[PATCH] D42447: [LV][VPlan] Detect outer loops for explicit vectorization.

[Diego Caballero via Phabricator via llvm-commits]

dcaballe created this revision.
dcaballe added reviewers: hfinkel, mkuper, rengolin, fhahn, aemerson, mssimpso.
Herald added a subscriber: bollu.

This is the patch #2 from the Patch Series #1 to introduce outer loop vectorization support in LV using the VPlan infrastructure.
RFC: http://lists.llvm.org/pipermail/llvm-dev/2017-December/119523.html
Patch #1: https://reviews.llvm.org/D40874

This patch introduces the basic infrastructure to detect, legality check and process outer loops annotated with hints for explicit vectorization:

1. Outer loop detection: only outer loops annotated with explicit vectorization hints, including the vector length, are collected for outer loop vectorization. This includes outer loops annotated with #pragma omp simd simdlen(#) or #pragma clang vectorize(enable) vectorize_width(#)*.

2. Outer loop legality check: only a restricted subset of simple outer loops are considered legal at this point. This subset includes outer loops that only contain uniform inner loops and uniform non-backedge branches. The uniformity property is also highly conservative (loop invariance) and will be relaxed in the future to support more complex cases.

3. Outer loop processing: legal outer loops are processed in a new vectorization path that will build the VPlan infrastructure upfront. We denote it as VPlan-native vectorization path. This new path is integrated in LV but it's independent of the inner loop vectorization path. We followed this approach to prevent the instability of the current inner loop vectorizer while reusing code and minimize divergence from the existing infrastructure. In the VPlan-native path, legal outer loops are fed into the LoopVectorizationPlanner which only prints a debug message for now. Actual vectorization will be introduced in the subsequent patches of this series.

It's important to remark that all these changes are protected under the feature flag -enable-vplan-native-path. This should make this patch NFC for the existing inner loop vectorizer.

(*) Pragma 'clang vectorize' and pragma 'omp simd' are currently implemented with the same metadata (llvm.loop.vectorize) even though the former has auto-vectorization semantics and the latter has explicit vectorization semantics. We temporarily abuse pragma 'clang vectorize' on outer loops to denote explicit vectorization due to the shared implementation of both pragmas. This will be fixed when the native representation for pragma 'omp simd' is introduce in LLVM (WIP).


https://reviews.llvm.org/D42447

Files:
  include/llvm/Transforms/Vectorize/LoopVectorize.h
  lib/Transforms/Vectorize/LoopVectorize.cpp
  test/Transforms/LoopVectorize/explicit_outer_detection.ll
  test/Transforms/LoopVectorize/explicit_outer_nonuniform_inner.ll
  test/Transforms/LoopVectorize/explicit_outer_uniform_diverg_branch.ll

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