Do not help the compiler at the expense of readability Unless you read the assembly code emitted at the bottleneck and did benchmarks

Posted on November 24, 2017 by admin

Compilers has gotten smarter and smarter nowadays that they’d be able to analyze our code for common patterns (or logically deduce away steps that doesn’t have to be performed at runtime).

Matt Godbolt gave a nice presentation at CppCon 2017 named “What Has My Compiler Done for Me Lately?”. Through observing the emitted assembly code at different optimization levels, he showed that the compiler doesn’t need to be micromanaged (through performance hacks in our code) anymore, as it will emit instructions as the performance-hacked code intended when it is better to do so.

It means the compiler writers already know our bag of performance hack tricks way better than we do. Their efforts spared us from premature optimization and leave us more time to find a better data structure or algorithm to solve the problem.

What I got from the lecture is NOT that we are free to write clumsy code and let the compiler sort it out (though it occasionally can, like factoring a loop doing simple arithmetic series into a one line closed form solution), but we don’t have to make difficult coding choices to accommodate performance anymore.

The most striking facts I learned from his lecture are

The compiler can emit a one-line CPU instruction that does not have a corresponding native operation C/C++ if your hardware architecture supports it. (e.g. clang can convert a whole loop that counts the number of set bits into just ‘popcnt eax, edi‘)
Through Link-Time Optimization (LTO), we don’t have to pay the performance penalty for language features that are ultimately necessary for the current compilation (e.g. virtuals are automatically dropped if the linker finds that nowhere in the output currently needs it)

With such LTO, why not do away the virtual specifier and make everything unspecified virtual by default anyway (like Java)? For decades, we’ve been making up stories that some classes are not meant to be derived (like STL containers), but the underlying motive is that we don’t want to pay for vtable if we don’t have to.

Instead of confusing new programmers about when should they make a method virtual (plenty of rule-of-thumbs became dogma), focus on telling them whenever they (choose to upcast a reference/pointer to the parent anywhere in their code and) invoke the destructor through the parent reference/pointer, they will pay a ‘hefty’ price of vtable and vptr.

I don’t think anybody (old codebase) will get harmed by turning on virtuals by default and let the linker decide if those virtuals can be dropped. If it changes anything, it might turn buggy code with the wrong destructor called into correct code which runs slower and takes up more space. In terms of correctness, this change might break low-level hacks that expects the objects to be of certain size (e.g. alignment) without vptr.

Even better, add a class specifier that mandates that all uses of its child must not invoke vtable (have the compiler catch that) unless explicitly overridden (the users decide to pay for the vtable). This way the compiler can warn about performance and space issues for the migration.

The old C++’s ideal was “you only pay for the language features you used (written)”, but as compilers gets better, we might be able change it to “you pay extra only for the language features that are actually used (in the finally generated executable) with your permission”.

I’d also like to add Return Value Optimization (RVO) into my list of compiler advances that changes the way we code. C++11 added move semantics, but I think it’s something that the compiler in the future could be able to manage themselves. Even with an old C++ compiler like the one shipped with VisualDSP 5.0, the copy constructor was not called (yes, skipping it is legal even if the copy constructor has side effects) when I do this:

Matrix operator+(const Matrix& a, const Matrix& b)
{
  Matrix c(a.dim);
  // ... for all element i, c.raw[i] = a.raw[i]+b.raw[i]
  return c;
}
Matrix c = a + b;

Actually, the compiler at that time was not that smart about RVO, the actual code I wrote originally had two return branches, which defeats RVO (it’s a defined behavior by the specs):

Matrix operator+(Matrix a, Matrix b)
{
  Dims m = a.dims;
  if( m == b.dims ) // Both inputs must have same dimensions
  {
    Matrix c(m); // Construct matrix c with same dimension as a
    // ... for all i, c.raw[i] = a.raw[i] + b.raw[i]
    return c;
  } 
  else 
  {
    return Matrix::dummy; // A static member, which is a Matrix object
  }
}

