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The various mpn subdirectories contain machine-dependent code, written in C or assembly. The mpn/generic subdirectory contains default code, used when there’s no machine-specific version of a particular file.
Each mpn subdirectory is for an ISA family. Generally 32-bit and 64-bit variants in a family cannot share code and have separate directories. Within a family further subdirectories may exist for CPU variants.
In each directory a nails subdirectory may exist, holding code with
nails support for that CPU variant. A NAILS_SUPPORT directive in each
file indicates the nails values the code handles. Nails code only exists
where it’s faster, or promises to be faster, than plain code. There’s no
effort put into nails if they’re not going to enhance a given CPU.