From e30f53e8a407a0d2dcbf7ee2a9a4a56bf4a2c5d2 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Ciro=20Santilli=20=E5=85=AD=E5=9B=9B=E4=BA=8B=E4=BB=B6=20?=
 =?UTF-8?q?=E6=B3=95=E8=BD=AE=E5=8A=9F?= <ciro.santilli@gmail.com>
Date: Wed, 26 Jun 2019 00:00:00 +0000
Subject: [PATCH] cpp: move atomic from cpp-cheat

---
 README.adoc             |  3 +++
 userland/cpp/atomic.cpp | 52 +++++++++++++++++++++++++++++++++++++++++
 userland/cpp/empty.cpp  |  2 +-
 3 files changed, 56 insertions(+), 1 deletion(-)
 create mode 100644 userland/cpp/atomic.cpp

diff --git a/README.adoc b/README.adoc
index 5b3018e..c4c9f16 100644
--- a/README.adoc
+++ b/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
 
diff --git a/userland/cpp/atomic.cpp b/userland/cpp/atomic.cpp
new file mode 100644
index 0000000..64a041f
--- /dev/null
+++ b/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
+}
diff --git a/userland/cpp/empty.cpp b/userland/cpp/empty.cpp
index cbd248e..71630b0 100644
--- a/userland/cpp/empty.cpp
+++ b/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() {}