[llvm-dev] [cfe-dev] 8.0 Regression with __builtin_constant_p.

[Bill Wendling via llvm-dev]

Ah! Cool. I'll let the release manager make that decision. :-)

On Fri, Mar 8, 2019 at 2:57 PM Eric Fiselier <eric at efcs.ca> wrote:

> Hi Bill,
>
> I committed a fix already as r355743 [1], and it fixes `test2` as well.
>
> I believe we should merge this into 8.0.
>
> /Eric
>
> [1]
> https://github.com/llvm/llvm-project/commit/680e865c313a80b6ec329abde61e1f0c66bdc103
>
> On Fri, Mar 8, 2019 at 5:46 PM Bill Wendling <isanbard at gmail.com> wrote:
>
>> Hi Eric,
>>
>> The attached patch may help matters, though it now fails because "test2"
>> doesn't exist (it's not enabled).
>>
>> -bw
>>
>>
>>
>> On Fri, Mar 8, 2019 at 11:38 AM Eric Fiselier via cfe-dev <
>> cfe-dev at lists.llvm.org> wrote:
>>
>>> Hi All,
>>>
>>> I know we're late in the release process, but I've discovered a
>>> regression I believe is serious enough to hold the release [1].
>>>
>>> The regression is in usages of `__builtin_constant_p` applied to a
>>> dereferenced pointer in a constant expression. In certain cases Clang now
>>> rejects this as a non-constant expression [2].
>>>
>>> This regression will have a larger impact than it initially appears.
>>>
>>> libstdc++ 7.1 and newer use `__builtin_constant_p` to implement C++17
>>> `std::char_traits` [4]. This means that any usage of `std::string_view` in
>>> a constant expression has the possibility to break. `__builtin_constant_p`
>>> may be rare, but `std::string_view` is not.
>>>
>>> This regression was first found because it breaks `absl::StrFormat` [4].
>>> This bug will break Abseil's LTS w/ libstdc++. Abseil cannot fully work
>>> around this bug.
>>>
>>> I would implore us to fix this bug in the upcoming 8 release.
>>>
>>> /Eric
>>>
>>> [1] https://reviews.llvm.org/D59038
>>> [2] https://godbolt.org/z/VOCCJF
>>> [3] https://github.com/abseil/abseil-cpp/issues/271
>>> [4]
>>> https://github.com/gcc-mirror/gcc/blob/gcc-8_1_0-release/libstdc++-v3/include/bits/char_traits.h#L229
>>> _______________________________________________
>>> cfe-dev mailing list
>>> cfe-dev at lists.llvm.org
>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>>>
>>
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