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

[Daniel Dunbar]

On Tue, Sep 17, 2013 at 11:29 AM, Reid Kleckner <rnk at google.com> wrote:

> Wait, I have a terrible idea.  Why don't we roll our own .init_array style
> appending section?  I think we can make this work for all toolchains we
> support.
>

Andy and I talked about this, but I don't think its worth it. My opinion is:
1. For tool options (the top-level llc, opt, llvm-as etc. opts) it doesn't
matter.
2. For experimental options (options that we would be happy if they were
compiled out of a production compiler/JIT client/whatever), it doesn't
matter.
3. For backend options that need to always be available, lots of them
probably already need to get promoted to real API.
4. For the remaining options (ones that don't need to become API, but also
aren't purely experimental), many of them can probably easily be
initialized by some existing initialization hook (pass initialization,
target initialization).
5. There aren't enough options left not in those categories to motivate
some kind of clever solution.

Another way of looking at it is: the implicitly initialized option syntax
is really convenient for experimental options, but those are exactly the
ones that don't cause problems because we could be happy just compiling
them out. For almost everything else, the implicitly initialized "feature"
of llvm::cl isn't all that useful, and is in some cases actively harmful.

 - Daniel

We'd have something like:
>
> struct PODOptData {
>   const char *FlagName;
> ...  // Common POD stuff, can be initialized at ParseCommandLine time.
> };
>
> LLVM_SECTION(".llvmopt")
> PODOptData OptionRegisterer = { "foo_flag", ... };
>
> I know the COFF magic to get the section bounds to form an array, and I
> know it exists for ELF, but I don't know how to do it on Darwin.
>
>
>
> 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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>>
>
>
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