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Comment by martincmartin | Hacker News Reader

  for (s = i = 0; i < NUMBER; ++i) {
      s += 1;
  }

I'm surprised gcc can't figure out that this is NUMBER*(NUMBER-1) and eliminate the loop entirely.

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> I'm surprised gcc can't figure out that this is NUMBER * (NUMBER-1)

Huh? Do you mean NUMBER instead? If it were s += i then that would be NUMBER * (NUMBER-1) / 2.

You're right, I forgot the / 2.

It probably can, at high-enough optimization level.

gcc and clang both perform this optimization for me at -O3.

Thanks. On godbolt, I see clang do it at -O3. But gcc still has a loop, where the body has add but no mul, although I don't fully understand the body. Where do you see it in gcc?

https://godbolt.org/z/sP194Y

They used -O2, which produces the straight forward loop. [0]

But with -O3, for NUMBER > 51, is uses some other odd loop. [1]

Seems to be O(n) no matter what though.

[0]: https://godbolt.org/z/czcaYf [1]: https://godbolt.org/z/sP194Y

Clang knows what’s up: https://godbolt.org/z/7zKffz. GCC is adding the numbers four at a time by stuffing 0, 1, 2, 3 in an XMM register, taking a packed addition within the register, and then doing a pairwise packed addition of the register with 4, 4, 4, 4 to get the next four numbers.

Did you mean s += i; ?

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