[PATCH] libstdc++: Implement std::unreachable() for C++23 (P0627R6)

[Marc Glisse]

On Thu, 31 Mar 2022, Jonathan Wakely wrote:

> On Thu, 31 Mar 2022 at 17:03, Marc Glisse via Libstdc++
> <libstdc++@gcc.gnu.org> wrote:
>>
>> On Thu, 31 Mar 2022, Matthias Kretz via Gcc-patches wrote:
>>
>>> I like it. But I'd like it even more if we could have
>>>
>>> #elif defined _UBSAN
>>>    __ubsan_invoke_ub("reached std::unreachable()");
>>>
>>> But to my knowledge UBSAN has no hooks for the library like this (yet).
>>
>> -fsanitize=undefined already replaces __builtin_unreachable with its own
>> thing, so I was indeed going to ask if the assertion / trap provide a
>> better debugging experience compared to plain __builtin_unreachable, with
>> the possibility to get a stack trace (UBSAN_OPTIONS=print_stacktrace=1),
>> etc? Detecting if (the right subset of) ubsan is enabled sounds like a
>> good idea.
>
> Does UBsan define a macro that we can use to detect it?

https://github.com/google/sanitizers/issues/765 seems to say no (it could 
be outdated though), but they were asking for use cases to motivate adding 
one. Apparently there is a macro for clang, although I don't think it is 
fine-grained.

Adding one to cppbuiltin.cc testing SANITIZE_UNREACHABLE looks easy, maybe 
we can do just this one, we don't need to go overboard and define macros 
for all possible suboptions of ubsan right now.

I don't think any of that prevents from pushing your patch as is for 
gcc-12.

-- 
Marc Glisse