When is ScopArrayInfo's isl_id.user uninitialised / NULL ? | illegal access results in segfault and crashes PolyBench.jl

[Sanjay Srivallabh Singapuram]

Hello,

I've been facing an issue with running PolyBench.jl: When "kernel_ludcmp" 's SCoP is being processed by PPCGCodeGeneration, getStmtAcesses queries for the number of dimensions of an array through a ScopArrayInfo object,

  gpu_stmt_access *getStmtAccesses(ScopStmt &Stmt) {

    gpu_stmt_access *Accesses = nullptr;

    for (MemoryAccess *Acc : Stmt) {

      auto Access = isl_alloc_type(S->getIslCtx(), struct gpu_stmt_access);

      Access->read = Acc->isRead();

      [...]

      Access->n_index = Acc->getScopArrayInfo()->getNumberOfDimensions();

      Accesses = Access;

    }

  return Accesses;

}

But, getScopArrayInfo returns NULL and crashes the program when getNumberOfDimensions attempts to access the object's Kind member (valgrind reported an "invalid read of 4 bytes" ). An assert(this) at the beginning of getNumberOfDimensions revealed the issue concretely.

const ScopArrayInfo *getScopArrayInfo() const {

  return getOriginalScopArrayInfo();

}

const ScopArrayInfo *MemoryAccess::getOriginalScopArrayInfo() const {

  isl::id ArrayId = getArrayId();

  void *User = ArrayId.get_user();

  const ScopArrayInfo *SAI = static_cast<ScopArrayInfo *>(User);

+  if( SAI==NULL )

+     dbgs() << __func__ << "SAI be NULL\n";

  return SAI; 

}

It happened that SAI had been NULL for cases where the Julia didn't crash, e.g. for other kernels in PolyBench and 85 times while processing kernel_ludcmp itself,

This seems surprising since there are asserts surrounding the value returned by getOriginalScopArrayInfo and member access (->) on the pointer returned by getScopArrayInfo, as far as I've seen.

What's even more surprising is that opt -polly-codegen-ppcg processes the file dumped by "-polly-dump-before" without issues,

opt -polly-codegen-ppcg -disable-output kernel_ludcmp-before.ll

Scop Region: %if20---%L73 | Function: japi1_kernel_ludcmp_66122 does not have permutable bands. Bailing out

Scop Region: %if20---%L73 | Function: japi1_kernel_ludcmp_66122 has empty PPCGGen->tree. Bailing out.

Scop Region: %if18---%L42 | Function: japi1_kernel_ludcmp_66122 does not have permutable bands. Bailing out

Scop Region: %if18---%L42 | Function: japi1_kernel_ludcmp_66122 has empty PPCGGen->tree. Bailing out.

I'd like to understand when and why a ScopArrayInfo object's isl_id.user would be unitialised ( set to NULL ).

Thanks,

Sanjay