[llvm-dev] RFC: Pass Execution Instrumentation interface

Fedor Sergeev via llvm-dev llvm-dev at lists.llvm.org
Thu Jun 7 13:07:14 PDT 2018

On 06/07/2018 07:14 PM, Philip Pfaffe wrote:
> Hi Fedor,
> 2018-06-07 17:48 GMT+02:00 Fedor Sergeev via llvm-dev 
> <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>>:
>     [...]
>     I consider LLVM context to be a good reference point for
>     "compilation-local singleton stuff".
>     My task is to provide a way to handle callbacks
>     per-compilation-context, and preferably have a single copy of those
>     (possibly stateful) callbacks per compilation.
>     In my implementation (linked at the end of RFC) I'm using
>     PassInstrumentationImpl to have a single copy of object.
>     What entity should *own* PassInstrumentationImpl object to make it
>     unique per-compilation?
> Both the PassBuilder and the AnalysisManager owning the analysis is 
> unique per compilation for all intents and purposes. Just making it an 
> analysis does not force you to extend the contract of IRUnitT to 
> access the context. The PassBuilder is also exposed to pass plugins, 
> so you get support for instrumentation plugins for free.
Conceptually I have always considered PassBuilder to be responsible only 
for construction of the pipeline.
Say, In our downstream usage we apply PassBuilder to construct a 
pipeline and get rid of it before even starting
the pipeline run. It appears to be a valid use as of right now.

If we enhance PassBuilder with bookkeeping capabilities then we will 
introduce the new requirement
for PassBuilder to stay alive till the end of compilation.
I'm not saying that it is a problem, it just breaks my view on a current 

Also, keep in mind that technically you can create a pipeline without 
PassBuilder at all.

On other hand, using PassBuilder to own InstrumentationImpl makes 
implementation rather simple since it can "seed" all the analyses
with instance that it owns.

AnalysisManager owning the InstrumentationImpl instance seems 
conceptually clearner to me, however to make this analysis unique
we need a way to make a single analysis manager responsible for that and 
to teach it how to feed other analyses (without transferring
the ownership). And that requires nontrivial implementation effort which 
I cant estimate right now.

Is it reasonable to enforce a requirement that ModulePassManager and 
ModuleAnalysisManager are always created?
Then we can put all this bookkeeping activity into ModuleAnalysisManager.

I'm kinda torn on this...


