[mli][vector] canonicalize vector.from_elements from ascending extracts (#139819)

Example:
```mlir
%0 = vector.extract %source[0, 0] : i8 from vector<1x2xi8>
%1 = vector.extract %source[0, 1] : i8 from vector<1x2xi8>
%2 = vector.from_elements %0, %1 : vector<2xi8>
```

becomes
```mlir
%2 = vector.shape_cast %source : vector<1x2xi8> to vector<2xi8>
```

It was decided that we should spill canonicalization tests into new
files (see
[discussion](#135096 (review)))
In view of this I added the new tests to a new file specifically for
canonicalization of from_elements. To be consistent in the location of
the tests, I moved existing tests `extract_scalar_from_from_element`,
`extract_1d_from_from_elements`, `extract_2d_from_from_elements` and
`from_elements_to_splat` from `canonicalize.mlir` to
`canonicalze/vector-from-elements.mlir`. In addition to moving I changed
the LIT variables to all be upper-case for consistency.

Author: newling

mlir/lib/Dialect/Vector/IR/VectorOps.cpp

@@ -33,6 +33,7 @@
 #include "mlir/IR/OpImplementation.h"
 #include "mlir/IR/PatternMatch.h"
 #include "mlir/IR/TypeUtilities.h"
+#include "mlir/IR/ValueRange.h"
 #include "mlir/Interfaces/SubsetOpInterface.h"
 #include "mlir/Interfaces/ValueBoundsOpInterface.h"
 #include "mlir/Support/LLVM.h"
@@ -2387,9 +2388,129 @@ static LogicalResult rewriteFromElementsAsSplat(FromElementsOp fromElementsOp,
   return success();
 }
 
