[llvm-dev] RFC: Pass Execution Instrumentation interface

Chandler Carruth via llvm-dev llvm-dev at lists.llvm.org
Thu Jun 7 08:11:02 PDT 2018

We had already talked about this, so unsurprisingly I'm generally in favor
of the direction. Some comments below.

On Thu, Jun 7, 2018 at 2:00 AM Fedor Sergeev <fedor.sergeev at azul.com> wrote:

>    - access through LLVM Context (allows to control life-time and scope
> in multi-context execution)
>    - wrap it into an analysis for easier access from pass managers

Why not simply make it an analysis, and leave LLVM context out?

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.

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".

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.

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
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