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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, Troll) by Anonymous Coward on Sunday August 25 2019, @02:20PM (8 children)
They could try using ProggyClean with blackboard theme for readability. Code-wise, they could use Contributor Covenant to squash any inclusivity bugs.
(Score: 0) by Anonymous Coward on Sunday August 25 2019, @02:46PM (4 children)
Here we go, was not enough to watch for null pointers and bounds but now also for minorities and LGTB code.
What's next, an FSM exception code?
(Score: 3, Funny) by The Mighty Buzzard on Sunday August 25 2019, @02:56PM
Nope, exceptions are problematic since they imply that the caller is somehow exceptional.
My rights don't end where your fear begins.
(Score: 0) by Anonymous Coward on Sunday August 25 2019, @07:28PM (1 child)
What should I do if my NullPointerException has completed hir transition from an ArrayOutOfBoundsException?
(Score: 2) by maxwell demon on Sunday August 25 2019, @08:55PM
Throw a SnowFlakeException.
The Tao of math: The numbers you can count are not the real numbers.
(Score: 0) by Anonymous Coward on Monday August 26 2019, @03:03PM
More like the owners and managers who recognize harassment and gay bashing don't hire people who create hostile work environments, and intelligent open source projects don't give a shit about your mad skillz until you learn to be a human being who doesn't hold people's sexual orientations or physiologic characteristics against them.
I suppose you're against working with all them cripples and darkies too.
(Score: 0) by Anonymous Coward on Sunday August 25 2019, @02:57PM (2 children)
Please don't let this guy on our team. Please don't let this guy on our team. Please don't let...
(Score: 0) by Anonymous Coward on Sunday August 25 2019, @06:48PM (1 child)
You are just scared the boss will find out your code is riddled with exclusivity.
(Score: 0) by Anonymous Coward on Monday August 26 2019, @10:50AM
Yes, it's scary. She may get promoted and asked to supervise snowflake's blackbox gay code, and that sucks.