Clang refuses to inline one-liners

[StreetwareGames]

https://godbolt.org/z/sKob5YhW3

You can uncomment either 1) or 2) and it will stop inlining. Seems to be a bug to me. Happens in both C and C++

[efriedma-quic]

We have a general heuristic that if a call is in the same block as an "unreachable", inlining is unlikely to be profitable because it's cold code. See 5390716 for reference.

Probably the heuristic is a little too aggressive here, though.

[StreetwareGames]

@efriedma-quic

The heuristic does indeed seem reasonable. From a performance point of view, inlining into a "cold" function is unlikely to be beneficial, and maybe even undesirable. But it definitely is too extreme, since it prevents the inlining of functions that other heuristics would say definitely should be inlined (local function called only once should obviously be inlined if possible). This heuristic also disregards that from a size standpoint, inlining can still be beneficial. Imagine you have a lot of error handling branches in your code that emit some diagnostic and then exit, and this diagnostic stuff uses some templated function to make the code more manageable. Now you potentially have hundreds of template instantiations of trivial functions that obviously should be inlined, but do not (unless you add __attribute__((always_inline)))