help: about how to use tblgen to constraint operand.

I define a pattern to move two 32bits gpr to 64bits fpr. like arm instructure fmdrr.
But I need to use an even/odd register pair to save its 2 operands.
I define in

SDTypeProfile<1, 2, [SDTCisVT<0, f64>, SDTCisVT<1, i32>,
SDTCisSameAs<1, 2>]>;
def my_fmdrr : …
def myFMDRR : …
(outs FPR: $result), ins(GPR: $op1, GPR:$op2 )
[(setFPR: $result, (my_fmdrr GPR: $op1, GPR:$op2) )]

I create myfmdrr instructure in mytargetISelLowering.cpp. and its operands are in R0 and R1.
But after optimization, the operands are save R2 and R1. I know optimization pass does not
know myfmdrr operands constraint. But How I tell optimzition pass by tblgen??

Could I can control operand constraint in mytargetiSelLowering.cpp? How do I control??


Currently there is no constraint that tells the register allocator to allocate a consecutive register pair. What I would suggest you do is to declare pseudo register pair registers (and corresponding register class, say PAIR_GPR). In this case, your myFMDRR would take one input of PAIR_GPR class. The asm printer should be taught to print a PAIR_GPR register as two GPR registers (you should also teach the JIT of the same thing).

A PAIR_GPR register should be a super register of two GPR registers. e.g. r0r1_pair is a super register of r0 and r1. In order to construct a PAIR_GPR register, you have to use two INSERT_SUBREG. To extract out a GPR from a PAIR_GPR, you need to issue EXTRACT_SUBREG. In most cases, these will be nop’s. In other cases, they are copies.