On 06/19/2018 12:55 PM, Paul Koning wrote:
> Gentlepeople,
>
> I have a two-operand divide instruction that takes a double length dividend
> in a register pair, and produces the quotient in the first register and
> remainder in the second.
>
> How do I write a divmod pattern for that? The quotient is easy enough, I
> write a match_operand for that register and a matching constraint ("0") for
> the input dividend. But what about the remainder? The remainder appears in
> a register that isn't explicitly mentioned in the RTL (it's the regnum one
> higher than the quotient, or if you like, the second subreg of the input
> (dividend) register.
You can generally allocate double-sized registers with appropriate
constraints and the like. And you could use matching constraints,
perhaps with subregs, but in the end, ewwwww.
>
> I can make it a define_expand that adds a move from the remainder register
> into a new register which is the output operand, and count on the optimizer
> to optimize away that move. Is that the best answer? The current "mod"
> pattern does that, and I could keep that approach.
But this would generally be better I think. I'd expect the move to be
optimized away the vast majority of the time.
jeff
