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

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

On 10/8/10 3:44 PM, Owen Anderson wrote:
> [snip]
>
>> 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.
Okay.  Thanks, Owen.

-- John T.
> --Owen
>

If gcc/ld sort sections alphabetically you could use a technique similar to
OBJECT_ENTRY_AUTO [1], perhaps like this:

Initializer* llvm_reg_start __attribute__ ((section "_llvm_reg__a")) =
NULL;
Initializer* llvm_reg_end __attribute__ ((section "_llvm_reg__z")) =
NULL;

#define INITIALIZE_LLVM_PASS (passname, pass) void* llvm_reg_passname
__attribute__ ((section "_llvm_reg__m")) = &pass;

void InitializeAllPasses() {
  // since a < m < z, all of the registered pass pointers should live in
(llvm_reg_start, llvm_reg_end) after linking
  for (Initializer** i = ++(&llvm_reg_start); i < &llvm_reg_end);
++i) {
    Pass::Initialize(*i);
  }
}

[1] http://msdn.microsoft.com/en-us/library/exx3wywe(VS.80).aspx

-----Original Message-----
From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] On
Behalf Of John Criswell
Sent: Friday, October 08, 2010 1:55 PM
To: Owen Anderson
Cc: llvmdev
Subject: Re: [LLVMdev] MAJOR API CHANGE: Pass initialization without static
constructors

  On 10/8/10 3:44 PM, Owen Anderson wrote:
> [snip]
>
>> 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.
Okay.  Thanks, Owen.

-- John T.
> --Owen
>
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