r11-2663 causes static_assert failure (was: Re: std:vec for classes with constructor?)

[Tobias Burnus]

Hi,

I got a bit lost in this thread – but the
commit r11-2663-g82c4b78dbef6f03838e3040688c934360a09513f
"Replace std::vector<> usage in ipa-fnsummary.c with GCC's vec<>."

Causes here:

gcc-mainline/gcc/vec.h:1287:3: error: static assertion failed:
    static_assert (alignof (vec_embedded) == alignof(vec), "");
    ^
for a '--build=i686-pc-linux-gnu --host=powerpc64le-linux-gnu --target=nvptx-none'
build.

Tobias


On 8/7/20 8:04 PM, Aldy Hernandez via Gcc-patches wrote:

> Is the attached patch what y'all suggest?
>
> The patch reverts the GCC vec<> usage in ipa-fnsummary.c and
> implements the above suggestion.
>
> Bootstraps, tests running...
>
> If tests pass, is this OK?
>
> Thanks.
> Aldy
>
> On Fri, Aug 7, 2020 at 11:17 AM Jonathan Wakely via Gcc-patches
> <gcc-patches@gcc.gnu.org> wrote:
>> On 07/08/20 10:55 +0200, Jakub Jelinek wrote:
>>> On Fri, Aug 07, 2020 at 09:34:38AM +0100, Jonathan Wakely via Gcc-patches wrote:
>>>>> Now that you say it, vec has a T[1] member so depending on T
>>>>> there might be a side-effect as invoking its CTOR?  Or it might
>>>>> even not compile if there is no default CTOR available...  Ick :/
>>>> Right.
>>>>
>>>> Does the GTY stuff add members to the struct, or otherwise alter its
>>>> layout?
>>> Or perhaps use offsetof on an alternate structure that should have the same
>>> layout.
>> Yes that's what I was going to suggest next.
>>
>>> So instead of
>>>   typedef vec<T, A, vl_embed> vec_embedded;
>>>   return offsetof (vec_embedded, m_vecdata) + alloc * sizeof (T);
>>> use
>>>   struct alignas (T) U { char data[sizeof (T)]; };
>>>   typedef vec<U, A, vl_embed> vec_embedded;
>> static_assert(sizeof(vec_embedded) == sizeof(vec), "");
>> static_assert(alignof(vec_embedded) == alignof(vec), "");
>>
>>>   return offsetof (vec_embedded, m_vecdata) + alloc * sizeof (T);
>>> where vec_embedded should have the same offset of m_vecdata as vec.
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