cpp: Support C mocked __atomic_compare_exchange_*

The builtin functions __atomic_compare_exchange_* are missing when
CONFIG_ATOMIC_OPERATIONS_C is set. Add them and verify they build/work
as expected.

Signed-off-by: Yuval Peress <peress@google.com>

lib/cpp/abi/CMakeLists.txt

@@ -3,3 +3,6 @@
 zephyr_sources_ifdef(CONFIG_STATIC_INIT_GNU
   cpp_dtors.c
 )
+zephyr_sources_ifdef(CONFIG_ATOMIC_OPERATIONS_C
+  cpp_atomics.c
+)

lib/cpp/abi/cpp_atomics.c

@@ -0,0 +1,51 @@
+/*
+ * Copyright (c) 2025 Google LLC
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/**
+ * @file
+ * @brief C-based implementation of GCC __atomic built-ins.
+ *
+ * This file provides a fallback implementation for the C++ atomic functions
+ * required by the compiler on architectures that do not have native atomic
+ * instructions and are using the generic C implementation of atomics.
+ * All operations are made atomic by using a global interrupt lock.
+ */
+
+#include <zephyr/kernel.h>
+#include <stdint.h>
+
+/*
+ * Note on memory ordering:
+ * The `memorder` parameters are ignored because the irq_lock() provides
+ * a full memory barrier, which is equivalent to the strongest memory order,
+ * __ATOMIC_SEQ_CST. This is always safe.
+ */
+
+/* === compare_exchange ======================================================= */
+
+#define DEFINE_ATOMIC_COMPARE_EXCHANGE(n, type)                                                    \
+	bool __atomic_compare_exchange_##n(volatile void *ptr, void *expected, type desired,       \
+					   bool weak, int success, int failure)                    \
+	{                                                                                          \
+		bool ret = false;                                                                  \
+		unsigned int key = irq_lock();                                                     \
+		volatile type *p = ptr;                                                            \
+		type *e = expected;                                                                \
+                                                                                                   \
+		if (*p == *e) {                                                                    \
+			*p = desired;                                                              \
+			ret = true;                                                                \
+		} else {                                                                           \
+			*e = *p;                                                                   \
+			ret = false;                                                               \
+		}                                                                                  \
+		irq_unlock(key);                                                                   \
+		return ret;                                                                        \
+	}
+
+DEFINE_ATOMIC_COMPARE_EXCHANGE(1, uint8_t)
+DEFINE_ATOMIC_COMPARE_EXCHANGE(2, uint16_t)
+DEFINE_ATOMIC_COMPARE_EXCHANGE(4, uint32_t)

tests/lib/cpp/atomics_c/CMakeLists.txt

@@ -0,0 +1,8 @@
+# Copyright (c) 2025 Google LLC
+# SPDX-License-Identifier: Apache-2.0
+
+cmake_minimum_required(VERSION 3.20.0)
+find_package(Zephyr REQUIRED HINTS $ENV{ZEPHYR_BASE})
+project(atomics_c)
+
+target_sources(app PRIVATE main.cpp)

