[Mlir-commits] [mlir] [mlir][acc] Effectively serial check should permit missing dimension (PR #194981)

[Razvan Lupusoru]

https://github.com/razvanlupusoru created https://github.com/llvm/llvm-project/pull/194981

The `isEffectivelySerial` check for `acc.compute_region` required that all parallelism dimensions be set to 1. However, missing dimensions means that the level of parallelism is not assigned and thus should not be considered. Fix the issue and add unit tests for the API.

>From 6ac2db0dab3aad7d6c5c0c1f150ee7488e7d86a3 Mon Sep 17 00:00:00 2001
From: Razvan Lupusoru <rlupusoru at nvidia.com>
Date: Wed, 29 Apr 2026 16:47:20 -0700
Subject: [PATCH] [mlir][acc] Effectively serial check should permit missing
 dimension

The `isEffectivelySerial` check for `acc.compute_region` required that
all parallelism dimensions be set to 1. However, missing dimensions
means that the level of parallelism is not assigned and thus should
not be considered. Fix the issue and add unit tests for the API.
---
 mlir/lib/Dialect/OpenACC/IR/OpenACCCG.cpp     |  4 ++
 .../Dialect/OpenACC/OpenACCCGOpsTest.cpp      | 67 +++++++++++++++++++
 2 files changed, 71 insertions(+)

diff --git a/mlir/lib/Dialect/OpenACC/IR/OpenACCCG.cpp b/mlir/lib/Dialect/OpenACC/IR/OpenACCCG.cpp
index fe806186ce25d..4d06b1f1ff90e 100644
--- a/mlir/lib/Dialect/OpenACC/IR/OpenACCCG.cpp
+++ b/mlir/lib/Dialect/OpenACC/IR/OpenACCCG.cpp
@@ -506,6 +506,10 @@ bool ComputeRegionOp::isEffectivelySerial() {
     return true;
 
   auto checkDim = [&](GPUParallelDimAttr dim) -> bool {
+    // Launch dimensions without an explicit `acc.par_width` for that dimension
+    // means that no such parallelism is assigned and thus defaults to width 1.
+    if (!getLaunchArg(dim))
+      return true;
     auto val = getKnownConstantLaunchArg(dim);
     return val && *val == 1;
   };
diff --git a/mlir/unittests/Dialect/OpenACC/OpenACCCGOpsTest.cpp b/mlir/unittests/Dialect/OpenACC/OpenACCCGOpsTest.cpp
index 44283c9091b4d..800d3b9a3108d 100644
--- a/mlir/unittests/Dialect/OpenACC/OpenACCCGOpsTest.cpp
+++ b/mlir/unittests/Dialect/OpenACC/OpenACCCGOpsTest.cpp
@@ -93,11 +93,78 @@ class OpenACCCGOpsTest : public ::testing::Test {
     YieldOp::create(regionBuilder, loc);
   }
 
+  /// Build a single `acc.compute_region` with the given launch arguments.
+  ComputeRegionOp makeComputeRegion(HostContext &host, ValueRange launchArgs) {
+    Region sourceRegion;
+    populateSourceRegionSingleBlock(sourceRegion, context, loc, std::nullopt,
+                                    false);
+    IRMapping mapping;
+    // Setting an empty origin since it should not be relevant for the tests.
+    ComputeRegionOp cr = buildComputeRegion(
+        loc, launchArgs, {}, "", sourceRegion, host.rewriter, mapping);
+    return cr;
+  }
+
   MLIRContext context;
   OpBuilder b;
   Location loc;
 };
 
+//===----------------------------------------------------------------------===//
+// ComputeRegionOp::isEffectivelySerial
+//===----------------------------------------------------------------------===//
+
+TEST_F(OpenACCCGOpsTest, IsEffectivelySerialNoLaunchArgs) {
+  HostContext host(context, loc, b);
+  ComputeRegionOp cr = makeComputeRegion(host, {});
+  EXPECT_TRUE(cr.isEffectivelySerial());
+  EXPECT_TRUE(succeeded(host.module->verify()));
+}
+
+TEST_F(OpenACCCGOpsTest, IsEffectivelySerialSparseDimsConstantOne) {
+  HostContext host(context, loc, b);
+  Value c1 = arith::ConstantIndexOp::create(host.rewriter, loc, 1);
+  Value pwBx = ParWidthOp::create(host.rewriter, loc, c1,
+                                  GPUParallelDimAttr::blockXDim(&context));
+  Value pwTy = ParWidthOp::create(host.rewriter, loc, c1,
+                                  GPUParallelDimAttr::threadYDim(&context));
+  Value pwTx = ParWidthOp::create(host.rewriter, loc, c1,
+                                  GPUParallelDimAttr::threadXDim(&context));
+  ComputeRegionOp cr = makeComputeRegion(host, {pwBx, pwTy, pwTx});
+  EXPECT_TRUE(cr.isEffectivelySerial());
+  EXPECT_TRUE(succeeded(host.module->verify()));
+}
+
+TEST_F(OpenACCCGOpsTest,
+       IsEffectivelySerialFalseWhenThreadXWidthGreaterThanOne) {
+  HostContext host(context, loc, b);
+  Value c2 = arith::ConstantIndexOp::create(host.rewriter, loc, 2);
+  Value pwTx = ParWidthOp::create(host.rewriter, loc, c2,
+                                  GPUParallelDimAttr::threadXDim(&context));
+  ComputeRegionOp cr = makeComputeRegion(host, pwTx);
+  EXPECT_FALSE(cr.isEffectivelySerial());
+  EXPECT_TRUE(succeeded(host.module->verify()));
+}
+
+TEST_F(OpenACCCGOpsTest, IsEffectivelySerialFalseWhenLaunchWidthUnknown) {
+  HostContext host(context, loc, b);
+  Value pwTx = ParWidthOp::create(host.rewriter, loc, Value(),
+                                  GPUParallelDimAttr::threadXDim(&context));
+  ComputeRegionOp cr = makeComputeRegion(host, pwTx);
+  EXPECT_FALSE(cr.isEffectivelySerial());
+  EXPECT_TRUE(succeeded(host.module->verify()));
+}
+
+TEST_F(OpenACCCGOpsTest, IsEffectivelySerialTrueWhenSeqLaunchPresent) {
+  HostContext host(context, loc, b);
+  Value c7 = arith::ConstantIndexOp::create(host.rewriter, loc, 7);
+  Value pwSeq = ParWidthOp::create(host.rewriter, loc, c7,
+                                   GPUParallelDimAttr::seqDim(&context));
+  ComputeRegionOp cr = makeComputeRegion(host, pwSeq);
+  EXPECT_TRUE(cr.isEffectivelySerial());
+  EXPECT_TRUE(succeeded(host.module->verify()));
+}
+
 //===----------------------------------------------------------------------===//
 // ComputeRegionOp::wireHoistedValueThroughIns
 //===----------------------------------------------------------------------===//