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

ChanMaxthon xcvista at me.com
Tue Sep 17 11:36:45 PDT 2013

So since you are rolling that then why don't just use the simpler ARM ELF ABI for that? armv7a ABI on i386 and AArch64 ABI in amd64? That init-array are dead simple to use.

Sent from my iPhone

> On 2013年9月18日, at 2:29, 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.
> 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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