gcc hacks: busy_loop and prevent_reorder

README.adoc

@@ -18452,6 +18452,13 @@ west build -b qemu_aarch64 samples/hello_world
 
 The build system of that project is a bit excessive / wonky. You need an edge CMake not present in Ubuntu 18.04, which I don't want to install right now, and it uses the weird custom `west` build tool frontend.
 
+== Compilers
+
+Argh, compilers are boring, let's learn a bit about them.
+
+* link:userland/gcc/busy_loop.c[]: https://stackoverflow.com/questions/7083482/how-to-prevent-gcc-from-optimizing-out-a-busy-wait-loop/58758133#58758133
+* link:userland/gcc/prevent_reorder.cpp[]: https://stackoverflow.com/questions/37786547/enforcing-statement-order-in-c/56865717#56865717
+
 == About this repo
 
 === Supported hosts

userland/gcc/busy_loop.c

@@ -0,0 +1,29 @@
+/* https://cirosantilli.com/linux-kernel-module-cheat#compilers */
+
+#include <stdlib.h>
+
+void __attribute__ ((noinline)) busy_loop(
+    unsigned long long max,
+    unsigned long long max2
+) {
+    for (unsigned i = 0; i < max; i++) {
+        for (unsigned j = 0; j < max2; j++) {
+            __asm__ __volatile__ ("" : "+g" (j), "+g" (j) : :);
+        }
+    }
+}
+
+int main(int argc, char **argv) {
+    unsigned long long max, max2;
+    if (argc > 1) {
+        max = strtoll(argv[1], NULL, 0);
+    } else {
+        max = 1;
+    }
+    if (argc > 2) {
+        max2 = strtoll(argv[2], NULL, 0);
+    } else {
+        max2 = 1;
+    }
+    busy_loop(max, max2);
+}

userland/gcc/prevent_reorder.cpp

@@ -0,0 +1,43 @@
+/* https://cirosantilli.com/linux-kernel-module-cheat#compilers */
+
+#include <chrono>
+#include <iostream>
+#include <string>
+
+// noinline ensures that the ::now() cannot be split from the __asm__
+template <class T>
+__attribute__((noinline)) auto get_clock(T& value) {
+    // Make the compiler think we actually use / modify the value.
+    // It can't "see" what is going on inside the assembly string.
+    __asm__ __volatile__ ("" : "+g" (value));
+    return std::chrono::high_resolution_clock::now();
+}
+
+template <class T>
+static T foo(T niters) {
+    T result = 42;
+    for (T i = 0; i < niters; ++i) {
+        result = (result * result) - (3 * result) + 1;
+    }
+    return result;
+}
+
+int main(int argc, char **argv) {
+    unsigned long long input;
+    if (argc > 1) {
+        input = std::stoull(argv[1], NULL, 0);
+    } else {
+        input = 1;
+    }
+
+    // Must come before because it could modify input
+    // which is passed as a reference.
+    auto t1 = get_clock(input);
+    auto output = foo(input);
+    // Must come after as it could use the output.
+    auto t2 = get_clock(output);
+    std::cout << "output " << output << std::endl;
+    std::cout << "time (ns) "
+              << std::chrono::duration_cast<std::chrono::nanoseconds>(t2 - t1).count()
+              << std::endl;
+}