[Flang][OpenMP] Fix update operation not found issue (#92165)

If there is only one non-terminator operation in the update region then
the update operation can be found and we can try to generate an
atomicrmw instruction. Otherwise use the cmpxchg loop.

Fixes #91929

Author: kiranchandramohan

mlir/lib/Target/LLVMIR/Dialect/OpenMP/OpenMPToLLVMIRTranslation.cpp

@@ -1623,31 +1623,33 @@ convertOmpAtomicUpdate(omp::AtomicUpdateOp &opInst,
 
   // Convert values and types.
   auto &innerOpList = opInst.getRegion().front().getOperations();
-  bool isRegionArgUsed{false}, isXBinopExpr{false};
+  bool isXBinopExpr{false};
   llvm::AtomicRMWInst::BinOp binop;
   mlir::Value mlirExpr;
-  // Find the binary update operation that uses the region argument
-  // and get the expression to update
-  for (Operation &innerOp : innerOpList) {
-    if (innerOp.getNumOperands() == 2) {
-      binop = convertBinOpToAtomic(innerOp);
-      if (!llvm::is_contained(innerOp.getOperands(),
-                              opInst.getRegion().getArgument(0)))
-        continue;
-      isRegionArgUsed = true;
-      isXBinopExpr = innerOp.getNumOperands() > 0 &&
-                     innerOp.getOperand(0) == opInst.getRegion().getArgument(0);
-      mlirExpr = (isXBinopExpr ? innerOp.getOperand(1) : innerOp.getOperand(0));
-      break;
+  llvm::Value *llvmExpr = nullptr;
+  llvm::Value *llvmX = nullptr;
+  llvm::Type *llvmXElementType = nullptr;
+  if (innerOpList.size() == 2) {
+    // The two operations here are the update and the terminator.
+    // Since we can identify the update operation, there is a possibility
+    // that we can generate the atomicrmw instruction.
+    mlir::Operation &innerOp = *opInst.getRegion().front().begin();
+    if (!llvm::is_contained(innerOp.getOperands(),
+                            opInst.getRegion().getArgument(0))) {
+      return opInst.emitError("no atomic update operation with region argument"
+                              " as operand found inside atomic.update region");
     }
+    binop = convertBinOpToAtomic(innerOp);
+    isXBinopExpr = innerOp.getOperand(0) == opInst.getRegion().getArgument(0);
+    mlirExpr = (isXBinopExpr ? innerOp.getOperand(1) : innerOp.getOperand(0));
+    llvmExpr = moduleTranslation.lookupValue(mlirExpr);
+  } else {
+    // Since the update region includes more than one operation
+    // we will resort to generating a cmpxchg loop.
+    binop = llvm::AtomicRMWInst::BinOp::BAD_BINOP;
   }
-  if (!isRegionArgUsed)
-    return opInst.emitError("no atomic update operation with region argument"
-                            " as operand found inside atomic.update region");
-
-  llvm::Value *llvmExpr = moduleTranslation.lookupValue(mlirExpr);
-  llvm::Value *llvmX = moduleTranslation.lookupValue(opInst.getX());
-  llvm::Type *llvmXElementType = moduleTranslation.convertType(
+  llvmX = moduleTranslation.lookupValue(opInst.getX());
+  llvmXElementType = moduleTranslation.convertType(
       opInst.getRegion().getArgument(0).getType());
   llvm::OpenMPIRBuilder::AtomicOpValue llvmAtomicX = {llvmX, llvmXElementType,
                                                       /*isSigned=*/false,

mlir/test/Target/LLVMIR/openmp-llvm.mlir

@@ -1467,6 +1467,28 @@ llvm.func @omp_atomic_update_intrinsic(%x:!llvm.ptr, %expr: i32) {
 
 // -----
 
+// CHECK-LABEL: @atomic_update_cmpxchg
+// CHECK-SAME: (ptr %[[X:.*]], ptr %[[EXPR:.*]]) {
+// CHECK:  %[[AT_LOAD_VAL:.*]] = load atomic i32, ptr %[[X]] monotonic, align 4
+// CHECK:  %[[LOAD_VAL_PHI:.*]] = phi i32 [ %[[AT_LOAD_VAL]], %entry ], [ %[[LOAD_VAL:.*]], %.atomic.cont ]
+// CHECK:  %[[VAL_SUCCESS:.*]] = cmpxchg ptr %[[X]], i32 %[[LOAD_VAL_PHI]], i32 %{{.*}} monotonic monotonic, align 4
+// CHECK:  %[[LOAD_VAL]] = extractvalue { i32, i1 } %[[VAL_SUCCESS]], 0
+// CHECK:  br i1 %{{.*}}, label %.atomic.exit, label %.atomic.cont
+
+llvm.func @atomic_update_cmpxchg(%arg0: !llvm.ptr, %arg1: !llvm.ptr) {
+  %0 = llvm.load %arg1 : !llvm.ptr -> f32
+  omp.atomic.update %arg0 : !llvm.ptr {
+  ^bb0(%arg2: i32):
+    %1 = llvm.sitofp %arg2 : i32 to f32
+    %2 = llvm.fadd %1, %0 : f32
+    %3 = llvm.fptosi %2 : f32 to i32
+    omp.yield(%3 : i32)
+  }
+  llvm.return
+}
+
+// -----
+
 // CHECK-LABEL: @omp_atomic_capture_prefix_update
 // CHECK-SAME: (ptr %[[x:.*]], ptr %[[v:.*]], i32 %[[expr:.*]], ptr %[[xf:.*]], ptr %[[vf:.*]], float %[[exprf:.*]])
 llvm.func @omp_atomic_capture_prefix_update(