[LLVMdev] Question about shouldMergeGEPs in InstructionCombining

[Francois Pichet]

Hello

I am not sure I understand the logic for merging GEPs in
InstructionCombining.cpp:

static bool shouldMergeGEPs(GEPOperator &GEP, GEPOperator &Src) {
  // If this GEP has only 0 indices, it is the same pointer as
  // Src. If Src is not a trivial GEP too, don't combine
  // the indices.
  if (GEP.hasAllZeroIndices() && !Src.hasAllZeroIndices() &&
      !Src.hasOneUse())
    return false;
  return true;
}

I have a case where
GEP: %arrayidx7 = getelementptr inbounds i32* %arrayidx, i32 %shl6
Src: %arrayidx = getelementptr inbounds [4096 x i32]* @phasor_4096, i32 0,
i32 %shl2

GEP.hasAllZeroIndices() will return false and the merge will occur
Why do we want to combine these 2 getelementptr?

On my out of tree target, combining these 2 GetElementPtr create a
performance regression because since GEP is in a loop (Src is out of loop),
GEP will lower to a more complicated address for a subsequent load. (the
complicated address needs to be calculated over and over in the loop)

Thanks.
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