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<!doctype html>
<html>
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0, maximum-scale=1.0, user-scalable=no">
<title>Runtime Polymorphism: Back to the Basics</title>
<meta name="author" content="Louis Dionne">
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<section data-markdown=""
data-separator="^====+$"
data-separator-vertical="^----+$"
data-notes="^Note:">
<script type="text/template">
## Runtime Polymorphism:<br>Back to the Basics
#### Louis Dionne, C++Now 2018
====
- These slides are available at: https://ldionne.com/cppnow-2018-runtime-polymorphism
- Their source code is available at: https://github.com/ldionne/cppnow-2018-runtime-polymorphism
====
<!-- .slide: class="slide-hidden" -->
### Font test
```c++
class ThisIsAClass {
};
int main() {
// this is a comment
std::vector<Foobar> foobar;
foobar.push_back(Foo{...});
foobar.push_back(Bar{...});
foobar.push_back(Baz{...});
for (auto& x : foobar) {
x.do_something();
}
}
```
==============================================================================
### What is runtime polymorphism <br> and when do you need it?
----
### Consider the following
```c++
struct Car {
void accelerate();
};
struct Truck {
void accelerate();
};
struct Plane {
void accelerate();
};
```
----
### Returning related types from a function
```c++
??? getVehicle(std::istream& user) {
std::string choice;
user >> choice;
if (choice == "car") return Car{...};
else if (choice == "truck") return Truck{...};
else if (choice == "plane") return Plane{...};
else die();
}
```
----
### Storing related types in a container
```c++
int main() {
// Should store anything that has an accelerate() method
std::vector<???> vehicles;
vehicles.push_back(Car{...});
vehicles.push_back(Truck{...});
vehicles.push_back(Plane{...});
for (auto& vehicle : vehicles) {
vehicle.accelerate();
}
}
```
----
### `variant` sometimes does the trick
- But it only works for closed set of types
- Using visitation is sometimes (often?) not convenient
----
### Bottom line:
#### Manipulating an open set of related types with different representations
====================
### C++ has a solution for that!
----
### Inheritance
<pre><code data-sample="code/inheritance.cpp#Vehicle"></code></pre>
----
### Under the hood
====================
### Aside
#### Inheritance has many problems
----
### Bakes in reference semantics
```c++
void foo(Vehicle* vehicle) {
Vehicle* copy = vehicle;
...
copy->accelerate();
...
}
```
----
### Heap allocations
```c++
std::unique_ptr<Vehicle> getVehicle(std::istream& user) {
std::string choice;
user >> choice;
if (choice == "car") return std::make_unique<Car>(...);
else if (choice == "truck") return std::make_unique<Truck>(...);
else if (choice == "plane") return std::make_unique<Plane>(...);
else die();
}
```
Note:
We don't really have a choice but to use pointers and allocate on the heap if
we want to put them in contiguous storage (vector, array, etc...), because the
objects have different sizes.
----
### Bakes in nullable semantics
```c++
std::unique_ptr<Vehicle> vehicle = getVehicle(std::cin);
// can vehicle be null?
```
----
### Ownership hell
```c++
Vehicle* getVehicle(std::istream& user);
std::unique_ptr<Vehicle> getVehicle(std::istream& user);
std::shared_ptr<Vehicle> getVehicle(std::istream& user);
```
----
### Doesn't play well with algorithms
```c++
std::vector<std::unique_ptr<Vehicle>> vehicles;
vehicles.push_back(std::make_unique<Car>(...));
vehicles.push_back(std::make_unique<Truck>(...));
vehicles.push_back(std::make_unique<Plane>(...));
std::sort(vehicles.begin(), vehicles.end()); // NOT what you wanted!
```
----
### Intrusive
```c++
namespace lib {
struct Motorcycle { void accelerate(); };
}
void foo(Vehicle& vehicle) {
...
vehicle.accelerate();
...
}
Motorcycle bike;
foo(bike); // can't work!
