[Mlir-commits] [mlir] [mlir][Vector] Add `vector.shuffle` tree transformation (PR #145740)

[James Newling]

================
@@ -0,0 +1,724 @@
+//===- VectorShuffleTreeBuilder.cpp ----- Vector shuffle tree builder -----===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements pattern rewrites to lower sequences of
+// `vector.to_elements` and `vector.from_elements` operations into a tree of
+// `vector.shuffle` operations.
+//
+//===----------------------------------------------------------------------===//
+
+#include "mlir/Dialect/Vector/IR/VectorOps.h"
+#include "mlir/Dialect/Vector/Transforms/LoweringPatterns.h"
+#include "mlir/Dialect/Vector/Transforms/Passes.h"
+#include "mlir/Rewrite/FrozenRewritePatternSet.h"
+#include "mlir/Transforms/GreedyPatternRewriteDriver.h"
+#include "llvm/ADT/DenseMap.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/Support/MathExtras.h"
+#include "llvm/Support/raw_ostream.h"
+
+namespace mlir {
+namespace vector {
+
+#define GEN_PASS_DEF_LOWERVECTORTOFROMELEMENTSTOSHUFFLETREE
+#include "mlir/Dialect/Vector/Transforms/Passes.h.inc"
+
+} // namespace vector
+} // namespace mlir
+
+#define DEBUG_TYPE "lower-vector-to-from-elements-to-shuffle-tree"
+
+using namespace mlir;
+using namespace mlir::vector;
+
+namespace {
+
+// Indentation unit for debug output formatting.
+constexpr unsigned kIndScale = 2;
+
+/// Represents a closed interval of elements (e.g., [0, 7] = 8 elements).
+using Interval = std::pair<unsigned, unsigned>;
+// Sentinel value for uninitialized intervals.
+constexpr unsigned kMaxUnsigned = std::numeric_limits<unsigned>::max();
+
+/// The VectorShuffleTreeBuilder builds a balanced binary tree of
+/// `vector.shuffle` operations from one or more `vector.to_elements`
+/// operations feeding a single `vector.from_elements` operation.
+///
+/// The implementation generates hardware-agnostic `vector.shuffle` operations
+/// that minimize both the number of shuffle operations and the length of
+/// intermediate vectors (to the extent possible). The tree has the
+/// following properties:
+///
+///   1. Vectors are shuffled in pairs by order of appearance in
+///      the `vector.from_elements` operand list.
+///   2. Each vector at each level is used only once.
+///   3. The number of levels in the tree is:
+///        ceil(log2(# `vector.to_elements` ops)).
+///   4. Vectors at each level of the tree have the same vector length.
+///   5. Vector positions that do not need to be shuffled are represented with
+///      poison in the shuffle mask.
+///
+/// Examples #1: Concatenation of 3x vector<4xf32> to vector<12xf32>:
+///
+///   %0:4 = vector.to_elements %a : vector<4xf32>
+///   %1:4 = vector.to_elements %b : vector<4xf32>
+///   %2:4 = vector.to_elements %c : vector<4xf32>
+///   %3 = vector.from_elements %0#0, %0#1, %0#2, %0#3, %1#0, %1#1,
+///                             %1#2, %1#3, %2#0, %2#1, %2#2, %2#3
+///                               : vector<12xf32>
+///   =>
+///
+///   %shuffle0 = vector.shuffle %a, %b [0, 1, 2, 3, 4, 5, 6, 7]
+///     : vector<4xf32>, vector<4xf32>
+///   %shuffle1 = vector.shuffle %c, %c [0, 1, 2, 3, -1, -1, -1, -1]
+///     : vector<4xf32>, vector<4xf32>
+///   %result = vector.shuffle %shuffle0, %shuffle1 [0, 1, 2, 3, 4, 5,
+///                                                  6, 7, 8, 9, 10, 11]
+///     : vector<8xf32>, vector<8xf32>
+///
+///   Comments:
+///     * The shuffle tree has two levels:
+///         - Level 1 = (%shuffle0, %shuffle1)
+///         - Level 2 = (%result)
+///     * `%a` and `%b` are shuffled first because they appear first in the
+///       `vector.from_elements` operand list (`%0#0` and `%1#0`).
+///     * `%c` is shuffled with itself because the number of
+///       `vector.from_elements` operands is odd.
+///     * The vector length for the first and second levels are 8 and 16,
+///       respectively.
