llvm dev - Sep 2017 - PEI::replaceFrameIndices() endless loop

Hi,

My backend  (based on version 3.8) was hanging in an infinite loop in the
Prolog/Epilog Inserter.
After investigation, it appears that it was looping in the first level loop of
the PEI::replaceFrameIndices() method: processing the second instruction of the
block again and again...
This loop never exits because the iterator is 'skipped backward' under
some condition in the middle of the loop (see yellow highlighted line).
I understand the rationale explained in the first comment, but I don't
understand how the condition ("not the first instruction") is related
to this rationale.
I have commented out the iterator decrement and then my back-end behave
correctly.
The handling of this loop iterator is quite complex: skipping backward or
forward in not so clearly raised conditions.
Could someone review the logic of this iterator to confirm its correctness? Or
is it a bug?

      // Some instructions (e.g. inline asm instructions) can have
      // multiple frame indices and/or cause eliminateFrameIndex
      // to insert more than one instruction. We need the register
      // scavenger to go through all of these instructions so that
      // it can update its register information. We keep the
      // iterator at the point before insertion so that we can
      // revisit them in full.
      bool AtBeginning = (I == BB->begin());
      if (!AtBeginning) --I;

      // If this instruction has a FrameIndex operand, we need to
      // use that target machine register info object to eliminate
      // it.
      TRI.eliminateFrameIndex(MI, SPAdj, i,
                              FrameIndexVirtualScavenging ?  nullptr : RS);

Regards,              Dominique Torette.



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Dominique Torette
System Architect
Rue des Chasseurs Ardennais - Liège Science Park - B-4031 Angleur
Tel: +32 (0) 4 361 81 11 - Fax: +32 (0) 4 361 81 20
www.spacebel.be<http://www.spacebel.be/>



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