Vincent Lefevre wrote:
If done in unsigned, this won't lead to any optimization, as unsigned arithmetic doesn't have overflows. So, if you write "a - 10" where a is unsigned, the compiler can't assume anything, whereas is a is signed, the compiler can assume that a >= INT_MIN + 10, reducing the range for a, and possibly allowing some optimizations.
Vincent, you missed the point, if we do this in unsigned, it works fine, and we don't WANT any optimization to intefere. That's what Richard was saying, if you want to do this kind of hand optimization of range checking, you do it in unsigned, and everything is fine. If you do it in signed expecting wrapping, then the optimization destroys your code. Yes, it is technically your fault, but this business of telling users "sorry, your code is non-standard, gcc won't handle it as you expect, go fix your code" is not very convincing to users who find other compilers that handle their code just fine. I remember that in Realia COBOL days, it was not just a matter of dealing with undefined situations, there were actually a couple of cases where the IBM compiler had semantics that were definitely non-conforming to the COBOL standard. Users depended on these bugs, so we carefully copied them in Realia COBOL. Sure, we could have explained to users that their code was junk, and the IBM compiler was wrong, but that would not have convinced them to move to using Realia COBOL!
