[LLVMdev] [polly] pass ordering

Hal Finkel
Wed, 17 Apr 2013 18:13:14 -0000

----- Original Message -----
> From: "Tobias Grosser" <tobias at grosser.es>
> To: "Sebastian Pop" <spop at codeaurora.org>
> Cc: llvmdev at cs.uiuc.edu
> Sent: Wednesday, April 17, 2013 12:45:26 PM
> Subject: Re: [LLVMdev] [polly] pass ordering
>
> On 04/17/2013 05:53 PM, Sebastian Pop wrote:
> > Hi,
> >
> > polly is run very early and schedules the following passes before
> > it runs:
> >
> > /// @brief Schedule a set of canonicalization passes to prepare for
> > Polly
> > ///
> > /// The set of optimization passes was partially taken/copied from
> > the
> > /// set of default optimization passes in LLVM. It is used to bring
> > the code
> > /// into a canonical form that simplifies the analysis and
> > optimization passes
> > /// of Polly. The set of optimization passes scheduled here is
> > probably not yet
> > /// optimal. TODO: Optimize the set of canonicalization passes.
> > static void registerCanonicalicationPasses(llvm::PassManagerBase
> > &PM) {
> >    PM.add(llvm::createPromoteMemoryToRegisterPass());
> >    PM.add(llvm::createInstructionCombiningPass());
> >    PM.add(llvm::createCFGSimplificationPass());
> >    PM.add(llvm::createTailCallEliminationPass());
> >    PM.add(llvm::createCFGSimplificationPass());
> >    PM.add(llvm::createReassociatePass());
> >    PM.add(llvm::createLoopRotatePass());
> >    PM.add(llvm::createInstructionCombiningPass());
> >
> >    if (!SCEVCodegen)
> >      PM.add(polly::createIndVarSimplifyPass());
> >
> >    PM.add(polly::createCodePreparationPass());
> >    PM.add(polly::createRegionSimplifyPass());
>
> Right.
>
> > Sergei was saying that on some benchmarks PromoteMemoryToRegister
> > was causing
> > performance regressions when it is run with and without Polly and
> > scheduled that
> > early.
>
> Are you saying these passes add compile time overhead or rather that
> they cause problems with the performance of the compiled binary?
>
> I assume when talking about regressions, you compare against a
> compilation with Polly disabled.
>
>  > Another remark is that these passes apply to all the functions,
> > transforming them without considering whether they contain loops or
> > whether
> > Polly could improve anything.
>
> True.
>
> > That brings the question: why do we run Polly that early?  Could we
> > move Polly
> > down after all these passes have been scheduled by LLVM's scalar
> > optimizer?
>
> For Polly we have basically two constraints:
>
> 1) We want to detect scops in the IR on which we run Polly
>
> This means the IR needs to be canonicalized enough to allow scalar
> evolution & Co to work.
>
> 2) The IR generated by Polly, should be well optimized through LLVM
>
> This means we do not only need to perform the optimizations that
> would
> have been necessary for the input code, but we also want to take
> advantage of optimization opportunities that show up after Polly
> regenerated code.
>
> When I generated the pass ordering, I did not spend a large amount of
> time to minimize it. I rather assumed, that to be sure the LLVM-IR is
> well optimized after Polly, it would be good to just run all passes
> LLVM
> passes over the output of Polly. So I just placed Polly at the very
> beginning. Now, to enable Polly to detect reasonably sized scops, I
> scheduled a set of canonicalization passes before Polly (taken from
> the
> beginning of the -O3 sequence).
>
> In terms of scop coverage and quality of the generated code this
> seems
> to be a good choice, but it obviously will increase the compile time
> compared to a run without Polly. What we could aim for is to run
> Polly
> at the beginning of the loop transformations e.g. by adding an
> extension
> point 'EP_LoopOptimizerStart'. Meaning before vectorization, before
> loop
> invariant code motion and before the loop idiom recognition. However,
> we
> would then need to evaluate what cleanup passes we need to run after
> Polly. For the classical code generation strategy we probably need a
> couple of scalar cleanups, with the scev based code generation, there
> is
> normally a lot less to do.
>
> If you can find a pass ordering that does not regress too much on the
> performance and scop coverage of the current one, but that has Polly
> integrated in the normal pass chain just before the loop passes, that
> would be a great improvement.

I thought that, when we discussed this in November, the goal was to have Polly
scheduled to run just prior to the loop vectorizer, etc. That way we could split
the analysis off and it could be (optionally) reused by the vectorization passes
without being invalidated by other transforms.

 -Hal

>
> Thanks,
> Tobias
>
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