[MLIR] Improve KernelOutlining to avoid introducing an extra block (#90359)

This fixes a TODO in the code.

Author: joker-eph

mlir/lib/Dialect/GPU/Transforms/KernelOutlining.cpp

@@ -241,24 +241,26 @@ static gpu::GPUFuncOp outlineKernelFuncImpl(gpu::LaunchOp launchOp,
     map.map(operand.value(), entryBlock.getArgument(operand.index()));
 
   // Clone the region of the gpu.launch operation into the gpu.func operation.
-  // TODO: If cloneInto can be modified such that if a mapping for
-  // a block exists, that block will be used to clone operations into (at the
-  // end of the block), instead of creating a new block, this would be much
-  // cleaner.
   launchOpBody.cloneInto(&outlinedFuncBody, map);
 
-  // Branch from entry of the gpu.func operation to the block that is cloned
-  // from the entry block of the gpu.launch operation.
-  Block &launchOpEntry = launchOpBody.front();
-  Block *clonedLaunchOpEntry = map.lookup(&launchOpEntry);
-  builder.setInsertionPointToEnd(&entryBlock);
-  builder.create<cf::BranchOp>(loc, clonedLaunchOpEntry);
-
-  outlinedFunc.walk([](gpu::TerminatorOp op) {
-    OpBuilder replacer(op);
-    replacer.create<gpu::ReturnOp>(op.getLoc());
-    op.erase();
-  });
+  // Replace the terminator op with returns.
+  for (Block &block : launchOpBody) {
+    Block *clonedBlock = map.lookup(&block);
+    auto terminator = dyn_cast<gpu::TerminatorOp>(clonedBlock->getTerminator());
+    if (!terminator)
+      continue;
+    OpBuilder replacer(terminator);
+    replacer.create<gpu::ReturnOp>(terminator->getLoc());
+    terminator->erase();
+  }
+
+  // Splice now the entry block of the gpu.launch operation at the end of the
+  // gpu.func entry block and erase the redundant block.
+  Block *clonedLaunchOpEntry = map.lookup(&launchOpBody.front());
+  entryBlock.getOperations().splice(entryBlock.getOperations().end(),
+                                    clonedLaunchOpEntry->getOperations());
+  clonedLaunchOpEntry->erase();
+
   return outlinedFunc;
 }
 

mlir/test/Dialect/GPU/outlining.mlir

@@ -54,12 +54,41 @@ func.func @launch() {
 // CHECK-NEXT: %[[BDIM:.*]] = gpu.block_dim x
 // CHECK-NEXT: = gpu.block_dim y
 // CHECK-NEXT: = gpu.block_dim z
-// CHECK-NEXT: cf.br ^[[BLOCK:.*]]
-// CHECK-NEXT: ^[[BLOCK]]:
 // CHECK-NEXT: "use"(%[[KERNEL_ARG0]]) : (f32) -> ()
 // CHECK-NEXT: "some_op"(%[[BID]], %[[BDIM]]) : (index, index) -> ()
 // CHECK-NEXT: = memref.load %[[KERNEL_ARG1]][%[[TID]]] : memref<?xf32, 1>
 
+// -----
+
+// Verify that we can outline a CFG
+// CHECK-LABEL:  gpu.func @launchCFG_kernel(
+// CHECK: cf.br
+// CHECK: gpu.return
+func.func @launchCFG() {
+  %0 = "op"() : () -> (f32)
+  %1 = "op"() : () -> (memref<?xf32, 1>)
+  %gDimX = arith.constant 8 : index
+  %gDimY = arith.constant 12 : index
+  %gDimZ = arith.constant 16 : index
+  %bDimX = arith.constant 20 : index
+  %bDimY = arith.constant 24 : index
+  %bDimZ = arith.constant 28 : index
+
+  gpu.launch blocks(%bx, %by, %bz) in (%grid_x = %gDimX, %grid_y = %gDimY,
+                                       %grid_z = %gDimZ)
+             threads(%tx, %ty, %tz) in (%block_x = %bDimX, %block_y = %bDimY,
+                                        %block_z = %bDimZ) {
+    "use"(%0): (f32) -> ()
+    cf.br ^bb1
+  ^bb1:
+    "some_op"(%bx, %block_x) : (index, index) -> ()
+    %42 = memref.load %1[%tx] : memref<?xf32, 1>
+    gpu.terminator
+  }
+  return
+}
+
+
 // -----
 
 // This test checks gpu-out-lining can handle gpu.launch kernel from an llvm.func
@@ -475,8 +504,6 @@ func.func @launch_cluster() {
 // CHECK-NEXT: %[[CDIM:.*]] = gpu.cluster_dim x
 // CHECK-NEXT: = gpu.cluster_dim y
 // CHECK-NEXT: = gpu.cluster_dim z
-// CHECK-NEXT: cf.br ^[[BLOCK:.*]]
-// CHECK-NEXT: ^[[BLOCK]]:
 // CHECK-NEXT: "use"(%[[KERNEL_ARG0]]) : (f32) -> ()
 // CHECK-NEXT: "some_op"(%[[CID]], %[[BID]], %[[BDIM]]) : (index, index, index) -> ()
 // CHECK-NEXT: = memref.load %[[KERNEL_ARG1]][%[[TID]]] : memref<?xf32, 1>