https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95264
--- Comment #8 from Freddie Witherden <freddie at witherden dot org> --- (In reply to rguent...@suse.de from comment #7) > > Instead of [[gnu::flatten]] you could use the > __attribute__((always_inline)) attribute on the foo function definition > if you didn't simplify the outline above too much to make that > infeasible. IIRC we do not have sth like > > [[gnu::inline]] foo(i, ...); > > to force inlining of a specific call, nor [[gnu::noinline]] foo(i, ...); > both which seem useful. Not sure if the C++ syntax would support > such placement of an attribute of course. So this is exactly what we had in the pre-flatten version of the code: https://github.com/PyFR/Polyquad/commit/f24366c059d2d693222985cdd9333238bd909ad3 The issue was while GCC would inline the annotated functions it would go no further. As such, if I recall correctly, all of the constructor calls to the relatively simple Eigen vector types were no longer inlined. Thus a line of code which should translate into a few register-to-memory mov instructions results in a a constructor call, an assignment call, and some cleanup. Since I could not add the force inline attribute to the library types I went in search of an alternative. For the T = bfloat eval_orthob instance is the "if (std::is_fundamental<T>::value)" considered before the body is inlined?
