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

[Owen Anderson]

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.

>>   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.

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

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