Saturday, December 22, 2018

When performance guarantees hurts performance - std::visit

The performance of std::visit came up in a discussion, and my first thought was that from reading generated assembly code, it's a jump table, so it should be pretty fast. But then it dawned on me, all examples I've seen have been jumps to functions. The compiler has not been able to inline the visitor. This is, of course, of no consequence if your visitor does any work worth speaking of, but if the visitor does trivial work, like incrementing the given variable, then it may be important if done in a hot path.

Looking at the (draft) standard, I found this on std::visit(visitor, variant...):

Click the image to read the (draft) std on eel.is
n, here, is the number of variants in the call to std::visit.

I believe the only way to achieve this is to implement visit more or less like this:

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
namespace detail {
template <std::size_t ... I, typename F, typename V>
auto visit(std::index_sequence<I...>, F&& f, V&& v)
{
    using ftype = void (*)(F&&, V&&);
    static constexpr ftype funcs [] = {
        [](F&& f, V&& v){ return f(std::get<I>(std::forward<V>(v)));}...
    };
    if (v.index() == std::variant_npos) throw std::bad_variant_access{};
    return funcs[v.index()](std::forward<F>(f), std::forward<V>(v));
}
}
template <typename F, typename V>
auto visit(F&& f, V&& v)
{
    using Vb = std::remove_cv_t<std::remove_reference_t<V>>;
    constexpr auto size = std::variant_size_v<Vb>;
    return detail::visit(std::make_index_sequence<size>{},std::forward<F>(f), std::forward<V>(v));
}

A table of functions is created at compile time, lines 5-8. At line 9 a check is made if the variant is invalid by exception, and if not, at line 10, the call is made via the jump table. In this case I've simplified the implementation to only work with visitors that do not return any value.

Unfortunately, it is extremely difficult for the compiler to inline the calls that go via the table. Look at the example on the compiler explorer godbolt.org. It is also a possible pessimization to have to check for the valueless by exception state first.

I began to wonder if it would be better if I ignore the O(1) requirement and let the compiler generate trivial if-else tests, since it can probably inline them. How many alternative types can the variant hold before the jump table becomes faster?

My idea was to use templates to generate code that compares the value of .index() on each variant, with the possible indexes, and make the call when it matches. So, if we imagine a call to visit, using two variants variant<int,char> v1 and variant<bool,void*,char> v2, then the index space is a 2x3 rectangle, since v1 has two valid indexes, and v3 has 3. The desired generated code should be something like this:


 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
template <typename F>
auto visit(F f, V1 v1, V2 v2)
{
  if (       v1.index() == 0 && v2.index() == 0) {
    return      f(std::get<0>(v1),   std::get<0>(v2));
  } else if (v1.index() == 1 && v2.index() == 0) {
    return      f(std::get<1>(v1),   std::get<0>(v2));
  } else if (v1.index() == 0 && v2.index() == 1) {
    return      f(std::get<0>(v1),   std::get<1>(v2));
  } else if (v1.index() == 1 && v2.index() == 1) {
    return      f(std::get<1>(v1),   std::get<1>(v2));
  } else if (v1.index() == 0 && v2.index() == 2) {
    return      f(std::get<0>(v1),   std::get<2>(v2));
  } else if (v1.index() == 1 && v2.index() == 2) {
    return      f(std::get<1>(v1),   std::get<2>(v2));
  } else {
    throw std::bad_variant_access{};
  }
}

Doing this requires some helpers dealing with index sequences:

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
template<std::size_t I, std::size_t ... Is>
constexpr
auto
prepend(std::index_sequence<Is...>) {
  return std::index_sequence<I, Is...>{};
}

constexpr
std::index_sequence<>
next_seq(std::index_sequence<>, std::index_sequence<>) {
  return {};
}

template<
  std::size_t I, std::size_t ... Is,
  std::size_t J, std::size_t ... Js
>
constexpr
auto
next_seq(std::index_sequence<I, Is...>, std::index_sequence<J, Js...>) {
  if constexpr (I + 1 == J) {
    return prepend<0>(next_seq(std::index_sequence<Is...>{},
                               std::index_sequence<Js...>{}));
  } else {
    return std::index_sequence<I + 1, Is...>{};
  }
}

