[mlir][acc] Add loop tiling utilities for OpenACC (llvm#171490)

Add utilities in OpenACCUtilsTiling.h/.cpp to support tiling
transformations on acc.loop operations:

- uncollapseLoops: Expand collapsed loops with multiple IVs into nested
loop structures when tile count exceeds collapse count
- tileACCLoops: Transform loop nests into tile and element loops based
on provided tile sizes, with automatic resolution of unknown tile sizes
(tile(*) represented as -1)

These utilities prepare for the ACCLoopTiling pass which handles the
OpenACC loop tile directive.

---------

Co-authored-by: Vijay Kandiah <vkandiah@nvidia.com>

Author: razvanlupusoru

mlir/include/mlir/Dialect/OpenACC/OpenACCUtilsTiling.h

@@ -0,0 +1,83 @@
+//===- OpenACCUtilsTiling.h - OpenACC Loop Tiling Utilities -----*- C++ -*-===//
+//
+// 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 contains utility functions for tiling OpenACC loops.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef MLIR_DIALECT_OPENACC_OPENACCUTILSTILING_H_
+#define MLIR_DIALECT_OPENACC_OPENACCUTILSTILING_H_
+
+#include "mlir/Dialect/OpenACC/OpenACC.h"
+#include "mlir/IR/PatternMatch.h"
+#include "llvm/ADT/SmallVector.h"
+
+namespace mlir {
+namespace acc {
+
+/// Uncollapse tile loops with multiple IVs and collapseCount < tileCount.
+/// This is used to prepare loops for tiling when the collapse count is less
+/// than the tile count.
+///
+/// \param origLoop The original loop operation to uncollapse.
+/// \param tileCount The number of tile dimensions.
+/// \param collapseCount The collapse count from the original loop.
+/// \param rewriter The rewriter to use for modifications.
+/// \return A vector of uncollapsed loop operations.
+llvm::SmallVector<mlir::acc::LoopOp>
+uncollapseLoops(mlir::acc::LoopOp origLoop, unsigned tileCount,
+                unsigned collapseCount, mlir::RewriterBase &rewriter);
+
+/// Tile ACC loops according to the given tile sizes.
+///
+/// Tiling a 2-level nested loop will create two 'tile' loops containing two
+/// 'element' loops. The transformation looks like:
+///
+/// Before Tiling:
+/// \code
+/// #pragma acc loop tile(tile_size1, tile_size2)
+///  for (i = lb1; i < ub1; i += step1) { // original loop
+///    for (j = lb2; j < ub2; j += step2) {
+///      a[i,j] = i + j;
+///    }
+///  }
+/// \endcode
+///
+/// After Tiling:
+/// \code
+///  for (i = lb1; i < ub1; i += (step1 * tile_size1)) { // tile loop 1
+///    for (j = lb2; j < ub2; j += (step2 * tile_size2)) { // tile loop 2
+///      for (ii = i; ii < min(ub1, (step1 * tile_size1) + i); ii += step1) {
+///      // element loop 1
+///        for (jj = j; jj < min(ub2, (step2 * tile_size2) + j); jj += step2)
+///        { // element loop 2
+///          a[ii,jj] = i + j;
+///        }
+///      }
+///    }
+///  }
+/// \endcode
+///
+/// Unknown tile sizes (represented as -1 in acc dialect for `tile(*)`) are
+/// resolved to the provided default tile size.
+///
+/// \param tileLoops The loops to tile (outermost first).
+/// \param tileSizes The tile sizes for each dimension. Values of -1 are
+///        treated as unknown and resolved to defaultTileSize.
+/// \param defaultTileSize The default tile size to use for unknown (*) tiles.
+/// \param rewriter The rewriter to use for modifications.
+/// \return The outermost loop after tiling.
+mlir::acc::LoopOp tileACCLoops(llvm::SmallVector<mlir::acc::LoopOp> &tileLoops,
+                               const llvm::SmallVector<mlir::Value> &tileSizes,
+                               int32_t defaultTileSize,
+                               mlir::RewriterBase &rewriter);
+
+} // namespace acc
+} // namespace mlir
+
+#endif // MLIR_DIALECT_OPENACC_OPENACCUTILSTILING_H_

mlir/lib/Dialect/OpenACC/Utils/CMakeLists.txt

@@ -1,4 +1,5 @@
 add_mlir_dialect_library(MLIROpenACCUtils
+  OpenACCUtilsTiling.cpp
   OpenACCUtils.cpp
 
   ADDITIONAL_HEADER_DIRS
@@ -14,7 +15,9 @@ add_mlir_dialect_library(MLIROpenACCUtils
   MLIROpenACCTypeInterfacesIncGen
 
   LINK_LIBS PUBLIC
+  MLIRArithDialect
   MLIROpenACCDialect
   MLIRIR
   MLIRSupport
+  MLIRTransformUtils
 )