[Mlir-commits] [mlir] [mlir] [XeGPU] Add XeGPU workgroup to subgroup pass (PR #139477)

[Frank Schlimbach]

================
@@ -0,0 +1,387 @@
+//===- XeGPUWgToSg.cpp - XeGPU WorkGroup to Subgroup Pass -------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+#include "mlir/Dialect/XeGPU/Transforms/Passes.h"
+
+#include "mlir/Dialect/Index/IR/IndexDialect.h"
+#include "mlir/Dialect/MemRef/IR/MemRef.h"
+#include "mlir/Dialect/Utils/IndexingUtils.h"
+#include "mlir/Dialect/XeGPU/IR/XeGPU.h"
+#include "mlir/Dialect/XeGPU/Transforms/Transforms.h"
+#include "mlir/Transforms/DialectConversion.h"
+#include "llvm/Support/Debug.h"
+#include <mlir/Dialect/GPU/IR/GPUDialect.h>
+#include <mlir/Dialect/Index/IR/IndexOps.h>
+#include <numeric>
+
+namespace mlir {
+namespace xegpu {
+#define GEN_PASS_DEF_XEGPUWGTOSG
+#include "mlir/Dialect/XeGPU/Transforms/Passes.h.inc"
+} // namespace xegpu
+} // namespace mlir
+
+#define DEBUG_TYPE "xegpu-wg-to-sg"
+#define DBGS() (llvm::dbgs() << "[" DEBUG_TYPE "]: ")
+#define LDBG(X) LLVM_DEBUG(DBGS() << X << "\n")
+
+using namespace mlir;
+
+namespace {
+
+// clang-format off
+/// This pattern transforms the CreateNdDescOp to create a subgroup descriptor
+/// from a workgroup descriptor. It replaces the offsets and sizes with
+/// appropriate values for the subgroup.
+/// It uses round-robin assignment to distribute the work to the subgroups.
+/// Following create_nd_desc operation:,
+///    %tdesc = xegpu.create_nd_tdesc %src[0, 0] : memref<24x24xf32>
+///       -> !xegpu.tensor_desc<24x24xf32, #xegpu.layout<sg_layout = [4, 4],
+///           sg_data = [2, 2], lane_layout = [2, 2], lane_data = [1, 1]>>
+/// is converted to 9 subgroup level operations based on the sg_layout & sg_data:
+///    %tdesc = xegpu.create_nd_tdesc %src[off1, off2] : memref<24x24xf32> ->
+///           !xegpu.tensor_desc<2x2xf32, #xegpu.layout<lane_layout = [2, 2], lane_data = [1, 1]>>
+///
+/// The sg_layout and sg_data attributes are dropped after the pass as they are no longer needed.
+///
+/// 24x24 matrix distribution example:
+/// sg_layout = [4, 4], sg_data = [2, 2]
+/// Each 8x8 matrix within the 24x24 matrix is called a distribution unit.
+/// dist_unit_shape = [8, 8] --> sg_layout[i] * sg_data[i]
+///
+/// +------------------------+
+/// | 8x8 | 8x8 | 8x8 |      <- 3 tiles across
+/// |-----+-----+-----|
+/// | 8x8 | 8x8 | 8x8 |      <- 3 tiles down
+/// |-----+-----+-----|
+/// | 8x8 | 8x8 | 8x8 |
+/// +------------------------+
+///
+/// Each 8x8 tile is further subdivided among subgroups:
+/// +------------------------+
+/// | 2x2 2x2 2x2 2x2 |  <- 4 subgroups across (each handles 2 columns)
+/// | 2x2 2x2 2x2 2x2 |  <- 4 subgroups down (each handles 2 rows)
+/// | 2x2 2x2 2x2 2x2 |
+/// | 2x2 2x2 2x2 2x2 |
+/// +------------------------+
+///
+/// Since the 24x24 matrix is divided into 8x8 distribution units, there will be 9
+/// distribution units (3x3) in total. Hence the 9 subgroup level operations.
+// clang-format on
+struct WgToSgCreateNdOp : public OpConversionPattern<xegpu::CreateNdDescOp> {
+  using OpConversionPattern<xegpu::CreateNdDescOp>::OpConversionPattern;
+
+  // Helper to extract mixed offsets into a Value array
+  SmallVector<Value> extractOffsets(ConversionPatternRewriter &rewriter,
+                                    xegpu::CreateNdDescOp op) const {
+    llvm::SmallVector<Value> offsets;
+    auto staticOffsets = op.getStaticOffsets();
+    auto dynamicOffsets = op.getOffsets();
+
+    for (size_t i = 0, j = 0; i != staticOffsets.size(); i++) {
+      if (ShapedType::isDynamic(staticOffsets[i]))
+        offsets.push_back(dynamicOffsets[j++]);
+      else
+        offsets.push_back(rewriter.create<arith::ConstantIndexOp>(
+            op.getLoc(), staticOffsets[i]));
+    }
+    return offsets;
+  }
----------------
fschlimb wrote:

In PassUtils.h we have a `getMixedAsValues` for this. 
For upstreaming, we could lift the version in MeshToMPI.cpp in upstream to a more prominent and re-usable place.

https://github.com/llvm/llvm-project/pull/139477