The type function next_seq takes a current index sequence and an upper limit for each index. Calls evaluates to:

next_seq(<0,0>,<2,3>)  ->  <1,0>
next_seq(<1,0>,<2,3>) -> <0,1>
next_seq(<0,1>,<2,3>) -> <1,1>
next_seq(<1,1>,<2,3>) -> <0,2>
next_seq(<0,2>,<2,3>) -> <1,2>
next_seq(<1,2>,<2,3>) -> <0,0>

The meat of the visit implementation can now be written:


 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
template<std::size_t ... I>
static
constexpr
std::size_t
sum(std::index_sequence<I...>) { return (I + ...); }

template<
  std::size_t ... Is,
  std::size_t ... Ms,
  typename F,
  typename ... Vs
>
constexpr
auto
visit(
  std::index_sequence<Is...> i,
  std::index_sequence<Ms...> m,
  F &&f,
  Vs &&... vs)
{
  constexpr auto n = next_seq(i, m);
  if (std::tuple(vs.index()...) == std::tuple(Is...)) {
    return f(std::get<Is>(std::forward<Vs>(vs))...);
  }
  if constexpr (sum(n) > 0) {
    return visit(n, m, std::forward<F>(f), std::forward<Vs>(vs)...);
  } else {
    throw std::bad_variant_access{};
  }
}

It's a compile time recursion over all possible indexes. Line 22 checks if the tuples current indexes matches the indexes of the recursion. If they do the call to the visitor function is made on line 23. The sum function, on lines 1-5 are a convenience to make the condition for recursion easier (line 25. If the sum of all indexes in the next sequence is 0, then it has wrapped.) If no match was found anywhere, at least one of the variants must have been invalid by exception, so an exception is thrown.

The real visit function is a wrapper that provides the index sequences:


 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
template<typename>
static constexpr std::size_t zero = 0;

template <typename T>
using remove_cv_ref_t = std::remove_const_t<std::remove_reference_t<T>>;

template <typename F, typename ... Vs>
auto visit(F&& f, Vs&& ... vs)
{
  return detail::visit(
    std::index_sequence<zero<Vs>...>{},
    std::index_sequence<std::variant_size_v<remove_cv_ref_t<Vs>>...>{},
    std::forward<F>(f),
    std::forward<Vs>(vs)...);
}

cppinsights.io does a reasonable job of showing how the compiler sees the code after template expansion.

Using this version of visit on the first example where the visitor does nothing, generates a function that does nothing, because now the compiler can inline the visitor. Here's a link to the godbolt.org compiler explorer.

So, how fast is this when trivial work is actually done?

The graph above shows the time in µs to count ones in a vector of 5000 elements. The X-axis is the number of types is the variant. The measurement is made with clang-7 using the flags '-O3 -std=c++17 -stdlib=libc++' on this machine:

Intel(R) Core(TM) i7 CPU       X 980  @ 3.33GHz
CPU Caches:
  L1 Data 32K (x6)
  L1 Instruction 32K (x6)
  L2 Unified 256K (x6)
  L3 Unified 12288K (x1)

Other compilers and standard library implementations shows similar results.

I think there are two interesting observations to be made from the graph. One is that the standard library implementation isn't strictly speaking O(1). It is substantially faster for very small numbers of types in the variant. The other is that for very simple visitors, it really pays to let the compiler be able to inline the call.

If you want to play with this, or shoot it down, the implementation and benchmark is available from github.com.

Thursday, November 29, 2018

How to speak at a conference

A former colleague of mine recently described the steps to speak at a conference as:

* Write a proposal and (optionally) a talk outline.
* Get accepted by the conference organisers.
* Write the talk.
* Deliver the talk.

Cool!

This is not wrong, by the way.

But... how do you get accepted? I'm sorry, but I don't have any great advice here, but I can tell you what little I know from my limited experience.

First, and very important thing to keep in mind: You're not being rejected, you're just not being accepted. Not being accepted means that competition is fierce, and other talks, on your chosen topic, were deemed more interesting than yours, or that your chosen topic did not quite match the conference. Not being accepted doesn't mean you suck. It means other talks were considered more interesting, and there's no shame in that. It obviously doesn't feel good if your proposal wasn't accepted, but keep at it, refine your proposal, and try again. Maybe with the same topic from another angle, or perhaps another topic, perhaps another conference?

Mind that not all conferences are alike, so there are differences, but the broad pictures is likely more or less the same.

Let's start with two examples.

