[LLVMdev] SCEV getMulExpr() not propagating Wrap flags

[Renato Golin]

Hi folks,

I'm trying to analyse this piece of IR:

for.body:                                         ; preds = %for.body,
%entry
  %indvars.iv = phi i64 [ 0, %entry ], [ %indvars.iv.next, %for.body ]
  %0 = shl nsw i64 %indvars.iv, 1
  %arrayidx = getelementptr inbounds i32* %b, i64 %0
  %1 = load i32* %arrayidx, align 4, !tbaa !1
  %add = add nsw i32 %1, %I
  %arrayidx3 = getelementptr inbounds i32* %a, i64 %0
  store i32 %add, i32* %arrayidx3, align 4, !tbaa !1
  %2 = or i64 %0, 1
  %arrayidx7 = getelementptr inbounds i32* %b, i64 %2
  %3 = load i32* %arrayidx7, align 4, !tbaa !1
  %add8 = add nsw i32 %3, %I
  %arrayidx12 = getelementptr inbounds i32* %a, i64 %2
  store i32 %add8, i32* %arrayidx12, align 4, !tbaa !1
  %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
  %exitcond = icmp eq i64 %indvars.iv.next, 512
  br i1 %exitcond, label %for.end, label %for.body

And, when going through the GEP:

  %0 = shl nsw i64 %indvars.iv, 1
  %arrayidx = getelementptr inbounds i32* %b, i64 %0

ScalarEvolution::createNodeForGEP() is losing the SCEV::FlagNSW, and I
believe it's because of this piece of code in getMulExpr():

      // Build the new addrec. Propagate the NUW and NSW flags if both the
      // outer mul and the inner addrec are guaranteed to have no overflow.
      //
      // No self-wrap cannot be guaranteed after changing the step size, but
      // will be inferred if either NUW or NSW is true.
      Flags = AddRec->getNoWrapFlags(clearFlags(Flags, SCEV::FlagNW));
      const SCEV *NewRec = getAddRecExpr(NewOps, AddRecLoop, Flags);

      // If all of the other operands were loop invariant, we are done.
      if (Ops.size() == 1) return NewRec;

I understand that you can't assume it won't wrap around with the new
stride, but I couldn't find anything else later that would re-assign
NSW/NUW. Maybe I need to add this code myself? Possibly in the GEP code, if
I can detect the range will be safe, even for large strides?

cheers,
--renato
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