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

[Owen Anderson]

On Oct 8, 2010, at 1:37 PM, John Criswell wrote:

> 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.

I was answering within the context of implementing a new pass in LLVM, where it's an API requirement that pass initializations are exposed in InitializePasses.h.  Within your own tool, it doesn't really matter where you declare it as long as it's visible to wherever you call it from. :-)

Also, as I pointed out, the RegisterPass<> templates do continue to exist and function without the need for explicit initialization(and are necessary for continued support of dynamically loadable plugins), but come with the cost of a static constructor.  For a pass wholly contained within an out-of-tree tool, it would be perfectly reasonable to use RegisterPass<> if the cost is acceptable to you.

> 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.

I don't especially like it either, but the abundance of static constructors in LLVM has been a long-standing performance concern.  I wouldn't be going this way if I had a better solution.

--Owen