> Cheers,
> Philip
>     Again, in my implementation with Analysis-managed
>     PassInstrumentation I put Impl into PassBuilder
>     which registers Analyses with a reference to its Impl.
>     However that makes Impl to be per-Builder unique, which is not the
>     same as per-compilation.
>>     Because this is very pass specific, I think it would be
>>     substantially cleaner for it to be more specifically based in the
>>     pass infrastructure.
>>     I also think that this can be more cleanly designed by focusing
>>     on the new PM. The legacy PM has reasonable solutions for these
>>     problems already, and I think the desgin can be made somewhat
>>     simpler if we don't have to support both in some way.
>     That I kind of agree with.
>     And I do not plan to implement both at once.
>     So in a good case we just switch to new PM and go forward.
>     And in a bad case of postponing the switch we can use experience
>     and details of implementation of new PM to solve problems with
>     legacy PM
>     (but that is definitely a much lower priority for me).
>>     My hope would be that there are two basic "layers" to this. Along
>>     side a particular PassManager, we would have an analysis that
>>     instruments the running passes. This would just expose the basic
>>     API to track and control pass behavior and none of the "business
>>     logic".
>     Yes. PassInstrumentation seems to provide that.
>>     Then I would hope that the Passes library can build an instance
>>     of this analysis with callbacks (or a type parameter that gets
>>     type erased internally) which handles all the business logic.
>     As an idea I do agree with this.
>     But practically I dont have  a clear picture on how to manage the
>     instance(s).
>     regards,
>       Fedor.
>>     I think this will also address the layering issues around IR
>>     units because I think that the generic code can use templates to
>>     generically lower the IR unit down to something that can be
>>     cleanly handled by the Passes library. I think it is generally
>>     fine for this layer to rapidly lose strong typing or only have
>>     limited typed facilities because this is about instrumenting
>>     things and shouldn't be having interesting (non-debug) behavioral
>>     effects.
>>         Details:
>>            1. introduce llvm::PassInstrumentation
>>               This is the main interface that handles the
>>         customization and
>>         provides instrumentation calls
>>               - resides in IR
>>               - is accessible through
>>         LLVMContext::getPassInstrumentation()
>>                 (with context owning this object).
>>            2. every single point of Pass execution in the (new)
>>         PassManager(s)
>>         will query
>>               this analysis and run instrumentation call specific to a
>>         particular point.
>>               Instrumentation points:
>>                  bool BeforePass (PassID, PassExecutionCounter);
>>                  void AfterPass (PassID, PassExecutionCounter);
>>                   Run before/after a particular pass execution
>>                       BeforePass instrumentation call returns true if
>>         this
>>         execution is allowed to run.
>>                      'PassID'
>>                           certain unique identifier for a pass (pass
>>         name?).
>>                      'PassExecutionCounter'
>>                           a number that uniquely identifies this
>>         particular pass
>>         execution
>>                           in current pipeline, as tracked by Pass
>>         Manager.
>>                  void StartPipeline()
>>                  void EndPipeline()
>>                   Run at the start/end of a pass pipeline execution.
>>                   (useful for initialization/finalization purposes)
>>            3. custom callbacks are registered with
>>         PassInstrumentation::register* interfaces
>>               A sequence of registered callbacks is called at each
>>         instrumentation point as appropriate.
>>            4. introduce llvm::ExecutionCounter to track execution of
>>         passes
>>               (akin to DebugCounter, yet enabled in Release mode as
>>         well?)
>>               Note: it is somewhat nontrivial to uniquely track pass
>>         executions
>>         with counters in new pass
>>               manager as its pipeline schedule can be dynamic. Ideas
>>         are welcome
>>         on how to efficiently
>>               implement unique execution tracking that does not break in
>>         presence of fixed-point iteration
>>               passes like RepeatedPass/DevirtSCCRepeatedPass
>>               Also, the intent is for execution counters to be able
>>         provide
>>         thread-safety in multi-threaded
>>               pipeline execution (though no work planned for it yet).
>>            5. introduce a new analysis llvm::PassInstrumentationAnalysis
>>               This is a convenience wrapper to provide an access to
>>         PassInstrumentation via analysis framework.
>>               If using analysis is not convenient (?legacy) then
>>         PassInstrumentation can be queried
>>               directly from LLVMContext.
>>         Additional goals
>>         ================
>>            - layering problem
>>              Currently OptBisect/OptPassGate has layering issue -
>>         interface
>>         dependencies on all the "IR units",
>>              even those that are analyses - Loop, CallGraphSCC.
>>              Generic PassInstrumentation facilitiy allows to inject
>>         arbitrary
>>         call-backs in run-time,
>>              removing any compile-time interface dependencies on
>>         internals of
>>         those callbacks,
>>              effectively solving this layering issue.
>>            - life-time/scope control for multi-context execution
>>              Currently there are issues with multi-context execution
>>         of, say,
>>         -time-passes which store
>>              their data in global maps.
>>              With LLVMContext owning PassInstrumentation there should
>>         be no
>>         problem with multi-context execution
>>              (callbacks can be made owning the instrumentation data).
>>         Open Questions
>>         ==============
>>            - whats the best way to handle ownership of
>>         PassInstrumentation
>>              Any problems with owning by LLVMContext?
>>              Something similar to TargetLibraryInfo (owned by
>>         TargetLibraryAnalysis/TargetLibraryInfoWrapperPass)?
>>            - using PassInstrumentationAnalysis or directly querying
>>         LLVMContext
>>              PassInstrumentationAnalysis appeared to be a nice idea,
>>         only until
>>         I tried querying it
>>              in new pass manager framework, and amount of hooplas to
>>         jump over
>>         makes me shiver a bit...
>>              Querying LLVMContext is plain and straightforward, but
>>         we do not
>>         have a generic way to access LLVMContext
>>              from a PassManager template (need to introduce generic
>>         IRUnit::getContext?)
>>         Implementation
>>         ==============
>>         PassInstrumentationAnalysis proof-of-concept unfinished
>>         prototype
>>         implementation:
>>         (Heavily under construction, do not enter without wearing a
>>         hard hat...)
>>         https://reviews.llvm.org/D47858 <https://reviews.llvm.org/D47858>
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