[LLVMdev] [RFC] Removing static initializers for command line options

[Chris Bieneman]

> On Aug 19, 2014, at 1:32 PM, Rafael Espíndola <rafael.espindola at gmail.com> wrote:
> 
>>> * Nobs for which there is not a single right answer for all users.
>>> There are very few of these currently and we expect it to remain like
>>> that. These should not use cl::opt or static storage at all. They
>>> should be an option passed to the constructor of the pass.  It may
>>> also involve exposing it to the C api and the PassManagerBuilder.
>> 
>> How would you suggest we expose cl::opts for modifying these options in tools like opt? A good example of this type of option would be the options in LoopUnrollPass.cpp.
> 
> Opt uses the PassManagerBuilder. Opt itself could have a command line
> options controlling its use of PassManagerBuilder. That is probably
> fine since we expect very few of these.
> 
>>> 
>>> * Options that we use during development of llvm. They are useful for
>>> testing, tracking a bug or enabling/disabling a feature that is still
>>> under development. These can use a static storage since external
>>> clients like webkit will never change them. We do have to avoid these
>>> options requiring static constructors, since otherwise the client is
>>> paying for something it will never use.
>> 
>> What about when we're debugging the WebKit JIT? For development of libraries using LLVM it would be nice to be able to toggle these values too, which is why Filip’s suggestion of an API like LLVMConfigSetBoolValue(Config, "ScalarizeLoadStore", 1) would be nice.
> 
> Most llvm bugs reproduce with just opt or llc, but if that is not the
> case, cl::ParseCommandLineOptions when debugging seems fine.

There are two reasons this doesn’t work:

(1) Cases where I might want to set a debug variable for the WebKit JIT but not for Mesa - if the option storage is global it will get overwritten for all users
(2) cl::ParseCommandLineOptions today is C++ API, WebKit restricts itself to the C API, so at the least we’d need to expose it as part of the C API

> 
> The advantages of this setup are
> 
> * Options that are exposed to the users are done so in a very natural
> way (constructor arguments).

I’m on board with this, but not to the exclusion of other use cases.

> * Internal options are still really easy to create, but now don't have
> static constructors.

We’re in agreement here

> * We don't need a LLVMConfig object that gets passed around.

For the sake of this discussion, let’s just scrap my phase two idea for a configuration object and treat that as a separate issue.

> * No stringly typed interface.

I think that there are advantages to a string-based interface. Sean Silva actually suggested that interface when I first sent out an email about our plans to rework command line options back on 8/6. Based on Sean’s feedback and a few discussions I had offline with other LLVM contributors I thought a stringly typed interface was the best approach to eliminating both the static constructors and the static storage which are explicit goals for our project.

Thanks,
-Chris

> 
> Cheers,
> Rafael