llvm dev - Oct 2010 - [LLVMdev] Fwd: Re: MAJOR API CHANGE: Pass initialization without static constructors

Forgot to reply to the list.

-- John T.


-------- Original Message --------
Subject: 	Re: [LLVMdev] MAJOR API CHANGE: Pass initialization without 
static constructors
Date: 	Fri, 8 Oct 2010 15:05:21 -0500
From: 	John Criswell <criswell at illinois.edu>
To: 	Owen Anderson <resistor at mac.com>



   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?
>     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.

-- John T.

>
>
>  * WHAT THIS MEANS FOR LLVM USERS
>
>  Users of LLVM will need to add explicit calls to the initialization
routines to their programs.  The trivial solution is to perform initialization
at the beginning of main(), and to call the batch initialization routines for
every LLVM library you link against.  Clients interested in improving launch
time and/or minimizing unnecessary linkage can defer initialization until it is
needs, and/or only call the initialization routines for the individual passes
that they depend on.
>
>  Dynamically loaded plugins containing passes will continue to use the
old-style style initialization scheme, and thus will continue to incur the cost
of static constructors.  Don't use them if you don't want static
constructors.
>
>  * WHAT THIS MEANS FOR LLVM DEVELOPERS
>
>  I have already done this work for all passes already in the LLVM tree, but
developers need to be aware for future work:
>
>  You MUST use the new-style INITIALIZE_PASS, INITIALIZE_AG_PASS, and
INITIALIZE_ANALYSIS_GROUP macros in place of the old-style RegisterPass<> 
templates.  These macros will create an initializeMyPass() method, which you
*MUST* declare in InitializePasses.h, and which you must add to your
library's batch initialization method.
>
>  --Owen Anderson
>  _______________________________________________
>  LLVM Developers mailing list
>  LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>  http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

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