[frutiger]

You don't have much choice with commodity hardware - but you could perform crypto on hardware/chips where the behavior is completely specified.

Note that this is not very practical, and impractical crypto is almost as good as no crypto.

[makomk]

This is often, though not always, the case with deeply embedded processors of the kind BearSSL seems to be designed for. There's generally some way to get predictable cycle-exact performance on them because it matters for some embedded applications.