```
----
### Listen to Sean Parent, not me
https://youtu.be/QGcVXgEVMJg
====================
### I just wanted this!
```c++
interface Vehicle { void accelerate(); };
namespace lib {
struct Motorcycle { void accelerate(); };
}
struct Car { void accelerate(); };
struct Truck { void accelerate(); };
int main() {
std::vector<Vehicle> vehicles;
vehicles.push_back(Car{...});
vehicles.push_back(Truck{...});
vehicles.push_back(lib::Motorcycle{...});
for (auto& vehicle : vehicles) {
vehicle.accelerate();
}
}
```
----
### How might that work?
----
### With inheritance
----
### Goal:
#### Independent storage and method dispatch
- Storage _policy_
- VTable _policy_
Note:
State motivation for the talk, i.e. we're going to play around with different
ways of implementing the `interface` keyword shown above.
====================
### Remote storage
----
### How that's implemented
<pre><code data-sample='code/remote_storage.cpp#Vehicle'></code></pre>
Note:
Quickly show a preview of the vtable, and then come back to explain.
----
### The vtable
<pre><code data-sample='code/vtable.hpp#vtable'></code></pre>
----
### With Dyno
<pre><code data-sample='code/remote_storage.dyno.cpp#Vehicle'></code></pre>
Note:
Explain that this is a compile-time string, and it's as efficient as a
member access in a struct (the vtable).
----
### Dyno's vtable
<pre><code data-sample='code/vtable.dyno.hpp#IVehicle'></code></pre>
----
### Strengths and weaknesses
<ul>
<li class="strength">Simple model, similar to classic inheritance</li>
<li class="weakness">Always requires an allocation</li>
</ul>
====================
### The _small buffer optimization_ (SBO)
----
### How that's implemented
<pre><code data-sample='code/sbo_storage.cpp#Vehicle'></code></pre>
Note:
Make sure to explain placement new.
----
### Alternative implementation 1
Note:
We can actually inline the bool into the vtable's function definitions, since
they always know whether the thing is going to be in the buffer or on the heap.
----
### Alternative implementation 2
(seems to be the fastest)
----
### With Dyno
<pre><code data-sample='code/sbo_storage.dyno.cpp#Vehicle'></code></pre>
----
### Strengths and weaknesses
<ul>
<li class="strength">Does not always require allocating</li>
<li class="weakness">Takes up more space</li>
<li class="weakness">Copy/move/swap is more complicated</li>
<li class="weakness">Dispatching may be more costly</li>
</ul>
Note:
We need to handle cases where the two types are not the same in swap.
====================
### Always-local storage
----
### Doesn't fit? Doesn't compile!
----
### How that's implemented
<pre><code data-sample='code/local_storage.cpp#Vehicle'></code></pre>
Note:
Mention that we're not checking for the alignment here.
----
### With Dyno
<pre><code data-sample='code/local_storage.dyno.cpp#Vehicle'></code></pre>
----
### Strengths and weaknesses
<ul>
<li class="strength">No allocation – ever</li>
<li class="strength">Simple dispatching</li>
<li class="weakness">Takes up more space</li>
</ul>
====================
### A quick benchmark
Creating many 16 bytes objects
<canvas data-chart="bar">
<!--
{
"data": {
"labels": ["inheritance",
"remote storage",
"SBO storage (4 bytes)",
"SBO storage (8 bytes)",
"SBO storage (16 bytes)",
"local storage (16 bytes)"],
"datasets": [{
"data": [55, 54, 54, 56, 3, 2],
"label": "Time (ns)",
"backgroundColor": "rgba(20, 220, 220, 1)"
}]
}
}
-->
</canvas>
Note:
- In some cases, similar benefits could be achieved by using a pool allocator
- However, if you shuffle the data with a pool allocator, the data and the pointer
to the data become in a different order. Traversing and dereferencing elements
in a shuffled sequence may become cache unfriendly. With the local buffer and
SBO approaches, the data and the handle stay together so this problem does not
arise.
