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Bluespec Compiler (BSC)
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implicit pack/unpack + custom Bits instance can result in extraneous logic. #731

Open rossc719g opened 3 weeks ago

rossc719g commented 3 weeks ago

The compiler inserts an implicit unpack when reading a port/register, and an implicit pack when writing. For "standard" types like "UInt" or "Bit", these are effectively no-ops. But when using a type with a custom Bits instance, they can generate extraneous logic. Given that the source and destination are both packed, these could be elided, and remove the extraneous logic that is generated.

See https://github.com/rossc719g/bsc_examples/blob/main/pack_unpack/README.md for a full example.

The example includes a workaround using a wrapper type.

krame505 commented 3 weeks ago

I think a fix for this would be to introduce some primitives:

primitive primPack :: (a - > Bit n) -> a -> Bit n
primitive primUnpack :: (Bit n -> a) -> Bit n -> a

and use these as wrappers around pack and unpack at synthesis boundaries. The evaluator would permit these to cancel each other out as a special case, otherwise it would just use the wrapped functions.