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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:

Original Submission