[llvm-dev] Differentiate array access at IR level

[Michael Meng via llvm-dev]

Hello Johannes,

Thanks for your suggestion! The ScalarEvolution analysis is very helpful.

I had a follow-up question. It would be great if you can take a look.

 %11 = load float*, float** %a.addr, align 8, !dbg !954
 %12 = load i32, i32* %i, align 4, !dbg !955
 %sub = sub nsw i32 %12, 1, !dbg !956
 %idxprom7 = sext i32 %sub to i64, !dbg !954
 %arrayidx8 = getelementptr inbounds float, float* %11, i64 %idxprom7, !dbg !954

For the above code, the result for %idxprom7 is: (-1 + (sext i32 %12 to i64))<nsw>. Just wondering if you have any suggestion that I can replace (sext i32 %12 to i64) with symbol %i


Thanks,
Michael

________________________________
From: Johannes Doerfert
Sent: Tuesday, February 18, 2020 5:34 PM
To: Michael Meng
Cc: Michael Kruse; via llvm-dev
Subject: Re: [llvm-dev] Differentiate array access at IR level

Hi Michael,

You should run mem2reg on your code, e.g. use the "create_ll.sh" script
in `polly/test/create_ll.sh` to transform little C files to '.ll' files.

The solution I would go for looks something like this, assuming this is
in a pass in which LoopInfo is available as LI and ScalarEvolution as SE.

```
  DenseMap<std::pair<Loop *, const SCEV *>, unsigned> Reads, Writes;

  void dealWithLoopAndSubLoops(Loop &L) {
    for (BasicBlock *BB : L.blocks()) {
      for (Instruction &I : *BB) {
        if (LoadInst *Load = dyn_cast<LoadInst>(&I)) {
          Reads[{&L, SE->getSCEVAtScope(Load->getPointerOperand())}] += 1
        } else if (StoreInst *Store = dyn_cast<StoreInst>(&I)) {
          Writes[{&L, SE->getSCEVAtScope(Store->getPointerOperand())}] += 1
        }
      }
    }
    for (Loop *SubL : L)
      dealWithLoopAndSubLoops(*SubL);
  }

  void run(...) {
    for (Loop *L : *LI)
      dealWithLoopAndSubLoops(*L);

    errs() << "Reads:\n";
    for (auto &It : Reads) {
      errs() << " - Location: " << *It.first.second << " Loop: " <<
      It.first.first->getName() << " Access count: " << It.second << "\n";
    }
    errs() << "Writes:\n";
    for (auto &It : Writes) {
      errs() << " - Location: " << *It.first.second << " Loop: " <<
      It.first.first->getName() << " Access count: " << It.second << "\n";
    }
  }
```

Note that this is not tested in any way.

Hope this helps.


Cheers,
  Johannes

On 02/19, Michael Meng via llvm-dev wrote:
> Hi Michael,
>
> Thanks for your reply!
>
> Since I want to collect the counts of unique array access to array elements in one iteration, so I consider a[i] and a[i - 1] as two accesses to different array elements. Would it be possible to get the notion of "i - 1" at IR level.
>
> The IR looks like:
>
> %11 = load float*, float** %a.addr, align 8, !dbg !954
> %12 = load i32, i32* %i, align 4, !dbg !955
> %sub = sub nsw i32 %12, 1, !dbg !956
> %idxprom7 = sext i32 %sub to i64, !dbg !954
> %arrayidx8 = getelementptr inbounds float, float* %11, i64 %idxprom7, !dbg !954
>
> Working the the source level might be easier, but I wonder if it is possible to know idxprom7 -> i - 1 with IR
>
>
> Thanks again!
> Michael
> ________________________________
> From: Michael Kruse <llvmdev at meinersbur.de>
> Sent: Tuesday, February 18, 2020 9:26 AM
> To: Michael Meng <overrainbow2013 at hotmail.com>
> Cc: via llvm-dev <llvm-dev at lists.llvm.org>
> Subject: Re: [llvm-dev] Differentiate array access at IR level
>
> Note that a[i] are a[i-1] are accessing the same memory addresses,
> just not in the same loop iteration. You may want to look into
> DependenceInfo.
>
> Michael
>
> Am Mo., 17. Feb. 2020 um 00:04 Uhr schrieb Michael Meng via llvm-dev
> <llvm-dev at lists.llvm.org>:
> >
> > Hi LLVM community,
> >
> > I am trying to differentiate access to different array elements, for example:
> >
> > for (int i = 1; i < 10; i++) {
> >
> >     a[i] = a[i] + 10;
> >
> >      b[i] = a[i - 1]  * 2;
> >
> > }
> >
> > If it is possible to tell it loads/stores 3 different array elements: a[i], b[i] and a[i - 1] at IR level?
> >
> >
> > Thanks for your time in advance!
> >
> >
> > Best,
> > Michael
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

> _______________________________________________
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--

Johannes Doerfert
Researcher

Argonne National Laboratory
Lemont, IL 60439, USA

jdoerfert at anl.gov
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