----
<!-- .slide: class="slide-hidden" -->
Creating many 4 bytes objects
<canvas data-chart="bar">
<!--
{
"data": {
"labels": ["inheritance",
"remote storage",
"SBO storage (4 bytes)",
"SBO storage (8 bytes)",
"SBO storage (16 bytes)",
"local storage (16 bytes)"],
"datasets": [{
"data": [56, 55, 3, 3, 3, 3],
"label": "Time (ns)",
"backgroundColor": "rgba(20, 220, 220, 1)"
}]
}
}
-->
</canvas>
----
<!-- .slide: class="slide-hidden" -->
Accessing many 4 bytes objects<br>
(10 x 3 method calls, SBO with bool)
<canvas data-chart="bar">
<!--
{
"data": {
"labels": ["inheritance",
"remote storage",
"SBO storage (4 bytes)",
"SBO storage (8 bytes)",
"SBO storage (16 bytes)",
"local storage (16 bytes)"],
"datasets": [{
"data": [49, 45, 59, 58, 58, 46],
"label": "Time (ns)",
"backgroundColor": "rgba(20, 220, 220, 1)"
}]
}
}
-->
</canvas>
----
<!-- .slide: class="slide-hidden" -->
Accessing many objects<br>
(half 8 bytes, half 16 bytes, SBO with bool)
<canvas data-chart="bar">
<!--
{
"data": {
"labels": ["inheritance",
"remote storage",
"SBO storage (4 bytes)",
"SBO storage (8 bytes)",
"SBO storage (16 bytes)",
"local storage (16 bytes)"],
"datasets": [{
"data": [46, 47, 56, 56, 58, 46],
"label": "Time (ns)",
"backgroundColor": "rgba(20, 220, 220, 1)"
}]
}
}
-->
</canvas>
----
<!-- .slide: class="slide-hidden" -->
Accessing many 4 bytes objects
<br>(10 x 3 method calls, SBO with pointer)
<canvas data-chart="bar">
<!--
{
"data": {
"labels": ["inheritance",
"remote storage",
"SBO storage (4 bytes)",
"SBO storage (8 bytes)",
"SBO storage (16 bytes)",
"local storage (16 bytes)"],
"datasets": [{
"data": [49, 46, 46, 45, 45, 46],
"label": "Time (ns)",
"backgroundColor": "rgba(20, 220, 220, 1)"
}]
}
}
-->
</canvas>
----
<!-- .slide: class="slide-hidden" -->
Accessing many objects<br>
(half 8 bytes, half 16 bytes, SBO with pointer)
<canvas data-chart="bar">
<!--
{
"data": {
"labels": ["inheritance",
"remote storage",
"SBO storage (4 bytes)",
"SBO storage (8 bytes)",
"SBO storage (16 bytes)",
"local storage (16 bytes)"],
"datasets": [{
"data": [49, 45, 53, 50, 45, 46],
"label": "Time (ns)",
"backgroundColor": "rgba(20, 220, 220, 1)"
}]
}
}
-->
</canvas>
----
### Guidelines
- Use local storage whenever you can afford it
- Otherwise, use SBO with the largest reasonable size
- Use purely-remote storage only when
+ Object sizes are so scattered SBO wouldn't help
====================
### Non-owning storage
(reference semantics, not value semantics)
----
### Basically a polymorphic view
```c++
void process(VehicleRef vehicle) {
...
vehicle.accelerate();
...
}
int main() {
Truck truck{...};
process(truck); // No copy!
}
```
----
### How that's implemented
<pre><code data-sample='code/non_owning_storage.cpp#VehicleRef'></code></pre>
----
### With Dyno
<pre><code data-sample='code/non_owning_storage.dyno.cpp#VehicleRef'></code></pre>
====================
### Shared remote storage
Note:
One use case is copy-on-write, where you share everything and make a copy
whenever you call a non-const function on it. Allows optimal sharing and
thread safety with minimal headache.
