[cfe-dev] 8.0 Regression with __builtin_constant_p.

[Eric Fiselier via cfe-dev]

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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>
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