[Mlir-commits] [mlir] [MLIR][GPUToLLVMSPV] Use global & local memory scope for GPUBarrierConversion (PR #169026)

[llvmlistbot at llvm.org]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-mlir

Author: Tomek Kuczyński (dev-tomek)

<details>
<summary>Changes</summary>

The MLIR [GPU dialect docs](https://mlir.llvm.org/docs/Dialects/GPU/#gpubarrier-gpubarrierop) specify that gpu::BarrierOp should make *all memory accesses* visible to all work items in the workgroup. 
Current implementation uses only CLK_LOCAL_MEM_FENCE, which per the [OpenCL specification](https://registry.khronos.org/OpenCL/sdk/3.0/docs/man/html/barrier.html) guarantees visibility of 
only *local memory accesses*.

This PR changes the barrier conversion to use CLK_LOCAL_MEM_FENCE | CLK_GLOBAL_MEM_FENCE, 
ensuring both local and global memory operations are properly synchronized per the MLIR spec.

This issue was discovered while investigating numerical instabilities on Intel Battlemage, 
where race conditions occurred due to incomplete memory synchronization.

---
Full diff: https://github.com/llvm/llvm-project/pull/169026.diff


2 Files Affected:

- (modified) mlir/lib/Conversion/GPUToLLVMSPV/GPUToLLVMSPV.cpp (+11-6) 
- (modified) mlir/test/Conversion/GPUToLLVMSPV/gpu-to-llvm-spv.mlir (+1-1) 


``````````diff
diff --git a/mlir/lib/Conversion/GPUToLLVMSPV/GPUToLLVMSPV.cpp b/mlir/lib/Conversion/GPUToLLVMSPV/GPUToLLVMSPV.cpp
index f143a9e505f4d..f62191425ba9f 100644
--- a/mlir/lib/Conversion/GPUToLLVMSPV/GPUToLLVMSPV.cpp
+++ b/mlir/lib/Conversion/GPUToLLVMSPV/GPUToLLVMSPV.cpp
@@ -96,12 +96,14 @@ namespace {
 // Barriers
 //===----------------------------------------------------------------------===//
 
-/// Replace `gpu.barrier` with an `llvm.call` to `barrier` with
-/// `CLK_LOCAL_MEM_FENCE` argument, indicating work-group memory scope:
+/// Replace `gpu.barrier` with an `llvm.call` to `barrier` using
+/// `CLK_LOCAL_MEM_FENCE | CLK_GLOBAL_MEM_FENCE`, ensuring that all memory
+/// accesses are visible to all work-items in the work-group.
 /// ```
 /// // gpu.barrier
-/// %c1 = llvm.mlir.constant(1: i32) : i32
-/// llvm.call spir_funccc @_Z7barrierj(%c1) : (i32) -> ()
+/// // 3 = CLK_LOCAL_MEM_FENCE | CLK_GLOBAL_MEM_FENCE
+/// %c3 = llvm.mlir.constant(3: i32) : i32
+/// llvm.call spir_funccc @_Z7barrierj(%c3) : (i32) -> ()
 /// ```
 struct GPUBarrierConversion final : ConvertOpToLLVMPattern<gpu::BarrierOp> {
   using ConvertOpToLLVMPattern::ConvertOpToLLVMPattern;
@@ -119,12 +121,15 @@ struct GPUBarrierConversion final : ConvertOpToLLVMPattern<gpu::BarrierOp> {
         lookupOrCreateSPIRVFn(moduleOp, funcName, flagTy, voidTy,
                               /*isMemNone=*/false, /*isConvergent=*/true);
 
-    // Value used by SPIR-V backend to represent `CLK_LOCAL_MEM_FENCE`.
+    // Values used by SPIR-V backend to represent a combination of 
+    // `CLK_LOCAL_MEM_FENCE` and `CLK_GLOBAL_MEM_FENCE`.
     // See `llvm/lib/Target/SPIRV/SPIRVBuiltins.td`.
     constexpr int64_t localMemFenceFlag = 1;
+    constexpr int64_t globalMemFenceFlag = 2;
+    constexpr int64_t localGlobalMemFenceFlag = localMemFenceFlag | globalMemFenceFlag;
     Location loc = op->getLoc();
     Value flag =
-        LLVM::ConstantOp::create(rewriter, loc, flagTy, localMemFenceFlag);
+        LLVM::ConstantOp::create(rewriter, loc, flagTy, localGlobalMemFenceFlag);
     rewriter.replaceOp(op, createSPIRVBuiltinCall(loc, rewriter, func, flag));
     return success();
   }
diff --git a/mlir/test/Conversion/GPUToLLVMSPV/gpu-to-llvm-spv.mlir b/mlir/test/Conversion/GPUToLLVMSPV/gpu-to-llvm-spv.mlir
index 0c2c021b9d43c..227287a9adcee 100644
--- a/mlir/test/Conversion/GPUToLLVMSPV/gpu-to-llvm-spv.mlir
+++ b/mlir/test/Conversion/GPUToLLVMSPV/gpu-to-llvm-spv.mlir
@@ -215,7 +215,7 @@ gpu.module @barriers {
 
   // CHECK-LABEL: gpu_barrier
   func.func @gpu_barrier() {
-    // CHECK:         [[FLAGS:%.*]] = llvm.mlir.constant(1 : i32) : i32
+    // CHECK:         [[FLAGS:%.*]] = llvm.mlir.constant(3 : i32) : i32
     // CHECK:         llvm.call spir_funccc @_Z7barrierj([[FLAGS]]) {
     // CHECK-SAME-DAG:  no_unwind
     // CHECK-SAME-DAG:  convergent

``````````

</details>


https://github.com/llvm/llvm-project/pull/169026