On: A Tiny Wolfenstein-Like Software Engine

[pedrow]

Interesting that it passes/returns structs by value quite a lot which is not usual in C (in my experience.) I assume there's a reason for this? e.g.

static Hero spin(Hero hero, const uint8_t* key)

https://github.com/glouw/littlewolf/blob/339cb5624462d4f2301...

[glouwbug → pedrow]

I do so to enforce a single return value from functions.

Given:

    static void damage(const Hero* hero, const Sprite* sprite);

The const can easily dropped later in production from both entities, potentionally creating two return values.

This is analougous to good use of privacy in C++ classes.

    hero = spin(hero, key);

Becomes:

    hero.spin(key);

Where external objects dictate the private state of the hero object.

Pointer aliasing is another thing. Inlining is easier with fewer pointers. A fun C++ experiment is to count your instruction and data cache points with valgrind's cachegrind with pod types both passed by value and by reference (&).

[jeffreyrogers → pedrow]

I think it's just for simplicity, since all that copying can't be faster than passing in a reference to it.

[munificent → jeffreyrogers]

You might be surprised. For small structs, copying is very fast and may give you a result with better locality. Passing around structs by pointer means more pointer dereferencing, which has its own overhead and can cause cache misses.

Performance of modern CPUs is very nonobvious.

[glouwbug → munificent]

You are correct. On a much larger project with a similar software engine, if I change all structs I pass by value to pass by reference my instruction count shoots up by 40% according to cachegrind (flto + ofast).

Amazing how powerful pass by value can be when it comes to pod structs.