[mlir][xegpu] Handle scalar uniform ops in SIMT distribution. (#138593)

This PR adds support for moving scalar uniform (gpu index ops, constants
etc) outside the `gpu.warp_execute_on_lane0` op. These kinds of ops do
not require distribution and are safe to move out of the warp op. This
also avoid adding separate distribution patterns for these ops.

Example:
```
   %1 = gpu.warp_execute_on_lane_0(%laneid) -> (index) {
     ...
     %block_id_x = gpu.block_id x
     gpu.yield %block_id_x
   }
  // use %1
```
To:
```
   %block_id_x = gpu.block_id x
   %1 = gpu.warp_execute_on_lane_0(%laneid) -> (index) {
     ...
     
     gpu.yield %block_id_x
   }
  // use %1

```

Author: charithaintc

mlir/lib/Dialect/XeGPU/Transforms/XeGPUSubgroupDistribute.cpp

@@ -1463,6 +1463,15 @@ void XeGPUSubgroupDistributePass::runOnOperation() {
       signalPassFailure();
       return;
     }
+    // At this point, we have moved the entire function body inside the warpOp.
+    // Now move any scalar uniform code outside of the warpOp (like GPU index
+    // ops, scalar constants, etc.). This will simplify the later lowering and
+    // avoid custom patterns for these ops.
+    getOperation()->walk([&](Operation *op) {
+      if (auto warpOp = dyn_cast<gpu::WarpExecuteOnLane0Op>(op)) {
+        vector::moveScalarUniformCode(warpOp);
+      }
+    });
   }
   // Finally, do the SIMD to SIMT distribution.
   RewritePatternSet patterns(&getContext());

mlir/test/Dialect/XeGPU/subgroup-distribution.mlir

@@ -1,4 +1,4 @@
-// RUN: mlir-opt -xegpu-subgroup-distribute -split-input-file %s | FileCheck %s
+// RUN: mlir-opt -xegpu-subgroup-distribute -cse -split-input-file %s | FileCheck %s
 
