llvm dev - Aug 2015 - Guidelines for pass initialization?

Let me rephrase to make sure I understand.  Once we have this settled, 
I'll update the comments in the file to summarize and possibly update 
the docs.

On 08/21/2015 10:27 AM, Owen Anderson wrote:
> The macro versions should be preferred, as they reduce static
initializaters.
Makes sense.  But the tradeoff is that they need to be baked into the 
list of known passes so that someone knows to call them?  So, if you 
have a pass which isn't part of LLVM itself, you should prefer 
RegisterPass?  Seems to make sense.  Is there also an implicit 
assumption here that any RegisterPass pass is a leaf in the dependency 
graph?  i.e. you couldn't use this to define your own
Analysis?
>
> INITIALIZE_PASS is used for a pass that is a leaf in the dependency graph,
whereas INITIALIZE_PASS_BEGIN is used for interior nodes.
Almost all transform passes - with the exception of LoopSimplify and the 
like - do not have passes which require them.  As a result, most 
transform passes are leaves in the dependency graph and should use the 
INITIALIZE_PASS mechanism.  All Analysis passes are expected to have 
consumers, so they are obvious not leaf and should use the 
INITIALIZE_PASS_BEGIN mechanism.  Is that a reasonable
summary?
>
> —Owen
>
>> On Aug 21, 2015, at 10:15 AM, Philip Reames via llvm-dev <llvm-dev
at lists.llvm.org> wrote:
>>
>> Does anyone know what the guidelines are supposed to be for properly
initializing a pass?  Looking around, we seem to have three styles of pass
registration in use.
>>
>> INITIALIZE_PASS(...)
>>
>> INITIALIZE_PASS_BEGIN(...)
>> INITIALIZE_PASS_DEPENDENCY(...)
>> ...
>> INITIALIZE_PASS_END(...)
>>
>> static RegisterPass<FooPass> X(...);
>> (This is the one encouraged in the docs, but seemingly the least widely
used in tree?)
>>
>> As far as I can tell, these often appear to work interchangeably. (At
least for passes with only "well known" dependencies.)  Can anyone
expose a set of guidelines as to when one should use each?
>>
>> Philip
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

> On Aug 21, 2015, at 10:40 AM, Philip Reames <listmail at
philipreames.com> wrote:
> 
>> 
>> INITIALIZE_PASS is used for a pass that is a leaf in the dependency
graph, whereas INITIALIZE_PASS_BEGIN is used for interior nodes.
> Almost all transform passes - with the exception of LoopSimplify and the
like - do not have passes which require them.  As a result, most transform
passes are leaves in the dependency graph and should use the INITIALIZE_PASS
mechanism.  All Analysis passes are expected to have consumers, so they are
obvious not leaf and should use the INITIALIZE_PASS_BEGIN mechanism.  Is that a
reasonable summary?
No, you have the edge directions backwards.  A pass that depends on something
else (a Transform that depends on an Analysis) needs to specify that dependency.
The Analysis pass itself does not need to specify the reverse dependency.

—Owen

On 08/21/2015 11:13 AM, Owen Anderson wrote:
>> On Aug 21, 2015, at 10:40 AM, Philip Reames <listmail at
philipreames.com> wrote:
>>
>>> INITIALIZE_PASS is used for a pass that is a leaf in the dependency
graph, whereas INITIALIZE_PASS_BEGIN is used for interior nodes.
>> Almost all transform passes - with the exception of LoopSimplify and
the like - do not have passes which require them.  As a result, most transform
passes are leaves in the dependency graph and should use the INITIALIZE_PASS
mechanism.  All Analysis passes are expected to have consumers, so they are
obvious not leaf and should use the INITIALIZE_PASS_BEGIN mechanism.  Is that a
reasonable summary?
> No, you have the edge directions backwards.  A pass that depends on
something else (a Transform that depends on an Analysis) needs to specify that
dependency.  The Analysis pass itself does not need to specify the reverse
dependency.
I realized I had that backwards not long after sending it.  :)  So, to 
rephrase one more time:
1) Analysis without dependencies are leaf and thus INITIALIZE_PASS 
eligible.  Most Analysis passes do not have dependencies.
2) Transforms without dependencies are leaf and thus INITIALIZE_PASS 
eligible.  Very few Transform passes have no dependencies!
3) Passes (Transform or Analysis) with dependencies are not leaf and 
must use INITIALIZE_PASS_BEGIN.
4) An INITIALIZE_PASS_BEGIN without a dependency is equivalent to 
INITIALIZE_PASS.

It would seem this makes the RegisterPass template essentially useless.  
It provides no way to register dependencies so any transform pass 
register through it is essentially relying on all Analysis passes it 
uses being previously registered.  Yuck.

Philip