First the abstract. This is what the committee decides from, and also what's visible to the attendees to the conference. The first example (so far two Nay and no Yay):


The Curiously Recurring Coupled Types Pattern.
Why can pointers be subtracted but not added? What do raw C pointers, STL
iterators, std::chrono types, and 2D/3D geometric primitives have in
common? In this talk we will present some curiously coupled data types that
frequently occur in your programs, together forming notions that you are
already intuitively familiar with. We will shine a light on the
mathematical notion of Affine Spaces, and how they guide stronger design.
We will review the properties of affine spaces and show how they improve
program semantics, stronger type safety and compile time enforcement of
these semantics. By showing motivational examples, we will introduce you to
the mathematical notion of affine spaces. The main focus will then be on
how affine space types and their well defined semantics shape expressive
APIs. We will give examples and guidelines for creating your own affine
types.


And the accompanying outline, as envisioned by the time of the submission. This outline is preliminary, and you won't be held accountable for it. It's an aid for the conference committee to decide. The conference committee knows that the talk is not written yet; that this is an idea for a talk. I find that writing the outline also helps with figuring out a structure for the talk:
Show familiar examples of situations of affine space semantics - pointer
arithmetic - iterators - chrono - coordinate systems 

  • Mathematical definitions/properties 
  • Describe properties of affine space types - operators and relations - [show a Concept for affine space types, tbd] 
  • Show how to write a simple affine space strong type template. 
  • Parallels to unit systems


Here's the abstract for another talk that was accepted:

Programming with Contracts in C++20 
Design by Contract is a technique to clearly express which parts of a
program has which responsibilities. In the case of bugs, contracts can
mercilessly point a finger at the part that violated the contract.
Contracts are coming as a language feature in C++20. I will show how they
can make your interfaces clearer with regards to use, but also point out
pitfalls and oddities in how contracts are handled in C++. Writing good
contracts can be difficult. I intend to give you guidance in how to think
when formulating contracts, and how to design your interfaces such that
good contracts can be written, because contracts have effects on interface
design. Warning: Parts of the contents are personal opinion.

and its outline:

Brief intro to the ideas behind Design by Contract Show what the current
draft standard supports, including strengths, weaknesses and missing
features. Propose rules of thumb for "best practices" with contracts in
C++20. Show some gotchas to look out for.
Is there a take away message from this?

I think there are two take away messages.

  1. It pays to think about how you want your talk to look like in the conference programme. This is difficult, and (at least for me) takes a disproportionate amount of time. It's only 100 or so words, after all, but expressing an idea very briefly is very hard.
  2. It also takes luck. It's not your fault if you're not lucky. A talk one conference didn't accept, another one might, and vice versa. Keep trying (and if not offered, ask for why the proposal wasn't accepted - chances are there's valuable information there.)
If you get email saying your talk has been accepted, then congratulations, it is time for the big work to begin. Think about how you best get your ideas across. Who is your audience? How knowledgeable are they about your topic? Watch a number of presentations you have liked, and study how the presenter does it. There are many different techniques. Shamelessly steal techniques you think works well, and note what's problematic so that you can avoid it. Writing the presentation material does (for me) take a huge amount of time, and I keep revisiting it over and over, polishing for better narrative, fixing bugs, improving visual style. One difficult thing to estimate is how long it takes to deliver the talk. Aim for filling your slot reasonably well. It's not nice to overshoot, but it's also rude to your audience and the conference organiser if you use up considerably less time than has been set aside for you. The only way to learn how long it takes is to do it (after you have done it a few times, you get a feel for your slide-rate, but for your first talk, you obviously have no idea.) Dry run the talk for yourself. Leave space for audience interaction. Practice on your colleagues. If it's a conference talk, try do do a practice run for your local user group. Solicit feedback, and improve your presentation even more. Some speakers like to rehearse the talk very much, others prefer to improvise more. Find out what works for you.

And, finally... you have things to say. You have experiences that are worth sharing. Your experiences will valuable to others, but only if you share them. Speaking at conferences or local user groups isn't the only way to share experiences, but don't dismiss it, OK?

Share your experiences.

Saturday, July 14, 2018

DRY multicomparisons

Now and then, I find myself writing something like if (x == a || x == b || x == c) ..., and every time the repetition of x == annoys me.

A number of people, me included, have reduced the repetition by writing code like:

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
template <typename T>
class is
{
public:
    constexpr is(T t_) : t(std::move(t_)) {}
    template <typename ... U>
    constexpr auto any_of(U const& ... u) const { return ((t == u) || ...); }
private:
  T t;
};

If you're not familiar with the C++17 constructs, I'll walk you through it.

