[llvm-dev] llvm combines "ADD frameindex, constant" to OR

[Kazushi Marukawa via llvm-dev]

Hi all,

I've been working on a backend of our architecture and noticed llvm performs
following combining although one of operands is FrameIndex.

    Combining: t114: i64 = add FrameIndex:i64<0>, Constant:i64<56>
    Creating new node: t121: i64 = or FrameIndex:i64<0>, Constant:i64<56>
     ... into: t121: i64 = or FrameIndex:i64<0>, Constant:i64<56>

This caused problem if frame pointer points 0x60000038 at run time.


I checked DAGCombiner::visitADD.  It folds ADD to OR by following code
without considering about FrameIndex.  This haveNoCommonBitsSet says
it's safe since FrameIndex(0) is 0.

      // fold (a+b) -> (a|b) iff a and b share no bits.
      if ((!LegalOperations || TLI.isOperationLegal(ISD::OR, VT)) &&
          DAG.haveNoCommonBitsSet(N0, N1))
        return DAG.getNode(ISD::OR, DL, VT, N0, N1);

I checked visitADD more and find that it performs some kind of undo
like bellow if the input is (+ (+ FI <const>) <const>).

        // Undo the add -> or combine to merge constant offsets from a frame index.
        if (N0.getOpcode() == ISD::OR &&
            isa<FrameIndexSDNode>(N0.getOperand(0)) &&
            isa<ConstantSDNode>(N0.getOperand(1)) &&
            DAG.haveNoCommonBitsSet(N0.getOperand(0), N0.getOperand(1))) {
          SDValue Add0 = DAG.getNode(ISD::ADD, DL, VT, N1, N0.getOperand(1));
          return DAG.getNode(ISD::ADD, DL, VT, N0.getOperand(0), Add0);
        }

I think this code may work fine on an architecture using FI + A + B.

However, our architecture seems to use only FI + A and this ADD is
converted to OR permanently.  As a result, generated code use OR and
it causes run time error depends on the address of frame pointer.

I also checked visitOR, but I cannot find any undoing there.


My question is:

  1. Is there any way to control this to avoid OR folding,
     like TargetLowering::preferNotCombineToOrFrameIndex?
     (or should we add new?)
  2. How to optimize FrameIndex?  I desire DAGCombiner use alignment
     information on data, stack frame, or something.

Best Regards,
-- Kazushi