[LLVMdev] MAJOR API CHANGE: Pass initialization without static constructors

[John Criswell]

  On 10/8/10 3:15 PM, Owen Anderson wrote:
>
> On Oct 8, 2010, at 1:05 PM, John Criswell wrote:
>
>> On 10/8/10 1:29 PM, Owen Anderson wrote:
>>> Hello fellow LLVM-ers,
>>>
>>> As those of you who follow llvm-commits have probably noticed, I've 
>>> been hard at work reworking our pass infrastructure to perform pass 
>>> initialization/registration without static constructors.  This is a 
>>> boon for clients of the LLVM libraries, as they can now control when 
>>> and if initialization occurs, as opposed to being forced to incur 
>>> the initialization cost at startup, negatively impacting their 
>>> startup times.
>>>
>>> The new-style solution consists of an llvm::initializeMyPass method 
>>> for every pass.
>>
>> What is this method of our passes supposed to do?  Is there a default 
>> implementation in the Pass class?
>
> This method is provided for you by INITIALIZE_PASS macros (and 
> friends).  All you have to do is add the declaration to 
> InitializePasses.h, and add a call of it to the library's batch 
> initialization method.

Is InitializePasses.h an LLVM header file?  If so, I assume modifying 
InitializePasses.h is only required if the pass is part of LLVM.  
Out-of-tree projects need to be able to create new passes without 
modifying the LLVM source.


>>>   These initialization routines must be invoked before the 
>>> PassManager will be able to resolve pipelines involving your pass. 
>>>  For convenience, LLVM now exposes per-library batch initialization 
>>> routines, i.e. initializeScalarOpts(), which initialize all the 
>>> passes within a given library in a single go.
>>
>> From an ease of programming perspective, this solution is not ideal 
>> as what we had before.  Each time I link in a new library into my 
>> program, I have to go find the method that initializes all of the 
>> passes and remember to call it from my tool.  Could you devise 
>> something better?  For example, could you extend PassManager with a 
>> method that calls all of the passes's initialization methods?  In 
>> other words, could you easily support something like:
>>
>> PassManager.add (Pass1);
>> PassManager.add (Pass2);
>> ...
>> PassManager.add (PassN);
>> PassManager.initializeAllPasses ();
>> PassManager.run();
>>
>>
>> ... or could the PassManager::add() method automatically initialize 
>> the pass?  Other possibilities may exists which are better as well; 
>> these are just the first two I've thought of.
>
> Neither of those will work.  These initialization have nothing to do 
> with initializing the pass, per se, and everything to do with the 
> PassManager being able to look it up.  In order for the PassManager to 
> be able to find a given pass (say, because some pass you explicitly 
> requested depends on it), that pass's corresponding PassInfo MUST have 
> been registered beforehand with the PassManager, enabling it to 
> perform recursive dependency chasing.  Without that pre-registration, 
> it has no way of knowing what passes exist, and is thus unable to 
> resolve dependencies.

Hrm.  I see.

I still don't like the idea of having every statically-linked tool 
explicitly initializing every library that gets linked in.  Just dumping 
the library into the Makefile and being done with it was much nicer.

If you can find a reasonable way to support that, it would be nice.  
However, if you can't, it's not that big a deal.  As I mentioned before, 
as long as out-of-tree passes don't have to modify LLVM source files to 
work properly, I'll live.

-- John T.

>
> At best, such an approach would allow you to avoid explicit 
> initialization of the passes you are explicitly instantiating, but 
> anything that must be found by dependency resolution needs to be 
> registered with the PassRegistry beforehand.
>
> --Owen
>