[PATCH][4.3] Deprecate -ftrapv

[Robert Dewar]

Florian Weimer wrote:
> * Robert Dewar:
> 
>> So here's exactly what happens now in Ada
>>
>> 1. If -gnato is not set (no overflow checking)
>>
>> In this case, the situation is exactly the same as C, and the front end
>> ignores the possibility of overflow.
> 
> It's C in -fwrapv mode, isn't it?  (Otherwise, you can't really get the
> other constraint checks to work, including bounds checking for array
> access.)

No, it is exactly C, if an arithmetic operation causes overflow
and you do not have -gnato set, and overflow occurs, the program
execution is erroneous. I reread the last para above more than once
but cannot guess what you are getting.
> 
>> The official Ada semantics are that if you suppress overflow checks,
>> and an overflow occurs (intermediate or otherwise), the executino is
>> erroneous (undefined in C-speak). So Ada with checks off is in all
>> respects identical to C semantics. No problem.
> 
> Uh-oh, this is really, really surprising.

Why is it surprising, if you suppress overflow checks you are saying
that overflow does not occur, just as you say this for *any* C program.

I don't understand the surprise or the concern here, can you
elucidate?

If you think your program might have integer overflow, and
you want to check for this, you must use -gnato.