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Why Const Doesn't Make C Code Faster

posted by Fnord666 on Sunday August 25 2019, @01:57PM   
from the quite-the-contrary dept.

upstart writes:

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:

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Original Submission

• Re:Learn the language! (Score: 2) by FatPhil on Monday August 26 2019, @07:51AM

  by FatPhil (863) <{pc-soylent} {at} {asdf.fi}> on Monday August 26 2019, @07:51AM (#885559) Homepage

  Not that you can't create counter-examples without, perhaps we should also be teaching programmers the evils of global variables, so that when the simplest counter-examples such as yours (yet, apparently, too complex for the AC to understand - what is the correct moderation for such a useless post?) are shown the young programmer knows that evil was being invoked during the process of breaking things.
  
  In order for const to help provide what newbs thinks const provides, you also need to teach them restrict and __attr__((pure)) (and thefore that in order to achieve their goals they need to escape the core language, and rely on individual implementations instead), and then hammer into them the fact that if you do end up with the desired optimisations it's only a side effect of what you've actually concretely got - namely an instruction that *you* are not permitted to alter the pointed-to object (via that pointer, modulo "restrict").
  
  It was a bad choice of word, and we're stuck with it, but that doesn't mean it's a bad concept, despite what some of the neighsayers are braying on about above.

  --
  Great minds discuss ideas; average minds discuss events; small minds discuss people; the smallest discuss themselves

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