[c++] Implement all atomics for AVR and Cortex-M

ext/gcc/atomic

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+/*
+ * Copyright (c) 2024, Niklas Hauser
+ *
+ * This file is part of the modm project.
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ */
+// ----------------------------------------------------------------------------
+
+#include_next <atomic>
+
+#include <modm_atomic.hpp>

ext/gcc/atomic.cpp.in

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+/*
+ * Copyright (c) 2020, 2024, Niklas Hauser
+ *
+ * This file is part of the modm project.
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ */
+// ----------------------------------------------------------------------------
+
+#include <modm_atomic.hpp>
+
+/* We are implementing the libary interface described here:
+ * See https://gcc.gnu.org/wiki/Atomic/GCCMM/LIbrary
+ *
+ * We ignore the memory order, since the runtime switching takes longer than
+ * the DMB instruction.
+ */
+
+// ============================ atomics for arrays ============================
+// These functions cannot be inlined, since the compiler builtins are named the
+// same. Terrible design really.
+extern "C" void
+__atomic_load(unsigned int size, const volatile void *src, void *dest, int /*memorder*/)
+{
+	__modm_atomic_pre_barrier(__ATOMIC_SEQ_CST);
+	{
+		modm::atomic::Lock _;
+		__builtin_memcpy(dest, (const void*)src, size);
+	}
+	__modm_atomic_post_barrier(__ATOMIC_SEQ_CST);
+}
+
+extern "C" void
+__atomic_store(unsigned int size, volatile void *dest, void *src, int /*memorder*/)
+{
+	__modm_atomic_pre_barrier(__ATOMIC_SEQ_CST);
+	{
+		modm::atomic::Lock _;
+		__builtin_memcpy((void*)dest, src, size);
+	}
+	__modm_atomic_post_barrier(__ATOMIC_SEQ_CST);
+}
+
+extern "C" void
+__atomic_exchange(unsigned int size, volatile void *ptr, void *val, void *ret, int /*memorder*/)
+{
+	__modm_atomic_pre_barrier(__ATOMIC_SEQ_CST);
+	{
+		modm::atomic::Lock _;
+		__builtin_memcpy(ret, (void*)ptr, size);
+		__builtin_memcpy((void*)ptr, val, size);
+	}
+	__modm_atomic_post_barrier(__ATOMIC_SEQ_CST);
+}
+
+extern "C" bool
+__atomic_compare_exchange(unsigned int len, volatile void *ptr, void *expected, void *desired,
+						  int /*success_memorder*/, int /*failure_memorder*/)
+{
+	bool retval{false};
+	__modm_atomic_pre_barrier(__ATOMIC_SEQ_CST);
+	{
+		modm::atomic::Lock _;
+		if (__builtin_memcmp((void*)ptr, expected, len) == 0) [[likely]]
+		{
+			__builtin_memcpy((void*)ptr, desired, len);
+			retval = true;
+		}
+		else __builtin_memcpy(expected, (void*)ptr, len);
+	}
+	__modm_atomic_post_barrier(__ATOMIC_SEQ_CST);
+	return retval;
+}
+
+%% macro atomic_fetch(len)
+	%% for name, op in [("add", "+"), ("sub", "-")]
+extern "C" {{len|u}}
+__atomic_fetch_{{name}}_{{len//8}}(volatile void *ptr, {{len|u}} value, int /*memorder*/)
+{
+	{{len|u}} previous{};
+	__modm_atomic_pre_barrier(__ATOMIC_SEQ_CST);
+	{
+		modm::atomic::Lock _;
+		previous = *reinterpret_cast<volatile {{len|u}}*>(ptr);
+		*reinterpret_cast<volatile {{len|u}}*>(ptr) = (previous {{op}} value);
+	}
+	__modm_atomic_post_barrier(__ATOMIC_SEQ_CST);
+	return previous;
+}
+	%% endfor
+%% endmacro
+
+%% for length in bit_lengths
+// ========================= atomics for {{length}} bit integers =========================
+// These functions cannot be inlined since the compiler refuses to find these
+// functions even if they are declared right at the call site. Unclear why.
+{{ atomic_fetch(length) }}
+%% endfor