Lower polygeist.subindex through memref.reinterpret_cast

This should be a (hopefully) foolproof method of performing indexing into a memref. A reintrepret_cast is inserted with a dynamic index calculated from the subindex index operand + the product of the sizes of the target type.

This has been added as a separate conversion pass instead of through the canonicalization drivers. When added as a canonicalization, the conversion may preemptively apply, resulting in sub-par IR. Nevertheless, i think it has its merits to have a polygeist op lowering pass which can be used as a fallback to convert the dialect operations, if canonicalization fails.

For now, just added support for statically shaped memrefs (enough to fix the regression on my side) but should be possible for dynamically shaped as well.

include/polygeist/Passes/Passes.h

@@ -19,6 +19,7 @@ std::unique_ptr<Pass> createParallelLowerPass();
 std::unique_ptr<Pass>
 createConvertPolygeistToLLVMPass(const LowerToLLVMOptions &options);
 std::unique_ptr<Pass> createConvertPolygeistToLLVMPass();
+std::unique_ptr<Pass> createLowerPolygeistOpsPass();
 
 } // namespace polygeist
 } // namespace mlir

include/polygeist/Passes/Passes.td

@@ -61,6 +61,12 @@ def RemoveTrivialUse : FunctionPass<"trivialuse"> {
   let constructor = "mlir::polygeist::createRemoveTrivialUsePass()";
 }
 
+def LowerPolygeistOps : FunctionPass<"lower-polygeist-ops"> {
+  let summary = "Lower polygeist ops to memref operations";
+  let constructor = "mlir::polygeist::createLowerPolygeistOpsPass()";
+  let dependentDialects = ["::mlir::memref::MemRefDialect"];
+}
+
 def ConvertPolygeistToLLVM : Pass<"convert-polygeist-to-llvm", "mlir::ModuleOp"> {
   let summary = "Convert scalar and vector operations from the Standard to the "
                 "LLVM dialect";

lib/polygeist/Ops.cpp

@@ -176,48 +176,6 @@ class SubToCast final : public OpRewritePattern<SubIndexOp> {
   }
 };
 
-// Simplify polygeist.subindex to memref.subview.
-class SubToSubView final : public OpRewritePattern<SubIndexOp> {
-public:
-  using OpRewritePattern<SubIndexOp>::OpRewritePattern;
-
-  LogicalResult matchAndRewrite(SubIndexOp op,
-                                PatternRewriter &rewriter) const override {
-    auto srcMemRefType = op.source().getType().cast<MemRefType>();
-    auto resMemRefType = op.result().getType().cast<MemRefType>();
-    auto dims = srcMemRefType.getShape().size();
-
-    // For now, restrict subview lowering to statically defined memref's
-    if (!srcMemRefType.hasStaticShape() | !resMemRefType.hasStaticShape())
-      return failure();
-
-    // For now, restrict to simple rank-reducing indexing
-    if (srcMemRefType.getShape().size() <= resMemRefType.getShape().size())
-      return failure();
-
-    // Build offset, sizes and strides
-    SmallVector<OpFoldResult> sizes(dims, rewriter.getIndexAttr(0));
-    sizes[0] = op.index();
-    SmallVector<OpFoldResult> offsets(dims);
-    for (auto dim : llvm::enumerate(srcMemRefType.getShape())) {
-      if (dim.index() == 0)
-        offsets[0] = rewriter.getIndexAttr(1);
-      else
-        offsets[dim.index()] = rewriter.getIndexAttr(dim.value());
-    }
-    SmallVector<OpFoldResult> strides(dims, rewriter.getIndexAttr(1));
-
-    // Generate the appropriate return type:
-    auto subMemRefType = MemRefType::get(srcMemRefType.getShape().drop_front(),
-                                         srcMemRefType.getElementType());
-
-    rewriter.replaceOpWithNewOp<memref::SubViewOp>(
-        op, subMemRefType, op.source(), sizes, offsets, strides);
-
-    return success();
-  }
-};
-
 // Simplify redundant dynamic subindex patterns which tries to represent
 // rank-reducing indexing:
 //   %3 = "polygeist.subindex"(%1, %arg0) : (memref<2x1000xi32>, index) ->
@@ -678,7 +636,7 @@ void SubIndexOp::getCanonicalizationPatterns(OwningRewritePatternList &results,
   results.insert<CastOfSubIndex, SubIndex2, SubToCast, SimplifySubViewUsers,
                  SimplifySubIndexUsers, SelectOfCast, SelectOfSubIndex,
                  RedundantDynSubIndex>(context);
-  // Disabled: SubToSubView
+  // Disabled:
 }
 
 /// Simplify pointer2memref(memref2pointer(x)) to cast(x)

lib/polygeist/Passes/CMakeLists.txt

@@ -10,6 +10,7 @@ add_mlir_dialect_library(MLIRPolygeistTransforms
   ParallelLower.cpp
   TrivialUse.cpp
   ConvertPolygeistToLLVM.cpp
+  LowerPolygeistOps.cpp
 
