std::unreachable doing nothing, currently

[AlexBAV]

It looks like with the current compiler implementation, std::unreachable does not affect optimizer. __assume(false), on the other hand, seems to do the job.

std::unreachable is implemented by means of __assume(false), however, as illustrated in the following issue:

https://developercommunity.visualstudio.com/t/Suboptimal-code-generation-for-std::unre/10071879
https://godbolt.org/z/58T55q7Mf

compiler misses the opportunity in this case.

According to the compiler team's feedback, they are currently not very interested in fixing this. However, it looks like if __forceinline is added to the function declaration, compiler is again capable of using the optimization hint, at least in simple tests:

STL/stl/inc/utility

Line 816 in febb643

[[noreturn]] inline void unreachable() noexcept /* strengthened */ {

[AlexBAV]

The following illustrates this (foo uses __assume(false), bar uses std::unreachable and good_bar uses patched unreachable):

https://godbolt.org/z/a9d33hc5b

[StephanTLavavej]

Thanks, this codegen helps me confidently approve #3055.