[PATCH] D79976: [LV] Handle Fold-Tail of loops with vectorizarion factor (VF) equal to 1

[Ayal Zaks via Phabricator via llvm-commits]

Ayal added inline comments.


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Comment at: llvm/test/Transforms/LoopVectorize/tail-folding-vectorization-factor-1-scalar.ll:15
+; CHECK-NEXT:    [[INDUCTION2:%.*]] = add i64 [[INDEX]], 2
+; CHECK-NEXT:    [[INDUCTION3:%.*]] = add i64 [[INDEX]], 3
+; CHECK:         [[INDEX_NEXT]] = add i64 [[INDEX]], 4
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continue CHECK-NEXTing the scalar `icmp ule`'s


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Comment at: llvm/test/Transforms/LoopVectorize/tail-folding-vectorization-factor-1-scalar.ll:17
+; CHECK:         [[INDEX_NEXT]] = add i64 [[INDEX]], 4
+; CHECK-NEXT:    [[TMP4:%.*]] = icmp eq i64 [[INDEX_NEXT]], 16
+; CHECK-NEXT:    br i1 [[TMP4]], label [[MIDDLE_BLOCK:%.*]], label [[VECTOR_BODY]], !llvm.loop !0
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bmahjour wrote:
> How is it that the original loop executes 15 iterations, but the vector loop iterates 16? It seems the minimum iteration count check branch at the top should branch to the scalar loop instead of vector.ph.
(Thanks for asking, reminded to check above and below that fold-tail emits the desired scalar `icmp ule`'s, which are the focus of this patch.)

Fold-tail is responsible for rounding-up the trip count from 15 to 16, see https://reviews.llvm.org/D50480.
Regarding minimum iteration count check branch, fold-tail is also responsible in general for branching directly to vector.ph w/o an "if (trip-count < VF*UF)", which in this case is known to be false anyhow.


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Comment at: llvm/test/Transforms/LoopVectorize/tail-folding-vectorization-factor-1-vector.ll:23
+; CHECK-NEXT:    [[VEC_IV5:%.*]] = add i64 [[INDEX]], 2
+; CHECK-NEXT:    [[VEC_IV6:%.*]] = add i64 [[INDEX]], 3
+; CHECK:         [[INDEX_NEXT]] = add i64 [[INDEX]], 4
----------------
continue CHECK-NEXTing the scalar icmp ule's


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D79976/new/

https://reviews.llvm.org/D79976