[PATCH] D72048: [InstCombine] Preserve nuw on sub of geps (PR44419)

[Nikita Popov via Phabricator via llvm-commits]

nikic marked an inline comment as done.
nikic added a comment.

I think this is still not quite right for the case where there are multiple GEP indexes. For example:

  define i64 @test_inbounds_nuw_multi_index([0 x [2 x i32]]* %base, i64 %idx, i64 %idx2) {
  ; CHECK-LABEL: @test_inbounds_nuw_multi_index(
  ; CHECK-NEXT:    [[P2_IDX:%.*]] = shl nuw nsw i64 [[IDX:%.*]], 3
  ; CHECK-NEXT:    [[P2_IDX1:%.*]] = shl nuw nsw i64 [[IDX2:%.*]], 2
  ; CHECK-NEXT:    [[P2_OFFS2:%.*]] = add i64 [[P2_IDX]], [[P2_IDX1]]
  ; CHECK-NEXT:    ret i64 [[P2_OFFS2]]
  ;
    %p1 = getelementptr inbounds [0 x [2 x i32]], [0 x [2 x i32]]* %base, i64 0, i64 0, i64 0
    %p2 = getelementptr inbounds [0 x [2 x i32]], [0 x [2 x i32]]* %base, i64 0, i64 %idx, i64 %idx2
    %i1 = ptrtoint i32* %p1 to i64
    %i2 = ptrtoint i32* %p2 to i64
    %d = sub nuw i64 %i2, %i1
    ret i64 %d
  }

Let's say %idx=-1, %idx2=4, then the overall result is `8 * %idx1 + 4 * %idx2 = 8`, which is positive, even though one of the intermediate indexes is negative and as such can't use `shl nuw`. I think in this case the `nuw` can't be on any of the instructions (including also not the `add`). Right?


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  https://reviews.llvm.org/D72048/new/

https://reviews.llvm.org/D72048