From 7b74328603db35958badff31a52f4def871e29ef 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: Mon, 2 Dec 2019 00:00:00 +0000
Subject: [PATCH] c++ parallel sort algorithm example

---
 README.adoc                    |  7 +++++
 path_properties.py             |  7 +++++
 userland/cpp/parallel_sort.cpp | 51 ++++++++++++++++++++++++++++++++++
 userland/cpp/random.cpp        |  3 +-
 4 files changed, 67 insertions(+), 1 deletion(-)
 create mode 100644 userland/cpp/parallel_sort.cpp

diff --git a/README.adoc b/README.adoc
index 80de56d..1d48c69 100644
--- a/README.adoc
+++ b/README.adoc
@@ -14020,6 +14020,13 @@ Bibliography:
 
 * https://stackoverflow.com/questions/31978324/what-exactly-is-stdatomic/58904448#58904448 "What exactly is std::atomic?"
 
+[[cpp-parallel-algorithms]]
+===== C++ parallel algorithms
+
+https://stackoverflow.com/questions/51031060/are-c17-parallel-algorithms-implemented-already/55989883#55989883
+
+link:userland/cpp/parallel_sort.cpp[]
+
 [[cpp-standards]]
 ==== C++ standards
 
diff --git a/path_properties.py b/path_properties.py
index 5203459..37e1d53 100644
--- a/path_properties.py
+++ b/path_properties.py
@@ -56,6 +56,7 @@ class PathProperties:
         # For some reason QEMU fails with SIGSEGV on int syscalls in x86_64.
         'qemu_x86_64_int_syscall': False,
         'interactive': False,
+        'minimum_gcc_version': (0, 0, 0),
         # The script takes a perceptible amount of time to run. Possibly an infinite loop.
         'more_than_1s': False,
         # The path should not be built. E.g., it is symlinked into multiple archs.
@@ -153,6 +154,9 @@ class PathProperties:
         },
     }
 
+    # TODO maybe extract automatically from GCC executable?
+    current_gcc_version = (7, 3, 0)
+
     '''
     Encodes properties of userland and baremetal paths.
     For directories, it applies to all files under the directory.
@@ -227,6 +231,8 @@ class PathProperties:
                     'cpus' in self['test_run_args'] and
                     self['test_run_args']['cpus'] > 1
                 )
+            ) and not (
+                self['minimum_gcc_version'] > self.current_gcc_version
             )
         )
 
@@ -631,6 +637,7 @@ path_properties_tuples = (
                             },
                         ),
                         'count.cpp': {'more_than_1s': True},
+                        'parallel_sort.cpp': {'minimum_gcc_version': (9, 0, 0)},
                         'sleep_for.cpp': {
                             'more_than_1s': True,
                         },
diff --git a/userland/cpp/parallel_sort.cpp b/userland/cpp/parallel_sort.cpp
new file mode 100644
index 0000000..b782594
--- /dev/null
+++ b/userland/cpp/parallel_sort.cpp
@@ -0,0 +1,51 @@
+// https://cirosantilli.com/linux-kernel-module-cheat#cpp-parallel-algorithms
+
+#include <algorithm>
+#include <cassert>
+#include <chrono>
+#include <execution>
+#include <random>
+#include <iostream>
+#include <vector>
+
+int main(int argc, char **argv) {
+    using clk = std::chrono::high_resolution_clock;
+    decltype(clk::now()) start, end;
+    std::vector<unsigned long long> input_parallel, input_serial;
+    unsigned int seed;
+    unsigned long long n;
+
+    // CLI arguments;
+    std::uniform_int_distribution<uint64_t> zero_ull_max(0);
+    if (argc > 1) {
+        n = std::strtoll(argv[1], NULL, 0);
+    } else {
+        n = 10;
+    }
+    if (argc > 2) {
+        seed = std::stoi(argv[2]);
+    } else {
+        seed = std::random_device()();
+    }
+
+    // Calculate random inpu.
+    std::mt19937 prng(seed);
+    for (unsigned long long i = 0; i < n; ++i) {
+        input_parallel.push_back(zero_ull_max(prng));
+    }
+    input_serial = input_parallel;
+
+    // Sort and time parallel.
+    start = clk::now();
+    std::sort(std::execution::par_unseq, input_parallel.begin(), input_parallel.end());
+    end = clk::now();
+    std::cout << "parallel " << std::chrono::duration<float>(end - start).count() << " s" << std::endl;
+
+    // Sort and time serial.
+    start = clk::now();
+    std::sort(std::execution::seq, input_serial.begin(), input_serial.end());
+    end = clk::now();
+    std::cout << "serial   " << std::chrono::duration<float>(end - start).count() << " s" << std::endl;
+
+    assert(input_parallel == input_serial);
+}
diff --git a/userland/cpp/random.cpp b/userland/cpp/random.cpp
index 742d582..9bc8ece 100644
--- a/userland/cpp/random.cpp
+++ b/userland/cpp/random.cpp
@@ -6,7 +6,8 @@
 int main(int argc, char **argv) {
     // Potentially expensive /dev/urandom read that waits for entropy.
     std::random_device dev;
-    std::mt19937 prng(dev());
+    auto seed = dev();
+    std::mt19937 prng(seed);
     std::uniform_int_distribution<uint64_t> zero_ull_max(0);
     std::uniform_int_distribution<uint8_t> one_to_six(1, 6);