+/// Rewrite from_elements on multiple scalar extracts as a shape_cast
+/// on a single extract. Example:
+///   %0 = vector.extract %source[0, 0] : i8 from vector<2x2xi8>
+///   %1 = vector.extract %source[0, 1] : i8 from vector<2x2xi8>
+///   %2 = vector.from_elements %0, %1 : vector<2xi8>
+///
+/// becomes
+///   %1 = vector.extract %source[0] : vector<1x2xi8> from vector<2x2xi8>
+///   %2 = vector.shape_cast %1 : vector<1x2xi8> to vector<2xi8>
+///
+/// The requirements for this to be valid are
+///
+///   i) The elements are extracted from the same vector (%source).
+///
+///  ii) The elements form a suffix of %source. Specifically, the number
+///      of elements is the same as the product of the last N dimension sizes
+///      of %source, for some N.
+///
+/// iii) The elements are extracted contiguously in ascending order.
+
+class FromElementsToShapeCast : public OpRewritePattern<FromElementsOp> {
+
+  using OpRewritePattern::OpRewritePattern;
+
+  LogicalResult matchAndRewrite(FromElementsOp fromElements,
+                                PatternRewriter &rewriter) const override {
+
+    // Handled by `rewriteFromElementsAsSplat`
+    if (fromElements.getType().getNumElements() == 1)
+      return failure();
+
+    // The common source that all elements are extracted from, if one exists.
+    TypedValue<VectorType> source;
+    // The position of the combined extract operation, if one is created.
+    ArrayRef<int64_t> combinedPosition;
+    // The expected index of extraction of the current element in the loop, if
+    // elements are extracted contiguously in ascending order.
+    SmallVector<int64_t> expectedPosition;
+
+    for (auto [insertIndex, element] :
+         llvm::enumerate(fromElements.getElements())) {
+
+      // Check that the element is from a vector.extract operation.
+      auto extractOp =
+          dyn_cast_if_present<vector::ExtractOp>(element.getDefiningOp());
+      if (!extractOp) {
+        return rewriter.notifyMatchFailure(fromElements,
+                                           "element not from vector.extract");
+      }
+
+      // Check condition (i) by checking that all elements have the same source
+      // as the first element.
+      if (insertIndex == 0) {
+        source = extractOp.getVector();
+      } else if (extractOp.getVector() != source) {
+        return rewriter.notifyMatchFailure(fromElements,
+                                           "element from different vector");
+      }
+
+      ArrayRef<int64_t> position = extractOp.getStaticPosition();
+      int64_t rank = position.size();
+      assert(rank == source.getType().getRank() &&
+             "scalar extract must have full rank position");
+
+      // Check condition (ii) by checking that the position that the first
+      // element is extracted from has sufficient trailing 0s. For example, in
+      //
+      //   %elm0 = vector.extract %source[1, 0, 0] : i8 from vector<2x3x4xi8>
+      //   [...]
+      //   %elms = vector.from_elements %elm0, [...] : vector<12xi8>
+      //
+      // The 2 trailing 0s in the position of extraction of %elm0 cover 3*4 = 12
+      // elements, which is the number of elements of %n, so this is valid.
+      if (insertIndex == 0) {
+        const int64_t numElms = fromElements.getType().getNumElements();
+        int64_t numSuffixElms = 1;
+        int64_t index = rank;
+        while (index > 0 && position[index - 1] == 0 &&
+               numSuffixElms < numElms) {
+          numSuffixElms *= source.getType().getDimSize(index - 1);
+          --index;
+        }
+        if (numSuffixElms != numElms) {
+          return rewriter.notifyMatchFailure(
+              fromElements, "elements do not form a suffix of source");
+        }
+        expectedPosition = llvm::to_vector(position);
+        combinedPosition = position.drop_back(rank - index);
+      }
+
+      // Check condition (iii).
+      else if (expectedPosition != position) {
+        return rewriter.notifyMatchFailure(
+            fromElements, "elements not in ascending order (static order)");
+      }
+      increment(expectedPosition, source.getType().getShape());
+    }
+
+    auto extracted = rewriter.createOrFold<vector::ExtractOp>(
+        fromElements.getLoc(), source, combinedPosition);
+
+    rewriter.replaceOpWithNewOp<vector::ShapeCastOp>(
+        fromElements, fromElements.getType(), extracted);
+
+    return success();
+  }
+
+  /// Increments n-D `indices` by 1 starting from the innermost dimension.
+  static void increment(MutableArrayRef<int64_t> indices,
+                        ArrayRef<int64_t> shape) {
+    for (int dim : llvm::reverse(llvm::seq<int>(0, indices.size()))) {
+      indices[dim] += 1;
+      if (indices[dim] < shape[dim])
+        break;
+      indices[dim] = 0;
+    }
+  }
+};
+
 void FromElementsOp::getCanonicalizationPatterns(RewritePatternSet &results,
                                                  MLIRContext *context) {
   results.add(rewriteFromElementsAsSplat);
+  results.add<FromElementsToShapeCast>(context);
 }
 
 //===----------------------------------------------------------------------===//

mlir/test/Dialect/Vector/canonicalize.mlir

@@ -2943,75 +2943,6 @@ func.func @extract_from_0d_splat_broadcast_regression(%a: f32, %b: vector<f32>,
 
