31

	REG_LAST = REG_FCC31 // the last defined register

	REG_SPECIAL = REG_FCSR0

	REGZERO = REG_R0 // set to zero
	REGLINK = REG_R1
	REGSP   = REG_R3
	REGRET  = REG_R20 // not use
	REGARG  = -1      // -1 disables passing the first argument in register
	REGRT1  = REG_R20 // reserved for runtime, duffzero and duffcopy
	REGRT2  = REG_R21 // reserved for runtime, duffcopy
	REGCTXT = REG_R29 // context for closures