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Re: Is it OK that gcc optimizes away overflow check?


On 26/07/11 10:38, Agner Fog wrote:

> We need to be constructive here. We want the optimizations, but we
> also want to check if the optimizations go wrong. I think the
> compiler should be able to generate warnings for every unsafe
> optimization,

Continuing to describe these as "unsafe optimizations" is tendentious
at best, and doesn't help the discussion to continue in a polite way.

> especially when removing a branch. The compiler generates a warning
> when optimizing away if (a+5 < a) but not when optimizing away if
> (abs(a)<0).

I think you'd be surprised at just how many warnings would be
generated.

Consider

  for (i = 0; i < limit; i++)
    f(i * 2);

which can be rewritten to

  tmp = limit * 2;
  for (i = 0; i < tmp; i++)
    f(i);

but only if you know integer overflow in i*2 can't occur.

This kind of thing happens all the time, in many programs.  It isn't
usually a problem because the programmer "just knows" that i*2 is less
than INT_MAX.

Andrew.


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