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Submitted via IRC for SoyCow3196
Why const Doesn't Make C Code Faster
In a post a few months back I said it's a popular myth that const is helpful for enabling compiler optimisations in C and C++. I figured I should explain that one, especially because I used to believe it was obviously true, myself. I'll start off with some theory and artificial examples, then I'll do some experiments and benchmarks on a real codebase: Sqlite.
Let's start with what I used to think was the simplest and most obvious example of how const can make C code faster. First, let's say we have these two function declarations:
void func(int *x);
void constFunc(const int *x);And suppose we have these two versions of some code:
void byArg(int *x)
{
printf("%d\n", *x);
func(x);
printf("%d\n", *x);
}void constByArg(const int *x)
{
printf("%d\n", *x);
constFunc(x);
printf("%d\n", *x);
}To do the printf(), the CPU has to fetch the value of *x from RAM through the pointer. Obviously, constByArg() can be made slightly faster because the compiler knows that *x is constant, so there's no need to load its value a second time after constFunc() does its thing. It's just printing the same thing. Right? Let's see the assembly code generated by GCC with optimisations cranked up:
(Score: 1, Funny) by Anonymous Coward on Sunday August 25 2019, @05:07PM (3 children)
C, Algol, and Fortran are obsolete languages for old people. Rockstar coders don't use old dead languages for social media marketing. Coding today is about selling an app to a billion idiot users, not about being clever with a computer.
(Score: 2) by barbara hudson on Sunday August 25 2019, @07:17PM (2 children)
Speaking of which, WTF is up with getting rid of the "volatile" keyword?
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(Score: 2) by Immerman on Monday August 26 2019, @04:56PM (1 child)
Say what?!? They're getting rid of volatile? I hadn't heard that, and it's madness. Not that I've used it very often, maybe once every few years, but when it's needed....
What's the alternative? Especially in a world where multi-threading has become compulsory if you want decent performance. Just assume everything is volatile and eliminate vast swaths of optimizations? Lock anything that might possibly be modified elsewhere behind mutexes? ...I suppose that might actually be good practice in a lot of cases, but I shudder at the thought of the performance penalties it would impose.
(Score: 2) by barbara hudson on Thursday August 29 2019, @02:27AM
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