[LLVMdev] Passes calling

[Andrew Trick]

On Jun 13, 2014, at 2:41 AM, Chandler Carruth <chandlerc at google.com> wrote:

> 
> On Fri, Jun 13, 2014 at 3:11 AM, Andrew Trick <atrick at apple.com> wrote:
> As for the Analysis passes, why do they need to be “passes”? I don’t think it should be required for a PassManager to schedule them. The static dependence between Analysis->Pass is often not what you want. A pass should be able to conditionally ask for an Analysis only if and when it’s needed. As a compiler matures, these conceptually stand-alone Analysis end up becoming utilities that can be invoked on-the-fly anyway. Furthermore, Analysis results can be built up incrementally as needed, and only valid within some scope.
> 
> An Analysis “result” just needs to be registered with the PassManager for reuse. The hard part of the problem is managing invalidation of the Analysis. I think it’s a bit crazy for example that when a pass currently invalidates an Analysis, the analysis remains valid until the pass completes and control returns to the PassManager. It would be nice to have a better framework for Analysis invalidation some day.
> 
> To be clear, there’s nothing wrong with having an AnalysisPass that can be explicitly scheduled. That’s useful for testing passes that only use an analysis “if available”. However, an Analysis should not need to be a pass in order to register its result with the PassManager.
> 
> So, the design actually makes precisely the distinction you're looking for. Analyses are different beasts from transformations at a reasonably firm level.
> 
> That said, they still are "passes" in some very vague sense -- they still benefit (sometimes) from having a phase where the run over the given unit of IR to build up the datastructure / information that they intend to vend.
> 
> The current design has two different but similar concepts: transformation passes and analysis passes. You can see the difference in the interface: a transformation pass returns the set of invalidated analyses, an analysis pass returns the results of that analysis which should be cached, queried, and (eventually) invalidated by the pass manager infrastructure.
> 
> And yes, the invalidation is already *much* more precise and I think we can make it even better. I still need to document some of this, but want to hold off on doing tons of documentation of guarantees and invariants until I have more things converted over (but not enabled of course) so that I'm more confident that the invariants promised can be upheld. =]

Sure, I’m just gently floating some ideas without spending much time understanding how the new design works.

The part that seems limiting to me is that a transform has to return a set of invalidated analysis. I supposed that set by default contains all the cached analyses and the transform selectively prunes what it preserves?

Here is a simple example of my ideal model—just a rough idea:

1. DomTree and LoopInfo analyses run. They register with the PassManager as CFG dependents.
2. Transform1 runs and acquires a reference to this function’s DomTree
3. Transform1 adds a CFG edge
4. Transform1 immediately notifies the PassManager that all analyses depending on the CFG must be invalidated
5. PassManager invalidates its reference to the DomTree and LoopInfo
6. Transform1 incrementally updates its copy of the DomTree and reregisters it with the PassManager.
7. Transform2 runs and reuses the DomTree but must recompute LoopInfo.

You could imagine other kinds of Analysis being dependent on the call graph or SSA values. You could also imagine that an analyses knows how to partially invalidate itself for example when a specific function, block, or value changes.

-Andy
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