Re: GCC optimizes integer overflow: bug or feature?

[Robert Dewar]

Gabriel Dos Reis wrote:

> I don't believe this particular issue of optimization based on
> "undefined behaviour" can be resolved by just telling people "hey
> look, the C standard says it is undefined, therefore we can optimize.
> And if you're not happy, just tell the compiler not to optimize".
> For not all undefined behaviour are equal, and undefined behaviour is
> also a latitude given to implementors to better serve their base
> users.

Right, and better service is a combination of doing what is expected
and generating fast code, and sometimes the two do conflict, and then
we have to do the best we can to balance the conlicting goals.

My concern is that in achieving this balance, we really need to have
data to show that we are indeed generating significantly faster code.
Compiler writers tend to want to optimize the last instruction out
in any situation, without necessarily showing it is significant to
do so.