Getting ScalarEvolution info inside a custom pass

Beginners
1

previous research:

I am trying to analyze every loop in a function and get the bounds of each loop via getBounds. I require an object of type ScalarEvolution to get that info.
I found that getAnalysis could be used in passes to get this info from some snippets online, but I get errors saying that getAnalysis is not defined, or that it is not a member of a static class. I assume there is some inaccuracy in how I am going about this.

This is the expansion of the line I use to compile my pass:

/home/share/llvm_build/bin/clang -shared -o hw1.so hw1.cpp -I/home/share/llvm-project-17.0.2.src/llvm/include -I/home/share/llvm_build/include -D_GNU_SOURCE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -fPIC -fno-rtti -L/home/share/llvm_build/lib

This is the code of the pass so far. I want to get the correct info in variable SE on line 16:

#include "llvm/IR/PassManager.h"
#include "llvm/Passes/PassBuilder.h"
#include "llvm/Passes/PassPlugin.h"

using namespace llvm;

namespace {

struct DependencyAnalysisPass : public PassInfoMixin<DependencyAnalysisPass> {
  PreservedAnalyses run(Function &F, FunctionAnalysisManager &FAM);
};

PreservedAnalyses DependencyAnalysisPass::run(Function &F, FunctionAnalysisManager &FAM) {

  auto &LI = FAM.getResult<LoopAnalysis>(F);
  auto &SE = getAnalysis<ScalarEvolutionWrapperPass>().getSE();
  for (const auto &L : LI) {
    if (auto LB = L->getBounds(*SE)) {

    } 
  }
  for (BasicBlock &BB : F) {
    for (Instruction &I : BB) {
        if (auto *LI = dyn_cast<LoadInst>(&I)) {
          outs() << "Load:  " << I << "\n" ;
        } else if (auto *SI = dyn_cast<StoreInst>(&I)) {
          outs() << "Store: " << I << "\n" ;
        } else {
          outs() << "Other: " << I << "\n" ;
        }
    }
  }
  return PreservedAnalyses::all();
}

} // end anonymous namespace

extern "C" ::llvm::PassPluginLibraryInfo LLVM_ATTRIBUTE_WEAK
llvmGetPassPluginInfo() {
  return {LLVM_PLUGIN_API_VERSION, "DependencyAnalysisPass", LLVM_VERSION_STRING,
          [](PassBuilder &PB) {
            PB.registerPipelineParsingCallback(
                [](StringRef Name, FunctionPassManager &FPM,
                   ArrayRef<PassBuilder::PipelineElement>) {
                  if (Name == "depanalyze") {
                    FPM.addPass(DependencyAnalysisPass());
                    return true;
                  }
                  return false;
                });
          }};
}
2

Have you tried FAM.getResult instead of getAnalysis?

3

thanks for the pointer. Using:

  auto &LI = FAM.getResult<LoopAnalysis>(F);
  auto &SE = FAM.getResult<ScalarEvolutionAnalysis>(F);
  for (const auto &L : LI) {
    if (auto LB = L->getBounds(SE)) {
      outs() << "Valid loop found" << "\n";
      outs() << LB->getInitialIVValue() << " "
             << LB->getStepInst() << "\n"; 
    } 
  }

compiles without any errors. However, it doesn’t seem to print any data related to loops in my test input. Is my usage of FAM.getResult correct?

4

The return value from getBounds is std::optional, and it can return nothing in a number of situations. You can look at the source to see what it does.

In terms of usage your code looks ok.

Another option is to use getBackedgeTakenCount from SE: https://github.com/llvm/llvm-project/blob/main/llvm/include/llvm/Analysis/ScalarEvolution.h#L889

5

I would recommend using that interface, as getBounds has a number of constraints which can cause it to fail. If you want to identify inductions, you check check if phis are AddRecurrences.

6

Thanks for the help. I’ve found that the loop isn’t particularly suitable for these analysis methods. I’ve tried using getBackedgeTakenCount with the result from FAM.getResult, which expectedly doesn’t work. I will look at how to get that working and ask in a separate thread if i have any other questions.