Architecture-specifc assembly code is stored in different subdirectories.
Currently (March 2013), these are
  powerpc64
  x86_64

In these subdirs, there is size-specific asm code for combined
multiplication and redc. The sizes are currently 1 to 20 limbs. If
needed, one could go to higher sizes, but is there a need?  There is also
a redc function coded in asm (without mul).

The files are automatically generated using a Python script. This
generation is not done at configure or compile time, to avoid a
dependency to Python. However, the script is given, for developpers to
play with.

At configure, if asm-redc is enabled, symbolic links are done to the
.asm files in the appropriate directory. 

Note (added March 11, 2013): on most architectures those asm files are not used
any more, since using mpn_mul_n or mpn_sqr and mpn_redc{1,2,n} is faster (see
the bench_mulredc program and the params.h files).