To take advantage of RVO, I had to reword my code

Matrix operator+(Matrix a, Matrix b)
{
  Dims m = a.dims;
  if( m == b.dims ) // Both inputs must have same dimensions
  {
    Matrix c(m); // Construct matrix c with same dimension as a
    // ... for all i, c.raw[i] = a.raw[i] + b.raw[i]
  } 
  else 
  {
    Matrix c = Matrix::dummy; // or just "Matrix c";
  }
  return c;
}

I think days are counting before C++ compilers can do “copy-on-write” like MATLAB does if independent compilation are no longer mandatory!

Given my extensive experience with MATLAB, I’d say it took me a while to get used designing my code with “copy-on write” behavior in mind. Always start with expressive, maintainable, readable and correct code keeping in mind the performance concerns only happens under certain conditions (i.e. passed object gets modified inside the function).

If people start embracing the mentality of letting the compiler do most of the mechanical optimization, we’ll move towards a world that debugging work are gradually displaced by performance-bottleneck hunting. In my view, anything that can be done systematically by programming (like a boilerplate code or idioms) can eventually be automated by better compiler/linker/IDE and language design. It’s the high-level business logic that needs a lot of software designers/engineers to translate fuzzy requirements into concrete steps.

Matt also developed a great website (http://godbolt.org/) that compiles your code repeatedly on the fly and shows you the corresponding assembly code. Here’s an example of how I use it to answer my question of “Should I bother to use std::div() if I want both the quotient and remainder without running the division twice?”:

The website also included a feature to share the pasted code through an URL.

As seen from the emitted assembly code, the answer is NO. The compiler can figure out that I’m repeating the division twice and do only one division and use the quotient (stored in eax) and remainder (stored in edx). Trying to enforce one division through std::div() requires an extra function call, which is strictly worse.

The bottom line: don’t help the compiler! Modern compiler does context free optimizations better than we do. Use the time and energy to rethink about the architecture and data structure instead!

C++ annoyances (and reliefs): operator[] in STL map-based containers

Posted on November 18, 2017 by admin

I recently watched Louis Brandy’s CppCon presentation “Curiously Recurring C++ Bugs at Facebook” on youtube.

For bug#2, which is a well-known trap for STL map-based containers, operator[] will insert the requested key (associated with a default-constructed value) if it is not found.

He mentioned a few workarounds and their disadvantages, like

use at() method: requires exception handling
const protect: noobs try to defeat that, transferred to non-const (stripped)
ban operator[] calls: makes the code ugly

but would like to see something neater. In bug#3, he added that a very common usage is to return a default when the key is not found. The normal approach requires returning a copy of the default (expensive if it’s large), which tempts noobs to return a local reference (to destroyed temporary variables: guaranteed bug).

Considering how much productivity drain a clumsy interface can cause, I think it’s worth spending a few hours of my time approaching it, since I might need to use STL map-based containers myself someday.

Here’s my thought process for the design choices:

Retain the complete STL interface to minimize user code/documentation changes
Endow a STL map-based container with a default_value (common use case), so that the new operator[] can return a reference without worrying about temporaries getting destroyed.
Give users a easy read-only access interface (make intentions clear with little typing)

The code (with detailed comment about design decisions and test cases) can be downloaded here: MapWithDefault. For the experienced, here’s the meat:

#include <unordered_map>
#include <map>

#include <utility>  // std::forward

// Legend (for extremely simple generic functions)
// ===============================================
// K: key
// V: value
// C: container
// B: base (class)
template <typename K, typename V, template <typename ... Args> class C = std::map, typename B = C<K,V> >
class MapWithDefault : private B 
{
public:
    // Make default_value mandatory. Everything else follows the requested STL container
    template<typename... Args>
    MapWithDefault(V default_value, Args&& ... args) : B(std::forward<Args>(args)...), default_value(default_value) {};

public:
    using B::operator=;
    using B::get_allocator;

    using B::at;

    using B::operator[];

