[llvm-dev] Potential infinite loop in MemorySSAUpdater

Godala, Bhargav-reddy via llvm-dev llvm-dev at lists.llvm.org
Sat Sep 23 09:55:40 PDT 2017

With regards
Bhargav Reddy Godala
Software Engineer 2
Bangalore, India
E-mail: Bhargav-reddy.Godala at amd.com<mailto:Bhargav-reddy.Godala at amd.com> Ext 30678

On 23-Sep-2017, at 9:27 PM, Daniel Berlin <dberlin at dberlin.org<mailto:dberlin at dberlin.org>> wrote:

On Sat, Sep 23, 2017 at 8:38 AM, Godala, Bhargav-reddy via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:

Can some one explain the intended behaviour of following loop in void MemorySSAUpdater::insertDef(MemoryDef *MD, bool RenameUses) function.

  while (!FixupList.empty()) {
    unsigned StartingPHISize = InsertedPHIs.size();
    // Put any new phis on the fixup list, and process them
    FixupList.append(InsertedPHIs.end() - StartingPHISize, InsertedPHIs.end());

With the latest code on trunk compilation of perlbench SPEC CPU 2017 INT benchmark with “-O3 -inline-threshold=1000 and -enable-gvn-hoist” options is looping infinitely on the above loop.

Above loop never terminates unless elements from InsertedPHIs are removed as and when they are processed.

Yes, the loop is slightly off.

The intention is to process any new phis added by fixupdefs.
However, it really should be InsertedPHIs.start() + StartingPHISize.

Even in that case it still is infinite, as there is no call to clear already processed elements(MemoryPhi) in InsertedPHIs. fixupDefs function only inserts new elements but not remove any that are processed.
That will insert only the phis that fixupdefs added, and it will terminate as soon as fixupdefs stops adding phis.

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