Different optimized IR with opt vs. ModulePassManager

Beginners
1

I’m currently seeing different results between running a ModulePassManager with the default O3 optimization passes and opt -passes='default<O3>' on the same IR.

Specifically, I have a ModulePassManager initialized as in Using the New Pass Manager — LLVM 16.0.0git documentation with

MPM = PB.buildPerModuleDefaultPipeline(llvm::PassBuilder::OptimizationLevel::O3);

that I apply to an IR module with MPM.run(module, MAM).

However, when I output the same module before optimization with

module->print(llvm::errs(), nullptr, false, true);

and run opt -passes='default<O3>' -S on the output, I get a different result - despite (from a cursory read of the opt source) using the same default pipeline without modifications!

Is this expected? Does opt modify the default pipelines in some way I haven’t caught, or am I doing something wrong in applying the default pipeline? I do see some IR changes from my MPM.run call, so the pipeline is having some effect, just not the same effect as running through opt.

2

For anyone else who encounters this problem: opt does a few things differently that seem to matter:

  • It initializes its PassBuilder with an explicit TargetMachine and PipelineTuningOptions with vectorization enabled
  • It sets its TargetMachine’s CodeGenOpt::Level to Aggressive
  • It explicitly adds an alias analysis pass to its FunctionAnalysisManager

With these changes in place, I generate IR that appears to be identical to opt’s output with the default O3 pipeline.

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