diff --git a/clang/include/clang/CIR/Dialect/IR/CIROps.td b/clang/include/clang/CIR/Dialect/IR/CIROps.td
index 4def17cd38e7..ff2a6d626bca 100644
--- a/clang/include/clang/CIR/Dialect/IR/CIROps.td
+++ b/clang/include/clang/CIR/Dialect/IR/CIROps.td
@@ -6063,6 +6063,68 @@ def CIR_AtomicCmpXchg : CIR_Op<"atomic.cmp_xchg", [
   let hasVerifier = 1;
 }
 
+def CIR_AtomicTestAndSetOp : CIR_Op<"atomic.test_and_set"> {
+  let summary = "Atomic test and set";
+  let description = [{
+    C/C++ atomic test and set operation. Implements the builtin function
+    `__atomic_test_and_set`.
+
+    The operation takes as its only operand a pointer to an 8-bit signed
+    integer. The operation atomically set the integer to an implementation-
+    defined non-zero "set" value. The result of the operation is a boolean value
+    indicating whether the previous value of the integer was the "set" value.
+
+    Example:
+    ```mlir
+      %res = cir.atomic.test_and_set seq_cst %ptr : !cir.ptr<!s8i> -> !cir.bool
+    ```
+  }];
+
+  let arguments = (ins
+    Arg<CIR_PtrToType<CIR_SInt8>, "", [MemRead, MemWrite]>:$ptr,
+    Arg<CIR_MemOrder, "memory order">:$mem_order,
+    OptionalAttr<CIR_MemScopeKind>:$syncscope,
+    OptionalAttr<I64Attr>:$alignment,
+    UnitAttr:$is_volatile);
+
+  let results = (outs CIR_BoolType:$result);
+
+  let assemblyFormat = [{
+    $mem_order $ptr
+    (`volatile` $is_volatile^)?
+    `:` qualified(type($ptr)) `->` qualified(type($result)) attr-dict
+  }];
+}
+
+def CIR_AtomicClearOp : CIR_Op<"atomic.clear"> {
+  let summary = "Atomic clear";
+  let description = [{
+    C/C++ atomic clear operation. Implements the builtin function
+    `__atomic_clear`.
+
+    The operation takes as its only operand a pointer to an 8-bit signed
+    integer. The operation atomically sets the integer to zero.
+
+    Example:
+    ```mlir
+      cir.atomic.clear seq_cst %ptr : !cir.ptr<!s8i>
+    ```
+  }];
+
+  let arguments = (ins
+    Arg<CIR_PtrToType<CIR_SInt8>, "", [MemRead, MemWrite]>:$ptr,
+    Arg<CIR_MemOrder, "memory order">:$mem_order,
+    OptionalAttr<CIR_MemScopeKind>:$syncscope,
+    OptionalAttr<I64Attr>:$alignment,
+    UnitAttr:$is_volatile);
+
+  let assemblyFormat = [{
+    $mem_order $ptr
+    (`volatile` $is_volatile^)?
+    `:` qualified(type($ptr)) attr-dict
+  }];
+}
+
 def CIR_AtomicFence : CIR_Op<"atomic.fence"> {
   let summary = "Atomic thread fence";
   let description = [{
diff --git a/clang/lib/CIR/CodeGen/CIRGenAtomic.cpp b/clang/lib/CIR/CodeGen/CIRGenAtomic.cpp
index be1995ac5aa5..20bde84d1395 100644
--- a/clang/lib/CIR/CodeGen/CIRGenAtomic.cpp
+++ b/clang/lib/CIR/CodeGen/CIRGenAtomic.cpp
@@ -746,11 +746,22 @@ static void emitAtomicOp(CIRGenFunction &CGF, AtomicExpr *E, Address Dest,
                                               cir::AtomicFetchKind::Nand);
     break;
   case AtomicExpr::AO__atomic_test_and_set: {
-    llvm_unreachable("NYI");
+    auto op = cir::AtomicTestAndSetOp::create(
+        builder, loc, Ptr.getPointer(), Order,
+        cir::MemScopeKindAttr::get(&CGF.getMLIRContext(), Scope),
+        builder.getI64IntegerAttr(Ptr.getAlignment().getQuantity()),
+        E->isVolatile());
+    builder.createStore(loc, op, Dest);
+    return;
   }
 
   case AtomicExpr::AO__atomic_clear: {
-    llvm_unreachable("NYI");
+    cir::AtomicClearOp::create(
+        builder, loc, Ptr.getPointer(), Order,
+        cir::MemScopeKindAttr::get(&CGF.getMLIRContext(), Scope),
+        builder.getI64IntegerAttr(Ptr.getAlignment().getQuantity()),
+        E->isVolatile());
+    return;
   }
   }
 
diff --git a/clang/lib/CIR/Lowering/DirectToLLVM/LowerToLLVM.cpp b/clang/lib/CIR/Lowering/DirectToLLVM/LowerToLLVM.cpp
index ce2564d6e031..404e0f52b0f4 100644
--- a/clang/lib/CIR/Lowering/DirectToLLVM/LowerToLLVM.cpp
+++ b/clang/lib/CIR/Lowering/DirectToLLVM/LowerToLLVM.cpp
@@ -3647,6 +3647,41 @@ mlir::LogicalResult CIRToLLVMAtomicFetchLowering::matchAndRewrite(
   return mlir::success();
 }
 
