[llvm-dev] ScalarEvolution in a ModulePass

[Hal Finkel via llvm-dev]

Hi, TB,

Just to be clear, are you using the new pass manager or the old one? You
can't do this from the old pass manager - this was one of the
motivations to create the new pass manager.

 -Hal


On 08/11/2018 09:16 AM, TB Schardl via llvm-dev wrote:
> Hey LLVMDev,
>
> I'm working on a ModulePass that uses ScalarEvolution along with
> several other analyses.  After some debugging, it looks to me like
> ScalarEvolutionWrapperPass does not handle memory correctly for this
> case.  Here's my current understanding of the problem.
>
> ScalarEvolutionWrapperPass maintains a unique_ptr to a
> ScalarEvolution.  Calling getSE() dereferences this pointer. 
> Meanwhile runOnFunction() resets the pointer to point to a new
> ScalarEvolution.  As a result, runOnFunction destructs and frees any
> ScalarEvolution the unique_ptr pointed to before.
>
> The ModulePass I'm working on uses ScalarEvolution and several other
> analysis FunctionPasses, including DominatorTree, LoopInfo,
> OptimizationRemarkEmitter, and a custom analysis pass I'm working on,
> which resembles LoopInfo.  To run ScalarEvolution and these other
> analysis FunctionPasses in a ModulePass, the ModulePass creates
> lambdas to get the analysis for a particular function, e.g.,
>
>     auto GetSE = [this](Function &F) -> ScalarEvolution & {
>       return this->getAnalysis<ScalarEvolutionWrapperPass>(F).getSE();
>     };
>
>
> Later, when the ModulePass examines a particular Function, it calls
> the appropriate lambda to get the analysis for that Function.
>
> The problem seems to arise when lambdas for other analysis
> FunctionPasses run after calling the GetSE lambda, e.g., when
> evaluating a statement like this:
>
>     for (Function &F : M)
>       Changed |= MyPassImpl(F, GetSE(F), GetDT(F), GetLI(F),
>     GetORE(F), ...).run();
>
>
> It appears that these other analysis FunctionPasses can cause
> ScalarEvolutionWrapperPass::runOnFunction() to rerun after GetSE()
> returns a pointer to a ScalarEvolution, which changes the underlying
> ScalarEvolution object that the wrapper pass points to.  As a result,
> the pointer originally obtained from GetSE() points to invalid memory,
> and chaos ensues.  Running valgrind on the "opt -mypass" indicates
> that the ScalarEvolution object used by MyPassImpl points to freed memory.
>
> I've been able to work around this problem, but I figured I should
> raise the issue and see if you might have more insights into this
> problem.  For example, I'm not totally sure why the other analysis
> FunctionPasses cause ScalarEvolutionWrapperPass::runOnFunction() to
> rerun.  I'm also concerned about other analysis FunctionPasses, that
> might suffer from the same problem.  MemorySSAWrapperPass, for
> example, uses a unique_ptr similarly to ScalarEvolution.
>
> Cheers,
> TB
>
>
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-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory

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