skipMask := off / goarch.PtrSize / 8
	skipBytes := skipMask * goarch.PtrSize * 8
	ptrmask := addb(gcbits, skipMask)
	src = add(src, skipBytes)
	off -= skipBytes
	size += off
	var bits uint32
	for i := uintptr(0); i < size; i += goarch.PtrSize {
		if i&(goarch.PtrSize*8-1) == 0 {
			bits = uint32(*ptrmask)
			ptrmask = addb(ptrmask, 1)
		} else {
			bits >>= 1
		}
		if off > 0 {
			off -= goarch.PtrSize

	MOVL	delta_lo+4(FP), SI
	MOVL	delta_hi+8(FP), DI
	// DX:AX = *addr
	MOVL	0(BP), AX
	MOVL	4(BP), DX
addloop:
	// CX:BX = DX:AX (*addr) + DI:SI (delta)
	MOVL	AX, BX
	MOVL	DX, CX
	ADDL	SI, BX
	ADCL	DI, CX

	// if *addr == DX:AX {
	//	*addr = CX:BX
	// } else {
	//	DX:AX = *addr
	// }
	// all in one instruction
	LOCK
	CMPXCHG8B	0(BP)

	JNZ	addloop

	// success

		Load(aX, RAX)
	default:
		Comment(fmt.Sprintf("%s += %d×%s×%s", r, i, aX, bX))
		IMUL3Q(Imm(i), Load(aX, GP64()), RAX)
	}
	MULQ(mustAddr(bX)) // RDX, RAX = RAX * bX
	ADDQ(RAX, r.lo)
	ADCQ(RDX, r.hi)
}

// shiftRightBy51 returns r >> 51 and r.lo.
//
// After this function is called, the uint128 may not be used anymore.
func shiftRightBy51(r *uint128) (out, lo GPVirtual) {
	out = r.hi