On Feb 4, 2012, at 1:19 PM, Howard Hinnant wrote:
> On Feb 4, 2012, at 1:16 PM, Sebastian Redl wrote:
>
>>
>> On 04.02.2012, at 19:06, Howard Hinnant wrote:
>>
>>> On Feb 4, 2012, at 12:52 PM, Howard Hinnant wrote:
>>>
>>>> On Feb 4, 2012, at 12:04 PM, Eli Friedman wrote:
>>>>>
>>>>> [expr.shift]p2: [...] if E1 has a signed type and non-negative value,
>>>>> and E1×2E2 is representable in the result type, then that is the
>>>>> resulting value; otherwise, the behavior is undefined.
>>>>>
>>>>> -Eli
>>>>
>>>> I see, you're point is that I've walked into undefined territory because I
>>>> set the sign bit on the long long? Does changing 1LL to 1ULL make the
>>>> compiler happy?
>>>
>>> Another question: Is there a motivation for giving the compile time
>>> behavior of these operations a different behavior than they would have at
>>> run time?
>>
>> The runtime behavior is undefined. Do you really want the compile time
>> behavior to be the same?
>>
>> As a side note, I think the diagnostics here could still be improved.
>>
>> Sebastian
>
> It is undefined by the standards committee which has not had the willpower to
> abandon 1's complement hardware. I believe it is well defined behavior on
> every platform we support (2's complement hardware). I believe this compile
> time behavior is overly pedantic, does not reveal any programming error, and
> will only serve up busy work for clang's clients.
>
> Howard
Just noticed:
On Feb 3, 2012, at 7:33 PM, Richard Smith wrote:
> constexpr:
> The recent support for potential constant expressions exposed a bug in the
> implementation of libstdc++4.6, where numeric_limits<int>::min() is defined
> as (int)1 << 31, which isn't a constant expression. Disable the 'constexpr
> function never produces a constant expression' error inside system headers
> to compensate.
So it appears already that this is an issue wider than just libc++. And I
would be surprised if the issue isn't wide spread. Just did a quick search of
Boost and found this:
static BOOST_LLT min BOOST_PREVENT_MACRO_SUBSTITUTION (){ return 1LL <<
(sizeof(BOOST_LLT) * CHAR_BIT - 1); }
Please reduce this to a pedantic warning and provide a way to turn it off
locally even then.
Howard
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