Re: GCC optimizes integer overflow: bug or feature?

[Paolo Bonzini]

> > We've optimized expressions such as (a*2)/2 on the basis of overflow 
> > being undefined for a very long time, not just loops.
>
> What is (a*2)/2 optimized to? certainly it has the value a if you wrap, 
> so you are not necessarily depending on undefined here.

No, it has not.  For example, if a is 0x40000000 in a 32 bit type and 
arithmetic wraps, a*2 = -0x80000000 (overflow), and hence (a*2)/2 = 
-0x40000000 = -1073741824.

Paolo