Proposal: Roundtrip serialization of Cmm (parser-compatible pretty-printer output)

[Hécate]

Hi Diego,

Thank you very much for your work in this direction, it's sorely needed.

I'm all for having proper roundtrip correctness for Cmm, but I am not 
sure altering the parser is the way to go.
In my opinion, GHC should produce valid textual Cmm, that can be 
ingested by the parser at it is today.

Have a nice day,
Hécate

Le 28/07/2025 à 02:16, Diego Antonio Rosario Palomino a écrit :
>
> Hello GHC devs,
>
> I'm currently working on Cmm documentation and tooling improvements as 
> part of my Google Summer of Code project. One of my core goals is to 
> make Cmm roundtrip serializable.
>
> Right now, the in-memory Cmm data structure—generated programmatically 
> (e.g., from STG via GHC)—can be pretty-printed, and Cmm can also be 
> parsed. However, the pretty-printed version is not compatible with the 
> parser. That is, we cannot take the output of the pretty printer and 
> feed it directly back into the parser.
>
> Example:
>
> Parseable version:
>
> |sum { cr: bits64 x; x = R1 + R2; R1 = x; jump %ENTRY_CODE(Sp(0))[R1]; } |
>
> Pretty-printed version:
>
> |sum() { // [] { info_tbls: [] stack_info: arg_space: 8 } {offset cf: 
> // global _ce::I64 = R1 + R2; R1 = _ce::I64; call (I64[Sp + 0 * 
> 8])(R1) args: 8, res: 0, upd: 8; } } |
>
> Another example:
>
> Parseable version:
>
> |simple_sum_4 { // [R2, R1] cr: // global bits64 _cq; _cq = R2; bits64 
> _cp; _cp = R1; R1 = _cq + _cp; jump (bits64[Sp])[R1]; } |
>
> Pretty-printed version:
>
> |simple_sum_4() { // [] { info_tbls: [] stack_info: arg_space: 8 } 
> {offset cs: // global _cq::I64 = R2; _cr::I64 = R1; R1 = _cq::I64 + 
> _cr::I64; call (I64[Sp])(R1) args: 8, res: 0, upd: 8; } } |
>
> While it’s possible to write parseable Cmm that resembles the 
> pretty-printed version (and hence the internal ADT), they don’t fully 
> match—mainly because the parser inserts inferred fields using 
> convenience functions.
>
> Proposal:
>
> To make roundtrip serialization possible, I propose supporting a new 
> syntax that matches the pretty printer output exactly.
>
> There are a couple of design options:
>
> 1.
>
>     Create a separate parser that accepts the pretty-printed syntax.
>     Files could then use either the current parser or the new strict one.
>
> 2.
>
>     Extend the current parser with a dedicated block syntax like:
>
> |low_level_unwrapped { ... } |
>
> This second option is the one my mentor recommends, as it may better 
> reflect GHC developers' preferences. In this mode, the parser would 
> not insert any inferred data and would expect the input to match the 
> pretty-printed form exactly.
>
> This would enable a true roundtrip:
>
>  *
>
>     Compile Haskell to Cmm (in-memory AST)
>
>  *
>
>     Pretty-print and write it to disk (wrapped in low_level_unwrapped
>     { ... })
>
>  *
>
>     Later read it back using the parser and continue with codegen
>
> Optional future direction:
>
> As a side note: currently the parser has both a “high-level” and a 
> “low-level” mode. The low-level mode resembles the AST more closely 
> but still inserts some inferred data.
>
> If we introduce this new “exact” low-level form, it's possible the 
> existing low-level mode could become redundant. We might then have:
>
>  *
>
>     High-level syntax
>
>  *
>
>     New low-level (exact)
>
>  *
>
>     And possibly deprecate the current low-level variant
>
> I’d be interested in your thoughts on whether that direction makes sense.
>
> Serialization libraries?
>
> One technically possible—but likely unacceptable—alternative would be 
> to derive serialization via a library like |aeson|. That would enable 
> serializing and deserializing the Cmm AST directly. However, I 
> understand that |aeson| adds a large dependency footprint, and likely 
> wouldn't be suitable for inclusion in GHC.
>
> Final question:
>
> Lastly—I’ve heard that parts of the Cmm pipeline may currently be 
> under refactoring. If that’s the case, could you point me to which 
> parts (parser, pretty printer, internal representation, etc.) are 
> being modified? I’d like to align my efforts accordingly and avoid 
> conflicts.
>
> Thanks very much for your time and input! I'm happy to iterate on this 
> based on your feedback.
>
> Best regards,
> Diego Antonio Rosario Palomino
> GSoC 2025 – Cmm Documentation & Tooling
>
>
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