Comment by PaulKeeble

16 hours ago

The thing is about these optimisations (assuming they test as higher performance) is that they can get applied in a library and then everyone benefits from the speedup that took some hard graft to work out. Very few people bake their own date API nowadays if they can avoid it since it already exists and techniques like this just speed up every programme whether its on the critical path or not.

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PaulKeeble

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codexb  16 hours ago

That's basically compilers these days. It used to be that you could try and optimize your code, inline things here and there, but these days, you're not going to beat the compiler optimization.

  • ryao  15 hours ago

    Meanwhile, GCC will happily implement bsearch() without cmov instructions and the result will be slower than a custom implementation on which it emits cmov instructions. I do not believe anyone has filed a bug report specifically about the inefficient bsearch(), but the bug report I filed a few years ago on inefficient code generation for binary search functions is still open, so I see no point in bothering:

    https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110001

    Binary searches on OpenZFS B-Tree nodes are faster in part because we did not wait for the compiler:

    https://github.com/openzfs/zfs/commit/677c6f8457943fe5b56d7a...

    Eliminating comparator function overhead via inlining is also a part of the improvement, which we would not have had because the OpenZFS code is not built with LTO, so even if the compiler fixes that bug, the patch will still have been useful.

  • godelski  14 hours ago

    Definitely not true.

    You aren't going to beat the compiler if you have to consider a wide range of inputs and outputs but that isn't a typical setting and you can actually beat them. Even in general settings this can be true because it's still a really hard problem for the compiler to infer things you might know. That's why C++ has all those compiler hints and why people optimize with gcc flags other than -O.

    It's often easy to beat Blas in matrix multiplication if you know some conditions on that matrix. Because Blas will check to find the best algo first but might not know (you could call directly of course and there you likely won't win, but you're competing against a person not the compiler).

    Never over estimate the compiler. The work the PL people do is unquestionably useful but they'll also be the first to say you can beat it.

    You should always do what Knuth suggested (the often misunderstood "premature optimization" quote) and get the profile.

  • matheusmoreira  15 hours ago

    These days optimizing compilers are your number one enemy.

    They'll "optimize" your code by deleting it. They'll "prove" your null/overflow checks are useless and just delete them. Then they'll "prove" your entire function is useless or undefined and just "optimize" it to a no-op or something. Make enough things undefined and maybe they'll turn the main function into a no-op.

    In languages like C, people are well advised to disable some problematic optimizations and explicitly force the compiler to assume some implementation details to make things sane.

    • ryao  14 hours ago

      If they prove a NULL check is always false, it means you have dead code.

      For example:

        if (p == NULL) return;

  if (p == NULL) doSomething();

      It is safe to delete the second one. Even if it is not deleted, it will never be executed.

      What is problematic is when they remove something like memset() right before a free operation, when the memset() is needed to sanitize sensitive data like encryption keys. There are ways of forcing compilers to retain the memset(), such as using functions designed not to be optimized out, such as explicit_bzero(). You can see how we took care of this problem in OpenZFS here:

      https://github.com/openzfs/zfs/pull/14544

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