 // -----
 
-// CHECK-LABEL: func @extract_scalar_from_from_elements(
-//  CHECK-SAME:     %[[a:.*]]: f32, %[[b:.*]]: f32)
-func.func @extract_scalar_from_from_elements(%a: f32, %b: f32) -> (f32, f32, f32, f32, f32, f32, f32) {
-  // Extract from 0D.
-  %0 = vector.from_elements %a : vector<f32>
-  %1 = vector.extract %0[] : f32 from vector<f32>
-
-  // Extract from 1D.
-  %2 = vector.from_elements %a : vector<1xf32>
-  %3 = vector.extract %2[0] : f32 from vector<1xf32>
-  %4 = vector.from_elements %a, %b, %a, %a, %b : vector<5xf32>
-  %5 = vector.extract %4[4] : f32 from vector<5xf32>
-
-  // Extract from 2D.
-  %6 = vector.from_elements %a, %a, %a, %b, %b, %b : vector<2x3xf32>
-  %7 = vector.extract %6[0, 0] : f32 from vector<2x3xf32>
-  %8 = vector.extract %6[0, 1] : f32 from vector<2x3xf32>
-  %9 = vector.extract %6[1, 1] : f32 from vector<2x3xf32>
-  %10 = vector.extract %6[1, 2] : f32 from vector<2x3xf32>
-
-  // CHECK: return %[[a]], %[[a]], %[[b]], %[[a]], %[[a]], %[[b]], %[[b]]
-  return %1, %3, %5, %7, %8, %9, %10 : f32, f32, f32, f32, f32, f32, f32
-}
-
-// -----
-
-// CHECK-LABEL: func @extract_1d_from_from_elements(
-//  CHECK-SAME:     %[[a:.*]]: f32, %[[b:.*]]: f32)
-func.func @extract_1d_from_from_elements(%a: f32, %b: f32) -> (vector<3xf32>, vector<3xf32>) {
-  %0 = vector.from_elements %a, %a, %a, %b, %b, %b : vector<2x3xf32>
-  // CHECK: %[[splat1:.*]] = vector.splat %[[a]] : vector<3xf32>
-  %1 = vector.extract %0[0] : vector<3xf32> from vector<2x3xf32>
-  // CHECK: %[[splat2:.*]] = vector.splat %[[b]] : vector<3xf32>
-  %2 = vector.extract %0[1] : vector<3xf32> from vector<2x3xf32>
-  // CHECK: return %[[splat1]], %[[splat2]]
-  return %1, %2 : vector<3xf32>, vector<3xf32>
-}
-
-// -----
-
-// CHECK-LABEL: func @extract_2d_from_from_elements(
-//  CHECK-SAME:     %[[a:.*]]: f32, %[[b:.*]]: f32)
-func.func @extract_2d_from_from_elements(%a: f32, %b: f32) -> (vector<2x2xf32>, vector<2x2xf32>) {
-  %0 = vector.from_elements %a, %a, %a, %b, %b, %b, %b, %a, %b, %a, %a, %b : vector<3x2x2xf32>
-  // CHECK: %[[splat1:.*]] = vector.from_elements %[[a]], %[[a]], %[[a]], %[[b]] : vector<2x2xf32>
-  %1 = vector.extract %0[0] : vector<2x2xf32> from vector<3x2x2xf32>
-  // CHECK: %[[splat2:.*]] = vector.from_elements %[[b]], %[[b]], %[[b]], %[[a]] : vector<2x2xf32>
-  %2 = vector.extract %0[1] : vector<2x2xf32> from vector<3x2x2xf32>
-  // CHECK: return %[[splat1]], %[[splat2]]
-  return %1, %2 : vector<2x2xf32>, vector<2x2xf32>
-}
-
-// -----
-
-// CHECK-LABEL: func @from_elements_to_splat(
-//  CHECK-SAME:     %[[a:.*]]: f32, %[[b:.*]]: f32)
-func.func @from_elements_to_splat(%a: f32, %b: f32) -> (vector<2x3xf32>, vector<2x3xf32>, vector<f32>) {
-  // CHECK: %[[splat:.*]] = vector.splat %[[a]] : vector<2x3xf32>
-  %0 = vector.from_elements %a, %a, %a, %a, %a, %a : vector<2x3xf32>
-  // CHECK: %[[from_el:.*]] = vector.from_elements {{.*}} : vector<2x3xf32>
-  %1 = vector.from_elements %a, %a, %a, %a, %b, %a : vector<2x3xf32>
-  // CHECK: %[[splat2:.*]] = vector.splat %[[a]] : vector<f32>
-  %2 = vector.from_elements %a : vector<f32>
-  // CHECK: return %[[splat]], %[[from_el]], %[[splat2]]
-  return %0, %1, %2 : vector<2x3xf32>, vector<2x3xf32>, vector<f32>
-}
-
-// -----
-
 // CHECK-LABEL: func @vector_insert_const_regression(
 //       CHECK:   llvm.mlir.undef
 //       CHECK:   vector.insert