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

[Agner Fog <agner@agner.org>]

On 25-07-2011 11:43, Andrew Haley wrote:
> On 23/07/11 21:13, Agner Fog wrote:
>> 5). I have tested many different C++ compilers, and gcc turned out to be
>> the one that optimizes best.
> Well yes, and one of the reasons for this is that we take advantage of
> integer overflow being undefined.  There is an entire class of
> optimizations (loop induction variable optimization) that is difficult
> of impossible without taking advantage of this.  We don't do this kind
> of thing without good reason.
Actually, I think we can have the cake and eat it here. If gcc behaves 
reasonably safe by default and then makes warnings in case of missed 
optimization opportunities. The programmer should then have the 
opportunity to enable the desired optimizations, for example with 
pragmas at the critical places in the code. If the programmer doesn't 
need the best optimization then he would not enable those warnings and 
would be safe without having to care.

We already have the opportunity to allow the optimizer to ignore pointer 
aliasing at specific places with the 'restrict' keyword. It would be 
nice to have a similar opportunity to tell the compiler where it can 
ignore overflow.