[Cryptography] GCC bug 30475 (was Re: bounded pointers in C)

[Bear]

There are in fact conformant checks for signed overflow. 

For example, you can say 

if (MAXINT - al >= be) halt(2); // overflow check - not using assert()
                                // because we want this check in
                                // production code.
ce = al + be;                   // addition guaranteed not to overflow.

What you can't do is check for overflow *AFTER* the operation that 
might commit an overflow.  The instant you actually perform an 
operation that might commit an overflow, the compiler is in fact 
free to create any old evil code it wants including ignoring your 
subsequent check.

This is why signed-overflow detection is one of the hardest things 
about writing a compiler that actually catches and warns about 
implementation-dependent behavior.  You have to take the condition
in the assertion and use it to make a proof that overflow cannot 
happen in the case of that addition, in order to avoid warning on 
the assignment of ce to the sum of al and be.  

			Bear