[LLVMdev] Undefined combinations of operands in ISel

[Dylan McKay]

Hello all,

I am working on an AVR backend <https://github.com/avr-llvm/llvm>. From the
instruction manual
<https://github.com/avr-llvm/architecture/raw/master/Manuals/avr-instruction-set-manual.pdf>,
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?

Thanks,
Dylan
​
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