To begin with, C++17 has a class template argument type deduction that works for constructors. This works automatically when the type used in the constructor is enough to deduce the template types, which is the case with the constructor on line 5, so is{3} is automatically deduced to be is<int>{3}, since the literal 3 is an int.

The member function template any_of(), on line 7, uses a variadic template parameter pack ...u, to accept any number of arguments of any type. This is then passed to a fold expression which will be expanded such that the pattern (t == u) is repeated, where u takes on each and every of the parameters called with, in the ....

Here's an example use of the construction:

1
if (is{x}.any_of(a,b,c)) ...

It will construct an is<int>{x}, and the call to any_of(a,b,c) will in effect return the result of ((x == a) || (x == b) || (x == c)).

This takes care of much of the repetition, but it doesn't read very nicely. It's awkward.

Enter ranges. Ranges have been in the works for C++ for some time now, and is available as open source libraries if you want to try them out. Chances are they will be a part of C++20, but that decision is not made yet.

With ranges, the algorithms from <algorithm> and <numeric> can work on whole ranges of values at once, so you don't have to fiddle with iterator pairs. Collections counts as ranges, including std::initializer_list<>. In my examples I'm using the range-v3 library, available from github.com. In the range-v3 library, the algorithms are in namespace ranges::v3.

So we can instead write:


1
if (ranges::v3::any_of({a,b,c}, [x](auto y){ return x == y; }) ...

It takes care of repetition, but at the cost of boiler plate to the degree it smells of obfuscation. It's not easy to read.


A trivial higher order function improves it:

1
2
3
4
5
template <typename T>
inline auto equals(T t)
{
    return [=](auto&& x) { return x == t;};
}

The call can now be made as:

1
if (ranges::v3::any_of({a,b,c}, equals(x)) ...

The call to equals(x) gives us the lambda that makes the test for equality. I guess that if you read it out, except for the namespace and punctuations, it kind of reads like normal English.

But there is a difference here. A subtle one, and one I didn't think of for a very long time. What if a, b and c, are not all of the same type?

Just a couple of days ago, I had a different idea. What if I make any_of to be a class template, that holds the values I want to compare, and implements an operator==? This would be sort of like the first example solution, but inside out.

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
template <typename ... T>
class any_of : std::tuple<T...>
{
public:
    using std::tuple<T...>::tuple;
    template <typename U>
    constexpr bool operator==(const U& u) const {
        return std::apply([&](const auto& ... a) { return ((a == u) || ...);},
                          get());
    }
    template <typename U>
    friend constexpr bool operator==(const U& u, const any_of& a)
    {
        return a == u;
    }
private:
    constexpr const std::tuple<T...> get() const { return *this;}
};

template <typename ... T>
any_of(T&& ...) -> any_of<T...>;

I'll walk you through the code.

First, any_of<T...> inherits privately from std::tuple<T...>, and on line 5 we say that we have all the constructors that tuple does.

Lines 6-10 implements operator==, that can be called with any type. std::apply, on line 8, calls a function (first parameter) with the individual values from a tuple (2nd parameter). The function is a lambda that can take any number of arguments, and compare them with the reference captured u, as explained above for the is<T>::any_of() function template, and the 2nd parameter is the tuple that any_of inherits from (as given by the private get() member function on line 17.)

The one thing missing now is that it is not obvious for the compiler what the types T are. It cannot deduce them automatically, so we need to help it. The deduction guide on lines 20-21 takes care of that. It say if we call any_of(a,b,c), it will deduce the Ts from the types of a, b and c. l-values will be deduced to l-value references, and r-values will be moved into the tuple.

This is enough to make it work. A call can now be made as:

1
if (any_of{a,b,c} == x)...

This is nice! A little bit Yoda-like grammar, but the friend operator== on lines 11-15 allows comparison with the arguments in the reversed order, like:

1
if (x == any_of{a,b,c})...

This is all and well. But since this can be used with any types for which the comparisons make sense, why not expand it to other operators too, and implement all_of and none_of? Why not allow an assertion like the following?:

1
assert(all_of(a,b,c,d) > " ");

This of course requires that a, b, c and d are all greater-than comparable with a c-string.

I think this is neat. It reads nicely, and is not very difficult to understand.

If you want to have a look at code generation, please toy with this link to gcc.godbolt.org. Spoiler - the compilers see through it all and generates amazingly efficient code.

If you want to try it in your own code, clone it from github.com.