----
### How that's implemented
<pre><code data-sample='code/shared_remote_storage.cpp#Vehicle'></code></pre>
----
### With Dyno
<pre><code data-sample='code/shared_remote_storage.dyno.cpp#Vehicle'></code></pre>
----
### Becomes interesting when mixed with copy on write
----
<pre><code data-sample='code/shared_remote_storage.dyno.cow.cpp#Vehicle'></code></pre>
----
<!-- .slide: class="slide-hidden" -->
### Concrete example
#### (stolen from Sean Parent)
```c++
int main() {
history_t h;
current(h).emplace_back(0);
current(h).emplace_back(std::string("Hello!"));
draw(current(h), std::cout);
commit(h);
current(h).emplace_back(current(h));
current(h).emplace_back(my_class_t());
draw(current(h), std::cout);
undo(h);
draw(current(h), std::cout);
}
```
----
<!-- .slide: class="slide-hidden" -->
```c++
using document_t = std::vector<Drawable>;
void draw(document_t const& x, std::ostream& out) {
// ...
}
using history_t = std::vector<document_t>;
void commit(history_t& x) {
x.push_back(x.back());
}
void undo(history_t& x) {
x.pop_back();
}
document_t& current(history_t& x) {
return x.back();
}
```
----
### Strengths and weaknesses
<ul>
<li class="strength">Allows sharing potentially expensive state</li>
<li class="strength">Interacts nicely with concurrency</li>
<li class="weakness">Allocates</li>
<li class="weakness">Uses reference counts</li>
</ul>
====================
### Now, let me show you why you care
----
### Have you heard of the following?
- [`std::function`](http://en.cppreference.com/w/cpp/utility/functional/function)
- [`inplace_function`](https://groups.google.com/a/isocpp.org/d/msg/std-proposals/vven2Om7Ha8/C7qQ_XwVCwAJ)
- [`function_view`](https://vittorioromeo.info/index/blog/passing_functions_to_functions.html)
----
### Consider this
<pre><code data-sample='code/functions.cpp#basic_function'></code></pre>
----
### Here's all of them:
<pre><code data-sample='code/functions.cpp#function'></code></pre>
<pre><code data-sample='code/functions.cpp#inplace_function'></code></pre>
<pre><code data-sample='code/functions.cpp#function_view'></code></pre>
<pre><code data-sample='code/functions.cpp#shared_function'></code></pre>
==============================================================================
### We've talked about storage
### What about vtables?
----
### Normally, it is remote
----
### Turns out we have some choices
====================
### Inlining the vtable in the object
<img src="img/local_vtable.png" style="width:600px; height:auto;">
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### How that's implemented
<pre><code data-sample='code/local_vtable.cpp#Vehicle'></code></pre>
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### With Dyno
<pre><code data-sample='code/local_vtable.dyno.cpp#Vehicle'></code></pre>
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### Usually a pessimization
(I did measure)
- If vtable in the cache, indirection does not matter
- Vtable in the object is more likely to be cold
====================
### Partial vtable inlining
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<!-- .slide: class="slide-hidden" -->
### The Vtable — remote part
<pre><code data-sample='code/joined_vtable.cpp#vtable'></code></pre>
