-funsafe-loop-optimizations

[Robert Dewar]

Geoffrey Keating wrote:

 >>But how useful? I would guess that virtually 100% of such warnings would
 >> be false positives, in the sense that they are not warning of situations
 >> in which the standard semantics would be fine. Let's take the example

>>void foo (unsigned int n)
>>> > {
>>> >   for (i = 0; i <= n; i++)
>>> >     bar ();
>>> > }
> 
>> 
>> I simply don't believe that anyone would write this code, expecting that
>> in the case where n was the max value, an infinite loop would occur, and
>> that's just what the programmer wants.

> They might not expect an infinite loop, but they might very well
> expect that the last case gets executed; for instance, if bar does
> 
> if (a[i] == i)
>   exit (0);
> 
> and 'a' happens to be an array with exactly 256 elements, and 'i' is
> of type 'unsigned char'.

Yes, of course, that's the buggy case.

> IMO, this kind of thing is a really slippery slope. 

We are talking about warnings here, so I don't see any slippery slope

 > There are
 > probably hundreds of optimisations that are much easier to do if you
 > just assume some apparently-trivial condition like "this procedure
 > terminates

Then they are not optimizations, they are examples of wrong code!
But we are talking warnings here, so this seems an irrelevant comment
anyway