[cfe-dev] Traps for signed arithmetic overflow

[Liu Hao via cfe-dev]

在 2018/11/24 5:36, Marc Glisse 写道:
> On Fri, 23 Nov 2018, Helmut Eller wrote:
> 
>> On Fri, Nov 23 2018, Marc Glisse wrote:
>>
>>> On Fri, 23 Nov 2018, Helmut Eller wrote:
>>>
>>>> when compiling this example with gcc -O2 -ftrapv:
>>>> [...]
>>> Try with "-fsanitize=undefined
>>> -fsanitize-undefined-trap-on-error". -ftrapv is quite broken and not
>>> really maintained. I think fixing it would mean making it an alias for
>>> the sanitizer version.
>>
>> Thanks for the quick reply.  That does indeed produce exactly the code I
>> want.
>>
>> It doesn't seem so detect overflow for short or char types, though:
> 
> That's because there is no overflow. Read about integer promotion.

I think I had better say something about this.

You are right; there is no overflow. But who needs such apparently
deficient (if not incomplete at all) behavior? People could start
arguing that integer promotion is mandated by the C standard and is what
'portable' code might rely on. Yes you are right, totally right about a
_very_ poor excuse: The so-called 'portable' code should not rely on the `int`
-> `short` conversion either, which is implementation-defined and not so
portable after all. It is a shame for C to have integer promotion. It is
a flaw for UBSan to discriminate overflows on `short`.


-- 
Best regards,
LH_Mouse