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<!-- .slide: class="slide-hidden" -->
### The Vtable — local part
<pre><code data-sample='code/joined_vtable.cpp#joined_vtable'></code></pre>
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<!-- .slide: class="slide-hidden" -->
### The polymorphic wrapper
<pre><code data-sample='code/joined_vtable.cpp#Vehicle'></code></pre>
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### With Dyno
<pre><code data-sample='code/joined_vtable.dyno.cpp#Vehicle'></code></pre>
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### Again, not really an optimization
====================
### Fun observation about vtables
----
<iframe id="godbolt" class="stretch" src="https://gcc.godbolt.org/e#z:OYLghAFBqd5QCxAYwPYBMCmBRdBLAF1QCcAaPECAKxAEZSAbAQwDtRkBSAJgCFufSAZ1QBXYskwgA5IILERyAgGoAagBUmAIwaYlHAOx8ADAEElSgG6o86JRABUAM1oBKCFZv2XHAMzGzlta2Do5cbh7oXr7%2B5hF2Tj7hQVF%2BHKaxQfGOACxJnt6ppgYAItFpJuUEmAC2AA7MVXo%2BAMIEAJ61mCxM1bpqvtjlmAAeVcQsqhraumgsspYEWjplRasmsvKKSsQ1qFUA%2BqxteoblGTZKzhAuJzyWtXL7ALQDV4KYDI77BXclZ4EXULXW73R4vHzYIHvT7faInUrpAG2Rw%2BYEGO4WB7EZ6vVHQr4/eH/OI5NGGUHY8GQ7IQfGwvxExHqJYzVBzAj2JSzeaYx4rAIRTl0/l/QoVUwbBTKPAsBCYYiEVgSQ4sY7o4l4YgEERMBhIy60NE%2BYpKIz82Ka7W6/VAm6%2BE1msUWrU6vUk1F242m82WS2um00z0OkX6Upi8qSrYMVDIXUqtWnREkw128kWAjafYAOjeHwJcL%2BScytpB6czOa4tLz9N%2BoeJxY9pYzDGzKKrMMJhYFxcDTfLpLpnbrTKmOi5bJ5zf2PsFSmF4br4dMJIgO2qe0w8e4ADYlExUzE9zmUz6mBXrqecx7L6SfuUu%2BUVzK5QrFixlUcd3uD/8z1c74if6VgBAR/teTpHreIYIuKJgrtGsYtp%2BXC7vuty/seF4QUBWGHmBuEYVBC4wVILiMNIACsUikCw0hGNRqDSM0/D8HOojiLo3A%2BLQ1EEHRpFkQA1iAFG0FmACc25GPoAAc2TcT4%2BhcBRFHkVI2TUbRUj0aQjFSNRgggEYpB8dppGkHAsBIGgdR4DoZAUBANm1HZ8ooMwbBScZjh2WMhkQJo/GkJoMpMMQbTSDxpA2b0LAEAA8iwDARWZpBYNUrDADoQX4Dsih4BYmCGalIyYMgIhVJF1EylUDBVSZCrVPVIgsFgPksJg6BVWRHnsCxvCMHgmiGZAZGoA8eATtITzxT4ShPLI6D2pwvD8LQGlCOxEh0D1lGaUFenDDJ25PNu2Rcr1SjblmRg3XYuCECQehcNxpBKM0qC2fZz3cTczGrbwvH8S4QkgNkMk3T44lcLQFHbsdXD6NkolqRtWk6XpBlGSZwO7VIXD7almM42ZIOkIVxCCJNtHZEAA%3D%3D"></iframe>
Note:
- First load is vtable load
- Second load is passing `this` as the first function argument
- Strictly speaking, the compiler would have the right not to reload the vtable pointer because we're not using `std::launder`
----
#### With `-fstrict-vtable-pointers`
<iframe id="godbolt" class="stretch" src="https://gcc.godbolt.org/e#z:OYLghAFBqd5QCxAYwPYBMCmBRdBLAF1QCcAaPECAKxAEZSAbAQwDtRkBSAJgCFufSAZ1QBXYskwgA5IILERyAgGoAagBUmAIwaYlHAOx8ADAEElSgG6o86JRABUAM1oBKCFZv2XHAMzGzlta2Do5cbh7oXr7%2B5hF2Tj7hQVF%2BHKaxQfGOACxJnt6ppgYAItFpJuUEmAC2AA7MVXo%2BAMIEAJ61mCxM1bpqvtjlmAAeVcQsqhraumgsspYEWjplRasmsvKKSsQ1qFUA%2BqxteoblGTZKzhAuJzyWtXL7ALQDV4KYDI77BXclZ4EXULXW73R4vHzYIHvT7faInUrpAG2Rw%2BYEGO4WB7EZ6vVHQr4/eH/OI5NGGUHY8GQ7IQfGwvxExHqJYzVBzAj2JSzeaYx4rAIRTl0/l/QoVUwbBTKPAsBCYYiEVgSQ4sY7o4l4YgEERMBhIy60NE%2BYpKIz82Ka7W6/VAm6%2BE1msUWrU6vUk1F242m82WS2um00z0OkX6Upi8qSrYMVDIXUqtWnREkw128kWAjafYAOjeHwJcL%2BScytpB6czOa4tLz9N%2BoeJxY9pYzDGzKKrMMJhYFxcDTfLpLpnbrTKmOi5bJ5zf2PsFSmF4br4dMJIgO2qe0w8e4ADYlExUzE9zmUz6mBXrqecx7L6SfuUu%2BUVzK5QrFixlUcd3uD/8z1c74if6VgBAR/teTpHreIYIuKJgrtGsYtp%2BXC7vuty/seF4QUBWGHmBuEYVBC4wVILiMNIACsUikCw0hGNRqDSM0/D8HOojiLo3A%2BLQ1EEHRpFkQA1iAFG0FmACc25GPoAAc2TcT4%2BhcBRFHkVI2TUbRUj0aQjFSNRgggEYpB8dppGkHAsBIGgdR4DoZAUBANm1HZ8ooMwbBScZjh2WMhkQJo/GkJoMpMMQbTSDxpA2b0LAEAA8iwDARWZpBYNUrDADoQX4Dsih4BYmCGalIyYMgIhVJF1EylUDBVSZCrVPVIgsFgPksJg6BVWRHnsCxvCMHgmiGZAZGoA8eATtITzxT4ShPLI6D2pwvD8LQ2TzY4Gx4IoTzpiyTy1NYcXyoIBnsRIdA9ZRmlBXpwwyduTzbhtyC9Uo25ZkYX12LghAkHoXDcaQSjNKgtn2YD3E3Mxq28Lx/EuEJIDZDJX0%2BOJXC0BR26PVw%2BjZKJakaTRd3SAZRkmYj11SFwt2pXpCNmUjpCFcQgiTbR2RAA%3D%3D%3D"></iframe>
