Rich,
Thank you for the extremely informative and quick response! I suspect in the small, understandable places that use this function in this package it should be possible to avoid multiple evaluations (well, at least for this version of the code; no g'tee that would hold).
Out of interest, I presume there's no guarantee that alloca is aligned? It's not a feature I've ever used - it seems like a micro-optimisation for tight loops that should be made as part of a decision by a higher-level language (eg Vala, which, as I understand, doesn't).
BTW, This package has a few more issues than just musl ones... it doesn't understand cross-compilation for starters, makes insane use of code generation (always a big smell in my book, especially when done using a compiled language). Looks like it suffers from not enough peer review / usage...