tests/lib/cpp/atomics_c/main.cpp

@@ -0,0 +1,102 @@
+/*
+ * Copyright (c) 2025 Google LLC
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <atomic>
+#include <cstdint>
+#include <zephyr/ztest.h>
+
+namespace
+{
+std::atomic<uint8_t> atomic_u8;
+std::atomic<uint16_t> atomic_u16;
+std::atomic<uint32_t> atomic_u32;
+} // namespace
+
+/**
+ * @brief This `before` function is run before each test in the suite.
+ *
+ * It ensures that all atomic variables are reset to a known state (0)
+ * so that the tests are independent and repeatable.
+ */
+static void cxx_atomic_before(void *fixture)
+{
+	atomic_u8.store(0);
+	atomic_u16.store(0);
+	atomic_u32.store(0);
+}
+
+/**
+ * @brief Tests the 1-byte (uint8_t) atomic implementation.
+ */
+ZTEST(cxx_atomic, test_u8_compare_exchange_weak)
+{
+	/* === Test 1: Successful exchange === */
+	/* We expect the value to be 0, so this exchange should succeed. */
+	uint8_t expected = 0;
+	const uint8_t desired = 42;
+
+	/* Loop until the weak exchange succeeds. */
+	while (!atomic_u8.compare_exchange_weak(expected, desired)) {
+	}
+	zassert_equal(atomic_u8.load(), desired, "Value should have been updated to 42");
+
+	/* === Test 2: Failed exchange === */
+	/* Now, the atomic value is 42. We will set `expected` to 0, so the exchange must fail. */
+	expected = 0;
+	const uint8_t new_desired = 99;
+	bool success = atomic_u8.compare_exchange_weak(expected, new_desired);
+
+	zassert_false(success, "Exchange should have failed");
+	zassert_equal(atomic_u8.load(), desired, "Value should remain 42");
+	/* Crucially, `expected` should be updated with the value that caused the failure. */
+	zassert_equal(expected, desired, "Expected should be updated to 42");
+}
+
+/**
+ * @brief Tests the 2-byte (uint16_t) atomic implementation.
+ */
+ZTEST(cxx_atomic, test_u16_compare_exchange_weak)
+{
+	/* === Test 1: Successful exchange === */
+	uint16_t expected = 0;
+	const uint16_t desired = 1337;
+	while (!atomic_u16.compare_exchange_weak(expected, desired)) {
+	}
+	zassert_equal(atomic_u16.load(), desired, "Value should have been updated to 1337");
+
+	/* === Test 2: Failed exchange === */
+	expected = 0;
+	const uint16_t new_desired = 9999;
+	bool success = atomic_u16.compare_exchange_weak(expected, new_desired);
+
+	zassert_false(success, "Exchange should have failed");
+	zassert_equal(atomic_u16.load(), desired, "Value should remain 1337");
+	zassert_equal(expected, desired, "Expected should be updated to 1337");
+}
+
+/**
+ * @brief Tests the 4-byte (uint32_t) atomic implementation.
+ */
+ZTEST(cxx_atomic, test_u32_compare_exchange_weak)
+{
+	/* === Test 1: Successful exchange === */
+	uint32_t expected = 0;
+	const uint32_t desired = 0xDEADBEEF;
+	while (!atomic_u32.compare_exchange_weak(expected, desired)) {
+	}
+	zassert_equal(atomic_u32.load(), desired, "Value should have been updated to 0xDEADBEEF");
+
+	/* === Test 2: Failed exchange === */
+	expected = 0;
+	const uint32_t new_desired = 0x12345678;
+	bool success = atomic_u32.compare_exchange_weak(expected, new_desired);
+
+	zassert_false(success, "Exchange should have failed");
+	zassert_equal(atomic_u32.load(), desired, "Value should remain 0xDEADBEEF");
+	zassert_equal(expected, desired, "Expected should be updated to 0xDEADBEEF");
+}
+
+ZTEST_SUITE(cxx_atomic, nullptr, nullptr, cxx_atomic_before, nullptr, nullptr);

tests/lib/cpp/atomics_c/prj.conf

@@ -0,0 +1,12 @@
+# Copyright (c) 2025 Google LLC
+# SPDX-License-Identifier: Apache-2.0
+
+# C++ support
+# ===========
+CONFIG_CPP=y
+CONFIG_STD_CPP20=y
+CONFIG_REQUIRES_FULL_LIBCPP=y
+
+# Test requirements
+# =================
+CONFIG_ZTEST=y

tests/lib/cpp/atomics_c/testcase.yaml

@@ -0,0 +1,12 @@
+# Copyright (c) 2025 Google LLC
+# SPDX-License-Identifier: Apache-2.0
+
+tests:
+  cpp.atomics_c:
+    tags: cpp kernel
+    arch_exclude: posix
+    build_only: true
+    filter: CONFIG_ATOMIC_OPERATIONS_C
+    integration_platforms:
+      - rpi_pico
+      - robokit1