[llvm-dev] RFC: Pass Execution Instrumentation interface
Philip Pfaffe via llvm-dev
llvm-dev at lists.llvm.org
Thu Jun 7 09:14:00 PDT 2018
2018-06-07 17:48 GMT+02:00 Fedor Sergeev via llvm-dev <
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
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.
> 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
> 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
> 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.
>> 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.
>> certain unique identifier for a pass (pass name?).
>> a number that uniquely identifies this particular pass
>> 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
>> - 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
>> PassInstrumentationAnalysis proof-of-concept unfinished prototype
>> (Heavily under construction, do not enter without wearing a hard hat...)
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