[PATCH] D108699: [LAA] Analyze pointers forked by a select

[Florian Hahn via Phabricator via llvm-commits]

fhahn added a comment.

In D108699#3150041 <https://reviews.llvm.org/D108699#3150041>, @fhahn wrote:

> I just realized how this may be a bit similar to how we handle pointers that are phi nodes. Currently those are handled by adding accesses for both incoming values (see D109381 <https://reviews.llvm.org/D109381>). Unfortunately the same approach cannot be directly used for selects, because we need to create 2 pointers that do not exist in the IR.
>
> But if `MemAccessInfo`/ would also carry the pointer SCEV directly, I think it would be possible to avoid adding another dimension to `RuntimePointerChecking::PointerInfo`. Instead we would add 2 PointerInfo entries with separate translated pointer SCEVs. I put up a rough sketch of what this may look like in D114480 <https://reviews.llvm.org/D114480>, D114479 <https://reviews.llvm.org/D114479> to see how this may look like

I also put up a sketch of a stripped down variant that only supports runtime check generation by adding multiple `PointerInfos` with different associated pointer SCEVs: D114487 <https://reviews.llvm.org/D114487>.

The variant that also extends `MemAccessInfo` should also be able to determine that certain loops are safe without runtime checks, e.g. like the one below I think:

  %s1 = type { [32000 x float], [32000 x float], [32000 x float] }
  define dso_local void @foo(%s1 * nocapture readonly %Base, i32* nocapture readonly %Preds) {
  entry:
    br label %for.body
  
  for.cond.cleanup:
    ret void
  
  for.body:
    %indvars.iv = phi i64 [ 0, %entry ], [ %indvars.iv.next, %for.body ]
    %arrayidx = getelementptr inbounds i32, i32* %Preds, i64 %indvars.iv
    %0 = load i32, i32* %arrayidx, align 4
    %cmp1.not = icmp eq i32 %0, 0
    %gep.1 = getelementptr inbounds %s1, %s1* %Base, i64 0, i32 1, i32 0
    %gep.2 = getelementptr inbounds %s1, %s1* %Base, i64 0, i32 2, i32 0
    %spec.select = select i1 %cmp1.not, float* %gep.1, float* %gep.2
    %.sink.in = getelementptr inbounds float, float* %spec.select, i64 %indvars.iv
    %.sink = load float, float* %.sink.in, align 4
    %1= getelementptr inbounds %s1, %s1 * %Base, i64 0, i32 0, i64 %indvars.iv
    store float %.sink, float* %1, align 4
    %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
    %exitcond.not = icmp eq i64 %indvars.iv.next, 100
    br i1 %exitcond.not, label %for.cond.cleanup, label %for.body
  }


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