Dead std::stack/std::deque is not being eliminated

[firewave]

An unused std::stack object with the default std::deque backend is not being eliminated. This is also true for GCC - see https://godbolt.org/z/749EGxjx1.

If you change the backend to std::vector it will be completely eliminated in Clang even if you pushed data into it. GCC will only completely eliminate it if it doesn't contain any data - see https://godbolt.org/z/fsacae58z.

[EugeneZelenko]

Could you please clarify which standard C++ library was used?

[firewave]

I am using libstdc++.

[fhahn]

At a first glance, it looks like the std::vector case gets optimized away because LLVM inlines both the std::vector constructor and the Stack.push(), while that's not the case for the default.

[llvmbot]

@llvm/issue-subscribers-clang-codegen

[firewave]

This also applies to other STL containers like std::map, std::list, std::set - std::vector appears to be the only outlier - https://godbolt.org/z/q68svnqfo.

Interestingly GCC is not doing it for std::vector.