 // CHECK-LABEL: gpu.func @store_nd_1d
 // CHECK: (%[[ARG0:[0-9a-zA-Z]+]]: memref<16xf32>) {
@@ -160,3 +160,50 @@ gpu.func @create_nd_tdesc_non_memref(%arg0: ui64, %arg1: ui64,
   gpu.return
 }
 }
+
+// -----
+// CHECK-LABEL: gpu.func @gemm_loop
+// CHECK: (%[[ARG0:[0-9a-zA-Z]+]]: memref<1024x1024xbf16>, %[[ARG1:[0-9a-zA-Z]+]]: memref<1024x1024xbf16>, %[[ARG2:[0-9a-zA-Z]+]]: memref<1024x1024xf32>) {
+// CHECK: %[[BLOCK_ID_X:.*]] = gpu.block_id x
+// CHECK: %[[BLOCK_ID_Y:.*]] = gpu.block_id y
+// CHECK: %[[Y_COORD:.*]] = arith.muli %[[BLOCK_ID_Y]], %c16 : index
+// CHECK: %[[X_COORD:.*]] = arith.muli %[[BLOCK_ID_X]], %c8 : index
+// CHECK: %[[T2:.*]] = xegpu.create_nd_tdesc %[[ARG2]][%[[X_COORD]], %[[Y_COORD]]] : memref<1024x1024xf32> -> !xegpu.tensor_desc<8x16xf32>
+// CHECK: %[[T3:.*]] = xegpu.load_nd %[[T2]] : !xegpu.tensor_desc<8x16xf32> -> vector<8xf32>
+// CHECK: %[[T4:.*]] = vector.shape_cast %[[T3]] : vector<8xf32> to vector<8x1xf32>
+// CHECK: %[[T5:.*]] = scf.for %[[K:.*]] = %{{.*}} to %{{.*}} step %{{.*}} iter_args(%[[ARG4:.*]] = %[[T4]]) -> (vector<8x1xf32>) {
+// CHECK: %[[T10:.*]] = xegpu.create_nd_tdesc %[[ARG1]][%[[K]], %[[Y_COORD]]] : memref<1024x1024xbf16> -> !xegpu.tensor_desc<16x16xbf16>
+// CHECK: %[[T11:.*]] = xegpu.load_nd %[[T10]] <{packed}> : !xegpu.tensor_desc<16x16xbf16> -> vector<16xbf16>
+// CHECK: %[[T12:.*]] = xegpu.create_nd_tdesc %[[ARG0]][%[[X_COORD]], %[[K]]] : memref<1024x1024xbf16> -> !xegpu.tensor_desc<8x16xbf16>
+// CHECK: %[[T13:.*]] = xegpu.load_nd %[[T12]] : !xegpu.tensor_desc<8x16xbf16> -> vector<8xbf16>
+// CHECK: %[[T14:.*]] = vector.shape_cast %[[ARG4]] : vector<8x1xf32> to vector<8xf32>
+// CHECK: %[[T15:.*]] = xegpu.dpas %[[T13]], %[[T11]], %[[T14]] : vector<8xbf16>, vector<16xbf16>, vector<8xf32> -> vector<8xf32>
+// CHECK: %[[T16:.*]] = vector.shape_cast %[[T15]] : vector<8xf32> to vector<8x1xf32>
+// CHECK: scf.yield %[[T16]] : vector<8x1xf32>
+// CHECK: }
+// CHECK: %[[T9:.*]] = vector.shape_cast %[[T5]] : vector<8x1xf32> to vector<8xf32>
+// CHECK: xegpu.store_nd %[[T9]], %[[T2]] : vector<8xf32>, !xegpu.tensor_desc<8x16xf32>
+gpu.module @test {
+gpu.func @gemm_loop(%arg0: memref<1024x1024xbf16>, %arg1: memref<1024x1024xbf16>, %arg2: memref<1024x1024xf32>){
+  %c0 = arith.constant 0 : index
+  %c16 = arith.constant 16 : index
+  %c8 = arith.constant 8 : index
+  %c1024 = arith.constant 1024 : index
+  %0 = gpu.block_id x
+  %1 = gpu.block_id y
+  %2 = arith.muli %0, %c8 : index
+  %3 = arith.muli %1, %c16 : index
+  %4 = xegpu.create_nd_tdesc %arg2[%2, %3] : memref<1024x1024xf32> -> !xegpu.tensor_desc<8x16xf32>
+  %5 = xegpu.load_nd %4 : !xegpu.tensor_desc<8x16xf32> -> vector<8x16xf32>
+  %6 = scf.for %arg3 = %c0 to %c1024 step %c16 iter_args(%arg4 = %5) -> (vector<8x16xf32>) {
+    %7 = xegpu.create_nd_tdesc %arg0[%2, %arg3] : memref<1024x1024xbf16> -> !xegpu.tensor_desc<8x16xbf16>
+    %8 = xegpu.create_nd_tdesc %arg1[%arg3, %3] : memref<1024x1024xbf16> -> !xegpu.tensor_desc<16x16xbf16>
+    %9 = xegpu.load_nd %7 : !xegpu.tensor_desc<8x16xbf16> -> vector<8x16xbf16>
+    %10 = xegpu.load_nd %8 : !xegpu.tensor_desc<16x16xbf16> -> vector<16x16xbf16>
+    %11 = xegpu.dpas %9, %10, %arg4 : vector<8x16xbf16>, vector<16x16xbf16>, vector<8x16xf32> -> vector<8x16xf32>
+    scf.yield %11 : vector<8x16xf32>
+  }
+  xegpu.store_nd %6, %4 : vector<8x16xf32>, !xegpu.tensor_desc<8x16xf32>
+  gpu.return
+}
+}