Move to a workgroup fence from a workgroup-one-as fence because things don't work how I thought they would

Author: krzysz00

mlir/lib/Conversion/AMDGPUToROCDL/AMDGPUToROCDL.cpp

@@ -543,7 +543,15 @@ struct LDSBarrierOpLowering : public ConvertOpToLLVMPattern<LDSBarrierOp> {
 
     Attribute mmra =
         rewriter.getAttr<LLVM::MMRATagAttr>("amdgpu-synchronize-as", "local");
-    StringRef scope = "workgroup-one-as";
+    // Note: while there *is* a workgroup-one-as scope, this, when combined with
+    // the MMRA, will lead to the fence having no effect. This is because
+    // the codepaths for an atomic load or store will observe that a
+    // one-address-space atomic to LDS requires no synchronization because
+    // operations on LDS are totally ordered with respect to each other,
+    // and so will not emit the correct waitcnt operations that these fences
+    // are intended to produce. Therefore, we use a broader type of fence
+    // and rely on the MMRA to relax it to the semantics we want.
+    StringRef scope = "workgroup";
 
     auto relFence = LLVM::FenceOp::create(rewriter, loc,
                                           LLVM::AtomicOrdering::release, scope);

mlir/test/Conversion/AMDGPUToROCDL/amdgpu-to-rocdl.mlir

@@ -416,7 +416,7 @@ func.func @amdgpu_raw_buffer_atomic_cmpswap_v2f16(%src : vector<2xf16>, %cmp : v
 
 // CHECK-LABEL: func @lds_barrier
 func.func @lds_barrier() {
-  // CHECK: llvm.fence syncscope("workgroup-one-as") release {llvm.mmra = #[[$MMRA_TAG]]}
+  // CHECK: llvm.fence syncscope("workgroup") release {llvm.mmra = #[[$MMRA_TAG]]}
   // GFX908: llvm.inline_asm has_side_effects asm_dialect = att
   // GFX908-SAME: ";;;WARNING: BREAKS DEBUG WATCHES\0As_barrier"
   // GFX90A-NEXT: rocdl.s.barrier
@@ -425,7 +425,7 @@ func.func @lds_barrier() {
   // GFX11-NEXT: rocdl.s.barrier
   // GFX12-NEXT: rocdl.s.barrier.signal -1
   // GFX12-NEXT: rocdl.s.barrier.wait -1
-  // CHECK-NEXT: llvm.fence syncscope("workgroup-one-as") acquire {llvm.mmra = #[[$MMRA_TAG]]}
+  // CHECK-NEXT: llvm.fence syncscope("workgroup") acquire {llvm.mmra = #[[$MMRA_TAG]]}
   amdgpu.lds_barrier
   func.return
 }