   ADDITIONAL_HEADER_DIRS
   ${MLIR_MAIN_INCLUDE_DIR}/mlir/Dialect/Affine

lib/polygeist/Passes/LowerPolygeistOps.cpp

@@ -0,0 +1,88 @@
+//===- TrivialUse.cpp - Remove trivial use instruction ---------------- -*-===//
+//
+// 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 a pass to lower gpu kernels in NVVM/gpu dialects into
+// a generic parallel for representation
+//===----------------------------------------------------------------------===//
+#include "PassDetails.h"
+
+#include "mlir/Dialect/Arithmetic/IR/Arithmetic.h"
+#include "mlir/Dialect/MemRef/IR/MemRef.h"
+#include "mlir/Dialect/StandardOps/IR/Ops.h"
+#include "mlir/Dialect/StandardOps/Transforms/Passes.h"
+#include "mlir/Rewrite/FrozenRewritePatternSet.h"
+#include "mlir/Transforms/DialectConversion.h"
+#include "polygeist/Dialect.h"
+#include "polygeist/Ops.h"
+
+using namespace mlir;
+using namespace polygeist;
+using namespace mlir::arith;
+
+namespace {
+
+struct SubIndexToReinterpretCast
+    : public OpConversionPattern<polygeist::SubIndexOp> {
+  using OpConversionPattern::OpConversionPattern;
+
+  LogicalResult
+  matchAndRewrite(polygeist::SubIndexOp op, OpAdaptor adaptor,
+                  ConversionPatternRewriter &rewriter) const override {
+    auto srcMemRefType = op.source().getType().cast<MemRefType>();
+    auto resMemRefType = op.result().getType().cast<MemRefType>();
+    auto shape = srcMemRefType.getShape();
+
+    if (!resMemRefType.hasStaticShape())
+      return failure();
+
+    int64_t innerSize = resMemRefType.getNumElements();
+    auto offset = rewriter.create<arith::MulIOp>(
+        op.getLoc(), op.index(),
+        rewriter.create<ConstantIndexOp>(op.getLoc(), innerSize));
+
+    llvm::SmallVector<OpFoldResult> sizes, strides;
+    for (auto dim : shape.drop_front()) {
+      sizes.push_back(rewriter.getIndexAttr(dim));
+      strides.push_back(rewriter.getIndexAttr(1));
+    }
+
+    rewriter.replaceOpWithNewOp<memref::ReinterpretCastOp>(
+        op, resMemRefType, op.source(), offset.getResult(), sizes, strides);
+
+    return success();
+  }
+};
+
+struct LowerPolygeistOpsPass
+    : public LowerPolygeistOpsBase<LowerPolygeistOpsPass> {
+
+  void runOnFunction() override {
+    auto op = getOperation();
+    auto ctx = op.getContext();
+    RewritePatternSet patterns(ctx);
+    patterns.insert<SubIndexToReinterpretCast>(ctx);
+
+    ConversionTarget target(*ctx);
+    target.addIllegalDialect<polygeist::PolygeistDialect>();
+    target.addLegalDialect<arith::ArithmeticDialect, mlir::StandardOpsDialect,
+                           memref::MemRefDialect>();
+
+    if (failed(applyPartialConversion(op, target, std::move(patterns))))
+      return signalPassFailure();
+  }
+};
+} // namespace
+
+namespace mlir {
+namespace polygeist {
+std::unique_ptr<Pass> createLowerPolygeistOpsPass() {
+  return std::make_unique<LowerPolygeistOpsPass>();
+}
+
+} // namespace polygeist
+} // namespace mlir

test/polygeist-opt/lower_polygeist_ops.mlir

@@ -0,0 +1,17 @@
+// RUN: polygeist-opt --lower-polygeist-ops --split-input-file %s | FileCheck %s
+
+// CHECK-LABEL:   func @main(
+// CHECK-SAME:               %[[VAL_0:.*]]: index) -> memref<30xi32> {
+// CHECK:           %[[VAL_1:.*]] = memref.alloca() : memref<30x30xi32>
+// CHECK:           %[[VAL_2:.*]] = arith.constant 30 : index
+// CHECK:           %[[VAL_3:.*]] = arith.muli %[[VAL_0]], %[[VAL_2]] : index
+// CHECK:           %[[VAL_4:.*]] = memref.reinterpret_cast %[[VAL_1]] to offset: {{\[}}%[[VAL_3]]], sizes: [30], strides: [1] : memref<30x30xi32> to memref<30xi32>
+// CHECK:           return %[[VAL_4]] : memref<30xi32>
+// CHECK:         }
+module {
+  func @main(%arg0 : index) -> memref<30xi32> {
+    %0 = memref.alloca() : memref<30x30xi32>
+    %1 = "polygeist.subindex"(%0, %arg0) : (memref<30x30xi32>, index) -> memref<30xi32>
+    return %1 : memref<30xi32>
+  }
+}