[LLVMdev] [RFC] Internal command line options should not be statically initialized.

[Andrew Trick]

On Sep 17, 2013, at 11:01 AM, Eric Christopher <echristo at gmail.com> wrote:

>>> I definitely agree with the desire to remove command line options and
>>> having them be initialized as part of the pass would be general
>>> goodness. However, a few possible issues:
>>> 
>>> a) a number of command line options aren't really connected to passes
>>> per-se (backend options)
>> 
>> We don’t have to ban the old-style options. They can live-on indefinitely for experimental purposes, but if test cases need to use those options, they would need a REQUIRES: asserts line.
> 
> Enh, I'm in favor of banning them. Even though I use a few.

Sure. They’re convenient though. I’d rather have temporary experimental options then no test cases at all for some feature during its development.

>> For options that we actually want to make available to tools (and general testing) we have a couple possibilities:
>> 
>> Create a hook that can be called before command line parsing, like initializeXXPass.
>> 
> 
> This seems meh as just a workaround for the current behavior. It'd
> probably be a step forward, but might be more work than just doing…

Fair enough. It’s a way to make options available for unit tests in -Asserts builds without being forced to declare the option in the tool API. The most attractive thing about this approach is that most experimental backend options could be easily converted. So even if it is just a workaround, it provides a lot of flexibility.

>> Raise those options into an API and define the command line interface at the tool level instead. This seems like something we want to do anyway, but I’m not proposing anything definite yet (I’m happy as long we’re moving in that direction). We may want to raise some options all the way to function attributes.
>> 
> 
> something like this. I think this is the way to go. I'm not sure how
> it would look other than passing option structs into a context, but
> that might work?

Ok, we all seem to want this. As I told Hal, we probably should have a declarative syntax that generates code for tool options, integrates function attributes and command line flags, and provides an central lookup with some reflection API. How we do this us not going to change how we solve the immediate problem of static initializers, and it will obviously take a lot of time to agree on the design. So I’ll declare it out of scope for now.

-Andy

>>> b) "As long as pass initialization happens before parseCommandLine,
>>> usage should be consistent." I'm thinking this isn't going to work for
>>> the opt tool at least :)
>>> 
>>> Thoughts?
>> 
>> I think all the standard passes will be initialized up front. The problem would be with plugins using -load, but I fail to see how that can work with -help today anyway.
>> 
> 
> Guess so. Seems weird, especially for slightly buggy cases like debug
> info where most things happen during initialization. (Unfortunate, but
> unraveling it is a pretty big project at the moment)
> 
> -eric
> 
>> -Andy
>> 
>>> 
>>> -eric
>>> 
>>> 
>>> On Tue, Sep 17, 2013 at 10:10 AM, Andrew Trick <atrick at apple.com> wrote:
>>>> LLVM's internal command line library needs to evolve. We have an immediate need to build LLVM as a library free of static initializers, but before brute-force fixing this problem, I'd like outline the incremental steps that will lead to a desirable long term solution. We want infrastructure in place to provide an evolutionary path.
>>>> 
>>>> In the near term, clients who need llvm-the-library with no static initializers will build with LLVM_NO_STATICINIT. In this mode, existing users of cl::opt will default to being optimized away as constant initializers, and those options will not be available for command line parsing.
>>>> 
>>>> A new option class will need be defined, cl::toolopt. Initially, this will share the implementation and API with cl::opt. The only difference will be that cl::toolopt is immune to LLVM_NO_STATICINIT. Options that are required for tool support can simply be type-renamed to toolopt. Since these are not defined in a library, their static initializers are irrelevant.
>>>> 
>>>> Eventually, we would like to eliminate the special LLVM_NO_STATICINIT build mode. This can be done by making the -Asserts build just as strict. In the meantime, we would like to run as many unit tests as possible with LLVM_NO_STATICINIT builds. This will be solved by gradually moving cl::opt definitions buried within LLVM libraries to to a new pattern that avoids static initialization.
>>>> 
>>>> One easy pattern to follow is to register the option during pass initialization with all the convenient flags and parameters, but refer to a globally defined option storage that enforces the singleton and provides visibility. As long as pass initialization happens before parseCommandLine, usage should be consistent.
>>>> 
>>>> Strawman:
>>>> 
>>>> cl::optval<bool> MyOption; // Just the storage, no initialization.
>>>> 
>>>> MyPass() {
>>>> // Only registers an option with the same optval once.
>>>> Option cl::registerOpt(MyOption, cl::init(false), cl::Hidden,
>>>>                        cl::desc("Descriptive string..."), );
>>>> }
>>>> 
>>>> -Andy
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>>