Undefined combinations of operands in ISel

Hello all,

I am working on an AVR backend. From the instruction manual, there are about a dozen instructions which have (seemingly arbitrary) constraints on their operands.

For example, take the “load from pointer P into register R and then postincrement P” instruction:

; Prototy
; LD R, P+

; load byte from Z pointer register into r20 and postincrement
LD r20, Z+

Here is a list of undefined operand combinations:

  • ELPM r30, Z+
  • ELPM r31, Z+
  • LD r26, X+
  • LD r27, X+
  • LD r26, -X
  • LD r27, -X
  • LD r28, Y+
  • LD r29, Y+
  • LD r28, -Y
  • LD r29, -Y
  • LD r30, Z+
  • LD r31, Z+
  • LD r30, -Z
  • LD r31, -Z
  • LPM r30, Z+
  • LPM r31, Z+
  • ST X+, r26
  • ST X+, r27
  • ST -X, r26
  • ST -X, r27
  • ST Y+, r28
  • ST Y+, r29
  • ST -Y, r28
  • ST -Y, r29
  • ST Z+, r30
  • ST Z+, r31
  • ST -Z, r30
  • ST -Z, r31

How can I tell LLVM to not generate these combinations during instruction selection?
Or maybe just a way to indicate to the register allocator that a set of operands for an instruction is undefined in a virtual function that can be overriden?