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From Tianqi Chen <notificati...@github.com>
Subject Re: [dmlc/tvm] [RFC] Register Relay VM design (#2915)
Date Sat, 30 Mar 2019 18:43:36 GMT
Some brain dumps from my side.

- Allocating a new frame for each function call can be not quite friendly for deep recursive
calls.
  - Possible alternative: simply use the stack as registers, e.g. the register number points
to an offset to the current stack pointer
- SSA vs register reuse
  - SSA is easy to generate but can need a lot of registers, which may not bad for deep recursion
  - Register alloca(with infinite registers) can provide reuse amount registers, would need

- Register ISA is indeed more intuitive during the dump.
- Control flow: the implementation of Phi seems to need some thoughts
   - Need to be able to explicit reset the not required slot to nullptr, need to think about
how that might interplay with register alloca.
  - Note:  if only relay primitive conditional is needed, do we need a generic Phi?

As mentioned above, one possible tradeoff is to simply use the stack as registers, so the
register number indicates an offset to the begin ptr of the current function. This could unify
quite a lot of things. 

## Cost of Register VM
The main argument against a register vm is the implementation complexity given that stack
vm is a "fine and concise impl", the question really boils down to do we need these additional
features and cost that comes with them, so far the costs are

- Cost of new frame alloca for new call
  - Can be resolved by using stack frame as register
- Cost of larger register slot size
   - Possibly need reg alloca, need to think about control flow handling
- Control flow handling





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