+///     * `%shuffle1` uses poison values to match the vector length of its
+///       tree level (8).
+///
+///
+/// Example #2: Arbitrary shuffling of 3x vector<5xf32> to vector<9xf32>:
+///
+///   %0:5 = vector.to_elements %a : vector<5xf32>
+///   %1:5 = vector.to_elements %b : vector<5xf32>
+///   %2:5 = vector.to_elements %c : vector<5xf32>
+///   %3 = vector.from_elements %2#2, %1#1, %0#1, %0#1, %1#2,
+///                             %2#2, %2#0, %1#1, %0#4 : vector<9xf32>
+///   =>
+///
+///   %shuffle0 = vector.shuffle %[[C]], %[[B]] [2, 6, -1, -1, 7, 2, 0, 6]
+///     : vector<5xf32>, vector<5xf32>
+///   %shuffle1 = vector.shuffle %[[A]], %[[A]] [1, 1, -1, -1, -1, -1, 4, -1]
+///     : vector<5xf32>, vector<5xf32>
+///   %result = vector.shuffle %shuffle0, %shuffle1 [0, 1, 8, 9, 4, 5, 6, 7, 14]
+///     : vector<8xf32>, vector<8xf32>
+///
+///   Comments:
+///     * `%c` and `%b` are shuffled first because they appear first in the
+///       `vector.from_elements` operand list (`%2#2` and `%1#1`).
+///     * `%a` is shuffled with itself because the number of
+///       `vector.from_elements` operands is odd.
+///     * The vector length for the first and second levels are 8 and 9,
+///       respectively.
+///     * `%shuffle0` uses poison values to mark unused vector positions and
+///       match the vector length of its tree level (8).
+///
+/// TODO: Implement mask compression to reduce the number of intermediate poison
+/// values.
+class VectorShuffleTreeBuilder {
+public:
+  VectorShuffleTreeBuilder() = delete;
+  VectorShuffleTreeBuilder(FromElementsOp fromElemOp,
+                           ArrayRef<ToElementsOp> toElemDefs);
+
+  /// Analyze the input `vector.to_elements` + `vector.from_elements` sequence
+  /// and compute the shuffle tree configuration. This method does not generate
+  /// any IR.
+  LogicalResult computeShuffleTree();
+
+  /// Materialize the shuffle tree configuration computed by
+  /// `computeShuffleTree` in the IR.
+  Value generateShuffleTree(PatternRewriter &rewriter);
+
+private:
+  // IR input information.
+  FromElementsOp fromElemsOp;
+  SmallVector<ToElementsOp> toElemsDefs;
+
+  // Shuffle tree configuration.
+  unsigned numLevels;
+  SmallVector<unsigned> vectorSizePerLevel;
+  /// Holds the range of positions each vector in the tree contributes to the
+  /// final output vector.
+  SmallVector<SmallVector<Interval>> intervalsPerLevel;
+
+  // Utility methods to compute the shuffle tree configuration.
+  void computeShuffleTreeIntervals();
+  void computeShuffleTreeVectorSizes();
+
+  /// Dump the shuffle tree configuration.
+  void dump();
+};
+
+VectorShuffleTreeBuilder::VectorShuffleTreeBuilder(
+    FromElementsOp fromElemOp, ArrayRef<ToElementsOp> toElemDefs)
+    : fromElemsOp(fromElemOp), toElemsDefs(toElemDefs) {
+  assert(fromElemsOp && "from_elements op is required");
+  assert(!toElemsDefs.empty() && "At least one to_elements op is required");
+}
+
+/// Duplicate the last operation, value or interval if the total number of them
+/// is odd. This is useful to simplify the shuffle tree algorithm given that
+/// vectors are shuffled in pairs and duplication would lead to the last shuffle
+/// to have a single (duplicated) input vector.
+template <typename T>
+static void duplicateLastIfOdd(SmallVectorImpl<T> &values) {
+  if (values.size() % 2 != 0)
+    values.push_back(values.back());
+}
+
+// ===---------------------------------------------------------------------===//
+// Shuffle Tree Analysis Utilities.
+// ===---------------------------------------------------------------------===//
+
+/// Compute the intervals for all the vectors in the shuffle tree. The interval
+/// of a vector is the range of positions that vector contributes to the final
----------------
newling wrote:

```suggestion
/// of a vector is the range of positions that the vector contributes to the final
```

https://github.com/llvm/llvm-project/pull/145740