Instructions with register constraints

For any instruction, a list of constraints can be specified, like:
let Constraints = “$dst = $lhs” in

I checked the existing backends and I didn’t notice any complex constraints: basically “Constraints” keyword is used to force 1 operand register to be the same as another one.
I am wondering if there is any way to express more complicated constraints like for example:

  • if $lhs is either r0,r1,r2,r3 then $dst should also be in the set {r0,r1,r2,r3}
  • if $lhs is either r4,r5,r6,r7 then $dst should also be in the set {r4, r5, r6, r7}

Thank you !


Not at the moment, no. The only constraints supported are the "tied-to" constraint you indicate above and the "@earlyclobber" constraint which indicates an output register that cannot also be allocated to one of the instruction inputs.

See CodeGenInstruction.cpp in utils/TableGen for the gory details.