LLVM command line options integrations

[mshockwave]

The proposed feature allows users of ponyc to supply LLVM-specific command line options from ponyc's command line interface in dev builds (i.e. Debug build). This should make development of ponyc easier by improving accessibilities of internal LLVM features during debugging and fine-grained tuning.

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from the Pony core team:

Everyone who has an opinion on Option 1 vs Option 2, please leave a reaction to this comment where you do:

hooray reaction if you favor option 1 and rocket reaction if you favor option 2.

[dipinhora]

Julia's solution seems like a good alternative: https://docs.julialang.org/en/v1/manual/environment-variables/index.html#JULIA_LLVM_ARGS-1

[mshockwave]

@dipinhora thanks for the feedback, I'll put into the alternative section.

[SeanTAllen]

Thanks for this @mshockwave.

We are going to discuss this on August 20th after trying to collect some feedback on the two options.

[dipinhora]

I'd prefer option 3: either an environment variable or a command line argument passed in using quotes ("to handle spaces") so that the entire blob can be passed off to LLVM directly without any pre-processing or thought about how to structure the arguments (to prefix them all with -mllvm or which separator to use instead of ).

From what I can tell, the Julia environment variable implementation boils down to 1 line of code (https://github.com/JuliaLang/julia/blob/master/src/codegen.cpp#L7613) calling into an existing LLVM function (https://llvm.org/doxygen/namespacellvm_1_1cl.html#ae5a3364c95fc71013039ef5fd7aa7fe0). I'm guessing there's a similar function for parsing all LLVM arguments from a single string also.

[jemc]

I like Dipin's suggestion.

[mshockwave]

Actually I see no problem implementing all of the options, including option 3 proposed by @dipinhora . Option 1 and option 2 can exist at the same time. If user also supply environment variable in option 3, we just concat it before or after those which are supplied from command line.

[SeanTAllen]

I'm not in favor of multiple ways to do this. I want 1 way to do it. Definitely not 2 or 3 possible ways.

[slfritchie]

Looking at the https://docs.julialang.org/en/v1/manual/environment-variables/index.html#JULIA_LLVM_ARGS-1 link, I don't see anything there that tells me what the contents of the JULIA_LLVM_ARGS env var is supposed to contain. Is it verbatim LLVM compiler args? Or something else. Some clarification and an example would be really helpful.

[mshockwave]

@slfritchie I think they simply use this API. Here is an usage example:

export MY_PONY_LLVM_ARGS="-debug-only=mem2reg,sroa -stats"
./ponyc <pony args>

somewhere in ponyc, we call the API like this:

llvm::cl::ParseEnvironmentOptions("ponyc", "MY_PONY_LLVM_ARGS");

In short, we put exactly the same CLI option string as we passed to other LLVM tools into the environment variable, and we can choose our own environment variable name.
However, as mentioned in the LLVM command line doc above: the API does not support quoting. That is, -some-args="foo bar" will be seen as two separated options -some-args=foo and bar with this API.

[SeanTAllen]

Ok, we are down to vote time. 3 possibilities. Please vote as a reaction to this comment:

• Command line option 2 as listed in the RFC add a 🚀
• Environment variable listed in comments here and in alternatives ❤️
• Allow for both of these 👀

[aturley]

The voting on this will close on September 10.

[SeanTAllen]

This has been accepted in principal. @mshockwave can you update the RFC to make option 2 as "the option" and have the others as alternatives that we didn't accept, but did exist?

Once that is done, we can merge and officially accept.

Thanks.

[mshockwave]

@SeanTAllen done.
Also just curious, after this RFC is accepted, are we going to proceed with the implementation?

[SeanTAllen]

@mshockwave when accepted, an issue will be created on the ponyc that anyone can pick up and start working on. if you want to do the work, please do.

[SeanTAllen]

This RFC has been accepted.

[SeanTAllen]

@mshockwave there's an issue open for this now: ponylang/ponyc#3327

[mshockwave]

@SeanTAllen awesome!