    // Read-only map (const object) uses only read-only operator[]
    const V& operator[](const K& key) const
    {
        auto it = this->find(key);
        return (it==this->end()) ? default_value : it->second;
    }

    using B::begin;
    using B::cbegin;
    using B::end;
    using B::cend;
    using B::rbegin;
    using B::crbegin;
    using B::rend;
    using B::crend;

    using B::empty;
    using B::size;
    using B::max_size;

    using B::clear;
    using B::insert;
    // using B::insert_or_assign;   // C++17
    using B::emplace;
    using B::emplace_hint;
    using B::erase;
    using B::swap;

    using B::count;
    using B::find;
    using B::equal_range;
    using B::lower_bound;
    using B::upper_bound;

public:
    const               V default_value;
    const MapWithDefault& read_only = static_cast<MapWithDefault&>(*this);
};

Note that this is private inheritance (can go without virtual destructors since STL doesn’t have it). I have not exposed all the private members and methods back to public with the ‘using’ keyword yet, but you get the idea.

This is how I normally want the extended container to be used:

int main()
{
    MapWithDefault<string, int> m(17);  // Endowed with default of 17
    cout << "pull rabbit from m.read_only:  " << m.read_only["rabbit"] << endl;   // Should read 17

    // Demonstrates commonly unwanted behavior of inserting requested key when not found
    cout << "pull rabbit from m:            " << m["rabbit"] << endl; // Should read 0 because the key was inserted (not default anymore)

    // Won't compile: demonstrate that it's read only
    // m.read_only["rabbit"] = 42;

    // Demonstrate writing
    m["rabbit"] = 42;

    // Confirms written value
    cout << "pull rabbit from m_read_only:  " << m.read_only["rabbit"] << endl;   // Should read 42
    cout << "pull rabbit from m:            " << m["rabbit"] << endl;             // Should read 42

    return 0;
}

Basically, for read-only operations, always operate directly on the chained ‘m.read_only‘ object reference: it will make sure the const protected version of the methods (including read-only operator[]) is called.

Please let me know if it’s a bad idea or there’s some details I’ve missed!

Super-simplified: Programming high performance code by considering cache

Posted on November 16, 2017 by admin

Code/data locality (compactness, % of each cache line that gets used)
Predictable access patterns: pre-fetch (instructions and data) friendly. This explains branching costs, why linear transversal might be faster than trees at smaller scales because of pointer chasing, why bubble sort is the fastest if the chunks fit in the cache.
Avoid false sharing: shared cache line unnecessarily with other threads/cores (due to how the data is packed) might have cache invalidating each other when anyone writes.

Super-simplified: What is a topology

Posted on November 16, 2017 by admin

‘Super-simplified’ is my series of brief notes that summarizes what I have learned so I can pick it up at no time. That means summarizing an hour of lecture into a few takeaway points.

These lectures complemented my gap in understanding open sets in undergrad real analysis, which I understood it under the narrow world-view of the real line.

$X$ : Universal set

Topology ≡ open + $\left\{\varnothing, X\right\}$

Open ≡ preserved under unions, and finite intersections.

Why finite needed for intersections only? Infinite intersections can squeeze open edge points to limit points, e.g. $\bigcap^{\infty}_{n}(-\frac{1}{n},\frac{1}{n}) = \left\{0\right\}$ .

Never forget that $\left\{\varnothing, X\right\}$ is always there because it might not have properties that the meat open set $B$ doesn’t have. e.g. a discrete topology of $\mathbb{Q}$ on $(0,1) = B$ $\subseteq$ universal set $X=\mathbb{R}$ means for any irrational point, $\mathbb{R}$ is the only open-neighborhood (despite it looks far away) because they cannot be ‘synthesized*’ from $\mathbb{Q}$ using operation that preserves openness.