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by teo_zero 86 days ago
> it is legal to optimize out any code paths that rely on this, even if they occur earlier in program order.

I don't think this is true. The compiler cannot remove or reorder instructions that have a visible effect.

  if (p == 0)
    printf("Ready?\n");
  *p++;
The printf() can't be omitted.
2 comments
aw1621107 86 days ago
> The compiler cannot remove or reorder instructions that have a visible effect.

You might be surprised! When it comes to UB compilers can and do reorder/eliminate instructions with side effects, resulting in "time travel" [0].

IIRC the upcoming version of the C standard bans this behavior, but the C++ standard still allows it (for now, at least).

[0]: https://devblogs.microsoft.com/oldnewthing/20140627-00/?p=63...

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saagarjha 86 days ago
No, this is explicitly legal. Most compilers will shy away from it these days since it made a lot of people upset, but it's definitely allowed.
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