cpp: move atomic from cpp-cheat

README.adoc

@@ -11791,7 +11791,10 @@ The implementation lives under `libgomp` in the GCC tree, and is documented at:
 
 Programs under link:userland/cpp/[] are examples of link:https://en.wikipedia.org/wiki/C%2B%2B#Standardization[ISO C] programming.
 
+* link:userland/cpp/empty.cpp[]
 * link:userland/cpp/hello.cpp[]
+* `<atomic>` 32 "Atomic operations library"
+** link:userland/cpp/atomic.cpp[]
 
 === POSIX
 

userland/cpp/atomic.cpp

@@ -0,0 +1,52 @@
+// https://github.com/cirosantilli/linux-kernel-module-cheat#atomic
+//
+// More restricted than mutex as it can only protect a few operations on integers.
+//
+// But if that is the use case, may be more efficient.
+//
+// On GCC 4.8 x86-64, using atomic is a huge peformance improvement
+// over the same program with mutexes (5x).
+
+#include <atomic>
+#include <cassert>
+#include <iostream>
+#include <thread>
+#include <vector>
+
+#if __cplusplus >= 201103L
+std::atomic_ulong my_atomic_ulong(0);
+unsigned long my_non_atomic_ulong = 0;
+size_t niters;
+
+void threadMain() {
+    for (size_t i = 0; i < niters; ++i) {
+        my_atomic_ulong++;
+        my_non_atomic_ulong++;
+    }
+}
+#endif
+
+int main(int argc, char **argv) {
+#if __cplusplus >= 201103L
+    size_t nthreads;
+    if (argc > 1) {
+        nthreads = std::stoull(argv[1], NULL, 0);
+    } else {
+        nthreads = 2;
+    }
+    if (argc > 2) {
+        niters = std::stoull(argv[2], NULL, 0);
+    } else {
+        niters = 1000;
+    }
+    std::vector<std::thread> threads(nthreads);
+    for (size_t i = 0; i < nthreads; ++i)
+        threads[i] = std::thread(threadMain);
+    for (size_t i = 0; i < nthreads; ++i)
+        threads[i].join();
+    assert(my_atomic_ulong.load() == nthreads * niters);
+    // Same as above through `operator T`.
+    assert(my_atomic_ulong == nthreads * niters);
+    std::cout << my_non_atomic_ulong << std::endl;
+#endif
+}

userland/cpp/empty.cpp

@@ -1,4 +1,4 @@
 // Sanity checking low level stuff, initially inspired by baremetal.
-// https://github.com/cirosantilli/linux-kernel-module-cheat#baremetal-cpp
+// https://github.com/cirosantilli/linux-kernel-module-cheat#cpp
 
 int main() {}