1==================================
2Unspecified Behavior Randomization
3==================================
4
5Background
6==========
7
8Consider the follow snippet which steadily happens in tests:
9
10
11.. code-block:: cpp
12
13    std::vector<std::pair<int, int>> v(SomeData());
14    std::sort(v.begin(), v.end(), [](const auto& lhs, const auto& rhs) {
15       return lhs.first < rhs.first;
16    });
17
18Under this assumption all elements in the vector whose first elements are equal
19do not guarantee any order. Unfortunately, this prevents libcxx introducing
20other implementatiosn because tests might silently fail and the users might
21heavily depend on the stability of implementations.
22
23Goal
24===================
25
26Provide functionality for randomizing the unspecified behavior so that the users
27can test and migrate their components and libcxx can introduce new sorting
28algorithms and optimizations to the containers.
29
30For example, as of LLVM version 13, libcxx sorting algorithm takes
31`O(n^2) worst case <https://llvm.org/PR20837>`_ but according
32to the standard its worst case should be `O(n log n)`. This effort helps users
33to gradually fix their tests while updating to new faster algorithms.
34
35Design
36======
37
38* Introduce new macro ``_LIBCPP_DEBUG_RANDOMIZE_UNSPECIFIED_STABILITY`` which should
39  be a part of the libcxx config.
40* This macro randomizes the unspecified behavior of algorithms and containers.
41  For example, for sorting algorithm the input range is shuffled and then
42  sorted.
43* This macro is off by default because users should enable it only for testing
44  purposes and/or migrations if they happen to libcxx.
45* This feature is only available for C++11 and further because of
46  ``std::shuffle`` availability.
47* We may use `ASLR <https://en.wikipedia.org/wiki/Address_space_layout_randomization>`_ or
48  static ``std::random_device`` for seeding the random number generator. This
49  guarantees the same stability guarantee within a run but not through different
50  runs, for example, for tests become flaky and eventually be seen as broken.
51  For platforms which do not support ASLR, the seed is fixed during build.
52* The users can fix the seed of the random number generator by providing
53  ``_LIBCPP_RANDOMIZE_UNSPECIFIED_STABILITY_SEED=seed`` definition.
54
55This comes with some side effects if any of the flags is on:
56
57* Computation penalty, we think users are OK with that if they use this feature.
58* Non reproducible results if they don't use the fixed seed.
59
60
61Impact
62------------------
63
64Google has measured couple of thousands of tests to be dependent on the
65stability of sorting and selection algorithms. As we also plan on updating
66(or least, providing under flag more) sorting algorithms, this effort helps
67doing it gradually and sustainably. This is also bad for users to depend on the
68unspecified behavior in their tests, this effort helps to turn this flag in
69debug mode.
70
71Potential breakages
72-------------------
73
74None if the flag is off. If the flag is on, it may lead to some non-reproducible
75results, for example, for caching.
76
77Currently supported randomization
78---------------------------------
79
80* ``std::sort``, there is no guarantee on the order of equal elements
81* ``std::partial_sort``, there is no guarantee on the order of equal elements and
82   on the order of the remaining part
83* ``std::nth_element``, there is no guarantee on the order from both sides of the
84   partition
85
86Patches welcome.
87