Dependence distance and basic polly usage

[Graman]

Hi,

I'm trying to use polly with llvm, my idea was to use the result of the analysis with polly on one of my pass.

After some problems with the library I managed to write an empty pass with the polly lib included  (I had to add some line in the cmakelist file of the polly project).

Now I want to do something like this, in particular I want to write a pass that identify all the loop carried dependence and prints the instruction associate to it and their distance.

I started with this piece of code just to get more comfortable using the library:

virtual void getAnalysisUsage(AnalysisUsage &AU) const {

    AU.addRequiredTransitive<DependenceInfoWrapperPass>();
    AU.addRequired<LoopInfoWrapperPass>();
    AU.addRequiredTransitive<ScopInfoWrapperPass>();
    AU.setPreservesAll();

   }

  bool runOnFunction(Function &F) override {
  DependenceInfoWrapperPass *DI = &getAnalysis<DependenceInfoWrapperPass>();
  ScopInfo *SI = getAnalysis<ScopInfoWrapperPass>().getSI();
  for(ScopInfo::iterator i = SI->begin(), e = SI->end(); i!=e; i++){
    errs() << "ok\n";
  }
  }

 I run it on my code, and I don't see anything in output, but I was expecting at least one "ok".

I'm sure that I have the scop in my c program since I only wrote one loop like this:

for(int i = 1; i < 10; i++){

a[i]+=a[i-1];

}

I've double checked also by following this and those commands prints all the information that I expect.

Do you know why it doesn't print anything or do you have a short working example that prints all the dependencies and distances in a loop?

PS a simple tutorial on the site like the one on llvm would be great for new comers, I'm still not so sure about what I did. It works, but it doesn't feel right.

[Michael Kruse]

Most likely you did not run the canonicalization passes before your
pass. These are required in order to detect scops. You can use
polly::registerCanonicalicationPasses. Also add
polly::createCodePreparationPass().

You should be able to print dependency info using

$ clang -O3 -mllvm -polly -mllvm -debug-only=polly-dependence

Or run the canonicalization separately:

$ clang -O3 -Xclang -disable-llvm-optzns -emit-llvm -S | opt
-polly-canonicalize -polly-prepare -polly-dependences -analyze

(did not try out any of them myself, might have forgotten something)

Michael

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[Graman]

Thank you for the response, sadly I did already try with something like the one you proposed, but it doesn't work.

I should try some different approach to get the Scops of a function

ps

-debug-only=polly-dependence doesn't seems to exist:

clang (LLVM option parsing): for the --debug-pass option: Cannot find option named 'polly-dependence'!

[Graman]

Ok I think I've found the problem.

Since the loop that I wrote was too simple and would not benefit from a polly optimization then it's scop was discarded.

To solve this I've added the -polly-process-unprofittable flag

[Michael Kruse]

Am Fr., 25. Juni 2021 um 10:34 Uhr schrieb Graman <ofey...@gmail.com>:
> Ok I think I've found the problem.
> Since the loop that I wrote was too simple and would not benefit from a polly optimization then it's scop was discarded.
> To solve this I've added the -polly-process-unprofittable flag

Good to hear you found the problem. Feel free to ask more questions if needed.

Am Fr., 25. Juni 2021 um 02:25 Uhr schrieb Graman <ofey...@gmail.com>:
> -debug-only=polly-dependence doesn't seems to exist:
> clang (LLVM option parsing): for the --debug-pass option: Cannot find option named 'polly-dependence'!

You need an assert build for this to work (LLVM_ENABLE_ASSERTIONS=ON).
The message does not match the option (`--debug-pass` vs
`-debug-only`). The -debug-only option will just not do anything if
the argument is never used. The `-debug` option prints everything.
Lookup LLVM_DEBUG to see how it works.

Michael

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