+mlir::LogicalResult CIRToLLVMAtomicTestAndSetOpLowering::matchAndRewrite(
+    cir::AtomicTestAndSetOp op, OpAdaptor adaptor,
+    mlir::ConversionPatternRewriter &rewriter) const {
+  mlir::LLVM::AtomicOrdering llvmOrder = getLLVMAtomicOrder(op.getMemOrder());
+  llvm::StringRef llvmSyncScope =
+      getLLVMSyncScope(adaptor.getSyncscope()).value_or(StringRef());
+
+  auto one = mlir::LLVM::ConstantOp::create(rewriter, op.getLoc(),
+                                            rewriter.getI8Type(), 1);
+  auto rmw = mlir::LLVM::AtomicRMWOp::create(
+      rewriter, op.getLoc(), mlir::LLVM::AtomicBinOp::xchg, adaptor.getPtr(),
+      one, llvmOrder, llvmSyncScope, adaptor.getAlignment().value_or(0),
+      op.getIsVolatile());
+  auto cmp = mlir::LLVM::ICmpOp::create(
+      rewriter, op.getLoc(), mlir::LLVM::ICmpPredicate::ne, one, rmw);
+
+  rewriter.replaceOp(op, cmp);
+  return mlir::success();
+}
+
+mlir::LogicalResult CIRToLLVMAtomicClearOpLowering::matchAndRewrite(
+    cir::AtomicClearOp op, OpAdaptor adaptor,
+    mlir::ConversionPatternRewriter &rewriter) const {
+  // FIXME: add syncscope.
+  mlir::LLVM::AtomicOrdering llvmOrder = getLLVMAtomicOrder(op.getMemOrder());
+  auto zero = mlir::LLVM::ConstantOp::create(rewriter, op.getLoc(),
+                                             rewriter.getI8Type(), 0);
+  auto store = mlir::LLVM::StoreOp::create(
+      rewriter, op.getLoc(), zero, adaptor.getPtr(),
+      adaptor.getAlignment().value_or(0), op.getIsVolatile(),
+      /*isNonTemporal=*/false, /*isInvariantGroup=*/false, llvmOrder);
+  rewriter.replaceOp(op, store);
+  return mlir::success();
+}
+
 mlir::LogicalResult CIRToLLVMAtomicFenceLowering::matchAndRewrite(
     cir::AtomicFence op, OpAdaptor adaptor,
     mlir::ConversionPatternRewriter &rewriter) const {
@@ -4603,8 +4638,10 @@ void populateCIRToLLVMConversionPatterns(
       CIRToLLVMAssumeAlignedOpLowering,
       CIRToLLVMAssumeOpLowering,
       CIRToLLVMAssumeSepStorageOpLowering,
+      CIRToLLVMAtomicClearOpLowering,
       CIRToLLVMAtomicCmpXchgLowering,
       CIRToLLVMAtomicFetchLowering,
+      CIRToLLVMAtomicTestAndSetOpLowering,
       CIRToLLVMAtomicXchgLowering,
       CIRToLLVMAtomicFenceLowering,
       CIRToLLVMBaseClassAddrOpLowering,
diff --git a/clang/lib/CIR/Lowering/DirectToLLVM/LowerToLLVM.h b/clang/lib/CIR/Lowering/DirectToLLVM/LowerToLLVM.h
index f13c37106665..4378da64ec4a 100644
--- a/clang/lib/CIR/Lowering/DirectToLLVM/LowerToLLVM.h
+++ b/clang/lib/CIR/Lowering/DirectToLLVM/LowerToLLVM.h
@@ -936,6 +936,26 @@ class CIRToLLVMAtomicFetchLowering
                   mlir::ConversionPatternRewriter &) const override;
 };
 