Note:
- Now there's only one load for the inheritance-based one (because the vptr can't change since we're not using `std::launder`)
- The handrolled remote vtable loads the pointer twice; nothing would prevent the code from changing the vptr during the call, since there's nothing special about the vtable.
====================
### Another story about inlining
```c++
template <typename AnyIterator, typename It>
__attribute__((noinline)) AnyIterator make(It it) {
return AnyIterator{std::move(it)};
}
template <typename AnyIterator>
void benchmark_any_iterator(benchmark::State& state) {
std::vector<int> input{...};
std::vector<int> output{...};
while (state.KeepRunning()) {
auto first = make<AnyIterator>(input.begin());
auto last = make<AnyIterator>(input.end());
auto result = make<AnyIterator>(output.begin());
for (; !(first == last); ++first, ++result) {
*result = *first;
}
}
}
```
----
<canvas data-chart="bar">
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"labels": ["static dispatch", "inheritance", "dyno's remote vtable"],
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"data": [14, 1126, 1089],
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</canvas>
----
### Now, just a small tweak
```c++
template <typename AnyIterator, typename It>
// __attribute__((noinline))
AnyIterator make(It it) {
return AnyIterator{std::move(it)};
}
```
----
<canvas data-chart="bar">
<!--
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"labels": ["static dispatch", "inheritance", "dyno's remote vtable"],
"datasets": [{
"data": [11, 1070, 11],
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</canvas>
----
### What happened?
<div align="left">Inheritance:</div>
<div align="left">Dyno's remote vtable:</div>
----
### What's the lesson?
- Reducing pointer hops can lead to unexpected inlining
- When that happens, giant optimizations become possible
----
### Guidelines
- By default, all methods are in the remote vtable
- Consider inlining some methods if you see a difference
- Watch out for places where you're a few hops away from devirtualization
==============================================================================
### Main problem with this talk:
### It's a giant pain to implement
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### Can we do something about it?
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### Perhaps with reflection?
```c++
struct Vehicle {
void accelerate();
};
int main() {
std::vector<std::poly<Vehicle>> vehicles;
vehicles.push_back(Car{...});
vehicles.push_back(Truck{...});
vehicles.push_back(lib::Motorcycle{...});
for (auto& vehicle : vehicles) {
vehicle.accelerate();
}
}
```
----
### Or with metaclasses? ([p0707r3](http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0707r3.pdf))
```c++
constexpr void interface(std::meta::type target,
std::meta::type const source)
{
// generates the type-erasure wrapper