[cfe-dev] Additional instantiations of anonymous enum inside a template

[Pavel Morozkin]

On 13 October 2014 23:51, Richard Smith <richard at metafoo.co.uk> wrote:

> On Mon, Oct 13, 2014 at 12:48 PM, Richard Smith <richard at metafoo.co.uk>
> wrote:
>
>> On Mon, Oct 13, 2014 at 12:02 PM, András Kucsma <r0maikx02b at gmail.com>
>> wrote:
>>
>>> Hi,
>>>
>>> I'm one of the developers of Metashell (
>>> https://github.com/sabel83/metashell), more specifically the developer
>>> of metadebugger, which was used to reproduce this issue.
>>>
>>> Metashell uses Templight (http://plc.inf.elte.hu/templight/) to gather
>>> information about the template instantiations.
>>>
>>> I put together a small test which reproduces the issue without
>>> Metashell, using only a clang patched with Templight:
>>> https://gist.github.com/r0mai/4d90fdc8007af96c2dd1
>>>
>>
>> I don't see any Clang bug here. Templight is not reporting template
>> instantiations; you're misinterpreting its output. 129 of the 130 instances
>> are of kind "Memoization", which is Templight's way of saying "Clang
>> checked whether the type was complete". There are no additional
>> instantiations here.
>>
>
> Sorry, I missed one; there are two TemplateInstantiations in that log, not
> one. One for fib<2> and one for int_<1>.
>

Thanks for the explanation. Could you briefly explain what Clang
understands under term «type completeness» regarding enum and why the
compiler does such amount of checks?


> You can find the trace that Templight produced in the gist as well.
>>> Memoization events of ENUM appear 130 times while instantiating
>>> int_<fib<2>::value>.
>>>
>>> I CC-ed in two guys (Zoltán Porkoláb and Zoltán Borók-Nagy). They will
>>> be able to help with Templight related questions.
>>>
>>> Regards,
>>> András
>>>
>>> On Mon, Oct 13, 2014 at 6:02 PM, Richard Smith <richard at metafoo.co.uk>
>>> wrote:
>>>
>>>>
>>>> On 13 Oct 2014 01:06, "Pavel Morozkin" <pavel.morozkin at gmail.com>
>>>> wrote:
>>>> >
>>>> > On 13 October 2014 03:53, Richard Smith <richard at metafoo.co.uk>
>>>> wrote:
>>>> >>
>>>> >> On Sun, Oct 12, 2014 at 3:29 AM, Pavel Morozkin <
>>>> pavel.morozkin at gmail.com> wrote:
>>>> >>>
>>>> >>> Hello cfe-dev group,
>>>> >>>
>>>> >>> Consider this template metaprogram:
>>>> >>> template <unsigned N> struct fib { enum { value = fib<N-1>::value +
>>>> fib<N-2>::value}; };
>>>> >>> template <> struct fib<1> { enum { value = 1 }; };
>>>> >>> template <> struct fib<0> { enum { value = 0 }; };
>>>> >>>
>>>> >>> During compilation of 'fib<2>::value' (after finally getting
>>>> memoized result of instantiation of struct fib<0>) Clang «instantiates the
>>>> anonymous enum inside fib<0> and fib<1>» 129 times in sum.
>>>> >>
>>>> >>
>>>> >> That sounds like a bug. How are you observing it?
>>>> >
>>>> >
>>>> > I debugged the metaprogram with use of Metashell tool.
>>>>
>>>> Possibly this is a bug in metashell? How does it get this information?
>>>>
>>>> >>> In case, when we use 'constexpr static auto' instead of 'enum', no
>>>> additional instantiations occurs. Why Clang performs additional
>>>> instantiations in the first case?
>>>> >>>
>>>> >>> Thank you,
>>>> >>> Pavel
>>>> >>>
>>>> >>> _______________________________________________
>>>> >>> cfe-dev mailing list
>>>> >>> cfe-dev at cs.uiuc.edu
>>>> >>> http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev
>>>> >>>
>>>> >>
>>>> >
>>>>
>>>
>>>
>>
>
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