+class CIRToLLVMAtomicTestAndSetOpLowering
+    : public mlir::OpConversionPattern<cir::AtomicTestAndSetOp> {
+public:
+  using mlir::OpConversionPattern<cir::AtomicTestAndSetOp>::OpConversionPattern;
+
+  mlir::LogicalResult
+  matchAndRewrite(cir::AtomicTestAndSetOp op, OpAdaptor,
+                  mlir::ConversionPatternRewriter &) const override;
+};
+
+class CIRToLLVMAtomicClearOpLowering
+    : public mlir::OpConversionPattern<cir::AtomicClearOp> {
+public:
+  using mlir::OpConversionPattern<cir::AtomicClearOp>::OpConversionPattern;
+
+  mlir::LogicalResult
+  matchAndRewrite(cir::AtomicClearOp op, OpAdaptor,
+                  mlir::ConversionPatternRewriter &) const override;
+};
+
 class CIRToLLVMAtomicFenceLowering
     : public mlir::OpConversionPattern<cir::AtomicFence> {
 public:
diff --git a/clang/test/CIR/CodeGen/atomic.cpp b/clang/test/CIR/CodeGen/atomic.cpp
index 3575bbf9e713..d033d2a45e07 100644
--- a/clang/test/CIR/CodeGen/atomic.cpp
+++ b/clang/test/CIR/CodeGen/atomic.cpp
@@ -1156,3 +1156,43 @@ extern "C" void test_op_and_fetch(void)
   // LLVM:  store i64 [[RET7]], ptr @ull, align 8
   ull = __sync_nand_and_fetch(&ull, uc);
 }
+
+// CHECK-LABEL: @_Z12test_and_setPvPVv
+// LLVM-LABEL: @_Z12test_and_setPvPVv
+void test_and_set(void *p, volatile void *vp) {
+  bool x = __atomic_test_and_set(p, __ATOMIC_SEQ_CST);
+  // CHECK:      %[[VOID_PTR:.+]] = cir.load align(8) %{{.+}} : !cir.ptr<!cir.ptr<!void>>, !cir.ptr<!void>
+  // CHECK-NEXT: %[[PTR:.+]] = cir.cast bitcast %[[VOID_PTR]] : !cir.ptr<!void> -> !cir.ptr<!s8i>
+  // CHECK:      %{{.+}} = cir.atomic.test_and_set seq_cst %[[PTR]] : !cir.ptr<!s8i> -> !cir.bool
+
+  // LLVM:      %[[PTR:.+]] = load ptr, ptr %{{.+}}, align 8
+  // LLVM-NEXT: %[[RES:.+]] = atomicrmw xchg ptr %[[PTR]], i8 1 seq_cst, align 1
+  // LLVM-NEXT: %{{.+}} = icmp ne i8 1, %[[RES]]
+
+  bool y = __atomic_test_and_set(vp, __ATOMIC_SEQ_CST);
+  // CHECK:      %[[VOID_PTR:.+]] = cir.load align(8) %{{.+}} : !cir.ptr<!cir.ptr<!void>>, !cir.ptr<!void>
+  // CHECK-NEXT: %[[PTR:.+]] = cir.cast bitcast %[[VOID_PTR]] : !cir.ptr<!void> -> !cir.ptr<!s8i>
+  // CHECK:      %{{.+}} = cir.atomic.test_and_set seq_cst %[[PTR]] volatile : !cir.ptr<!s8i> -> !cir.bool
+
+  // LLVM:      %[[PTR:.+]] = load ptr, ptr %{{.+}}, align 8
+  // LLVM-NEXT: %[[RES:.+]] = atomicrmw volatile xchg ptr %[[PTR]], i8 1 seq_cst, align 1
+  // LLVM-NEXT: %{{.+}} = icmp ne i8 1, %[[RES]]
+}
+
+// CHECK-LABEL: @_Z5clearPvPVv
+// LLVM-LABEL: @_Z5clearPvPVv
+void clear(void *p, volatile void *vp) {
+  __atomic_clear(p, __ATOMIC_SEQ_CST);
+  // CHECK:      %[[VOID_PTR:.+]] = cir.load align(8) %{{.+}} : !cir.ptr<!cir.ptr<!void>>, !cir.ptr<!void>
+  // CHECK-NEXT: %[[PTR:.+]] = cir.cast bitcast %[[VOID_PTR]] : !cir.ptr<!void> -> !cir.ptr<!s8i>
+  // CHECK:      cir.atomic.clear seq_cst %[[PTR]] : !cir.ptr<!s8i>
+
+  // LLVM: store atomic i8 0, ptr %{{.+}} seq_cst, align 1
+
+  __atomic_clear(vp, __ATOMIC_SEQ_CST);
+  // CHECK:      %[[VOID_PTR:.+]] = cir.load align(8) %{{.+}} : !cir.ptr<!cir.ptr<!void>>, !cir.ptr<!void>
+  // CHECK-NEXT: %[[PTR:.+]] = cir.cast bitcast %[[VOID_PTR]] : !cir.ptr<!void> -> !cir.ptr<!s8i>
+  // CHECK:      cir.atomic.clear seq_cst %[[PTR]] volatile : !cir.ptr<!s8i>
+
+  // LLVM: store atomic volatile i8 0, ptr %{{.+}} seq_cst, align 1
+}