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

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

 
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  • (Score: 2, Informative) by Anonymous Coward on Sunday August 25 2019, @06:49PM

    by Anonymous Coward on Sunday August 25 2019, @06:49PM (#885299)

    Look at his command line.

    gcc -S -Wall -O3 test.c

    That is the most aggressive optimization switch. Two of those are basic constant folding and basic constant propagation. When compiling, it can look and determine both of the variables, despite only one being marked, are not changed during actual execution and makes such a substitution, when it believes it is safe to do so. To your point, when GCC compilers were dumber "const" was important because it would signal the compiler you could handle the constants that way and it didn't have to guess whether or not it was safe.

    And that doesn't get into any other black magic the -O3 switch will do. If anything, he should have shown what a bare command line, -O, -O2, and -Os, all against the same code. He is smart enough to know the difference between a t test and a U test (although not that these are dependent samples and a different test should be used). I reckon he is smart enough to try and show the difference that way.

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