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

[Hans Wennborg via cfe-dev]

Okay, sounds like this is important enough, and now that there's a fix
committed I'll make sure it gets into the release. Let's have it bake
in trunk a little bit first though.

On Sat, Mar 9, 2019 at 12:08 AM Bill Wendling via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
>
> 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
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