[Mlir-commits] [mlir] [MLIR][GPU] Add a pattern to rewrite gpu.subgroup_id (PR #137671)

[Jakub Kuderski]

================
@@ -0,0 +1,82 @@
+//===- SubgroupIdRewriter.cpp - Implementation of SubgroupId rewriting ----===//
+//
+// 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 in-dialect rewriting of the gpu.subgroup_id op for archs
+// where:
+// subgroup_id = (tid.x + dim.x * (tid.y + dim.y * tid.z)) / subgroup_size
+//
+//===----------------------------------------------------------------------===//
+
+#include "mlir/Dialect/GPU/IR/GPUDialect.h"
+#include "mlir/Dialect/GPU/Transforms/Passes.h"
+#include "mlir/Dialect/Index/IR/IndexOps.h"
+#include "mlir/IR/Builders.h"
+#include "mlir/IR/PatternMatch.h"
+#include "mlir/Pass/Pass.h"
+
+using namespace mlir;
+
+namespace {
+struct GpuSubgroupIdRewriter final : OpRewritePattern<gpu::SubgroupIdOp> {
+  using OpRewritePattern<gpu::SubgroupIdOp>::OpRewritePattern;
+
+  LogicalResult matchAndRewrite(gpu::SubgroupIdOp op,
+                                PatternRewriter &rewriter) const override {
+    // Calculation of the thread's subgroup identifier.
+    //
+    // The process involves mapping the thread's 3D identifier within its
+    // block (b_id.x, b_id.y, b_id.z) to a 1D linear index.
+    // This linearization assumes a layout where the x-dimension (w_dim.x)
+    // varies most rapidly (i.e., it is the innermost dimension).
+    //
+    // The formula for the linearized thread index is:
+    // L = tid.x + dim.x * (tid.y + (dim.y * tid.z))
+    //
+    // Subsequently, the range of linearized indices [0, N_threads-1] is
+    // divided into consecutive, non-overlapping segments, each representing
+    // a subgroup of size 'subgroup_size'.
+    //
+    // Example Partitioning (N = subgroup_size):
+    // | Subgroup 0      | Subgroup 1      | Subgroup 2      | ... |
+    // | Indices 0..N-1  | Indices N..2N-1 | Indices 2N..3N-1| ... |
+    //
+    // The subgroup identifier is obtained via integer division of the
+    // linearized thread index by the predefined 'subgroup_size'.
+    //
+    // subgroup_id = floor( L / subgroup_size )
+    //             = (tid.x + dim.x * (tid.y + dim.y * tid.z)) /
+    //             subgroup_size
+
+    auto loc = op->getLoc();
----------------
kuhar wrote:

```suggestion
    Location loc = op->getLoc();
```
See https://llvm.org/docs/CodingStandards.html#use-auto-type-deduction-to-make-code-more-readable

https://github.com/llvm/llvm-project/pull/137671