[llvm-dev] Backend implementation for an architecture with only majority operation instruction

[Matthias Braun via llvm-dev]

> On Jun 1, 2017, at 8:13 PM, Sreejita saha via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> Hello everyone,
>  
> I was trying to create an LLVM backend for a processor with a very simple architecture and that does all instructions like load, store, arithmetic and logical instructions using a bunch of majority functions. The processor has only one instruction(majority function) in its ISA and breaks down all other instructions into a number of majority instructions depending on what instruction it is. All the instructions have different combinations of majority operations. Is there any way to implement this without creating a new Selection DAG node for the majority operation?
> I was thinking of creation of a new Selection DAG node and mapping all the other instructions like loads, stores as pseudo instructions and breaking them up. Can someone please help me with this?

If your architecture is alien enough (and so far this sounds pretty alien to me) it may make sense to just create a custom backend that goes from the IR representation to whatever machine format/assembly you are using.
While CodeGen gives you a lot of infrastructure that fits "normal" CPUs well (ABI lowering, instruction selection, register allocation, scheduling, object file formats, debug info). On the other hand if your target is a hypothetical CPU (just making guesses from what little you wrote) you may not need all that:
- Register allocation usually only makes sense if you have a bounded number of registers or special constraints.
- Scheduling usually only makes sense if you have a machine model with different latencies or want to optimize register pressure to minimize spill reloads (for bounded number of regs).
- If your object file format isn't ELF/macho/coff then you won't gain much from the MC infrastructure and debug generation infrastructure.

If you don't need most of these things then you are probably better off manually implementing a TargetMachine without using CodeGen (instead of implementing an LLVMTargetMachine with all the CodeGen utilities).

- Matthias
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