Thread

C++26: Trivial infinite loops are no longer undefined behaviour

173 points · 290 comments · ibobev

oleganza · 1789744680 · focus · HN ↗
Why is null-terminated C string considered a "billion dollar mistake", but UB isn't?
1. fooker · 1789753105 · focus · HN ↗
  This comes from a fundamental misunderstanding of what UB is.
  Think of this piece of code - `y * x / y`.
  Would you like to simplify it to just `x` ?
  You need to either lean on UB to do so or have some magical way to prove that y can not be 0.
  Otherwise this transformation changes behavior, and is illegal.
  1. colejohnson66 · 1789757114 · focus · HN ↗
    Why not just execute what I wrote? Maybe I'm doing some rounding?
    layer8 · 1789767368 · focus · HN ↗
    Because we want compilers to perform the optimizations, not programmers.
    fooker · 1789773060 · focus · HN ↗
    Because we want people to write simple idiomatic code that runs fast today and also in ten years on alien hardware.
    > Maybe I'm doing some rounding?
    Compilers won't do this optimization when it is illegal to. If you disagree with the compiler's idea of what is legal, you can either write inline assembly, or put this code into an always inlined, but never optimized function.

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