[Cryptech Tech] Headsup: Perfopt branch merged into modexp master

[Joachim Strömbergson]

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Aloha!

I've just merged a first set of changes to the modexp core to improve
the performance. The main changes are:

* init and add parts of the montprod inner loop fused. This removes
about 30-40% of the cycles in a modexp operation.


* operand size in montprod, memories and all sub modules are now generic
and can be changed during build time. Testbenches needs to be updated
and operand size used now is 32 bit. There is a wrapper in rtl/support
to allow test implementation of montprod with different operand sizes.
I've tested with 64 and 128 bit operands. Timing is no problem, size
scales linearly. On the Novena we might have resource problems using 128
bit operands, but 64 bits seems to be possible. 64 bits means cutting
the number of cycles in half.

Note that the API memories needs to be adjusted manually due to
replication that varies with size. The generic size needs to be tested
before being used.


* Modexp now supports explicit exponent length. For short exponents
(e=65537) the performance gain is huge compared to having the same
operand length as the modulus.

NOTE: This changes means that padding of the exponent should NOT be done
anymore. For e=65537 the length should be set to one (1) word.

This change provides some performance gains for big exponents too since
the padding has been eliminated. For 1024 bit exponent the performance
gain is about 3%.


I've built a Novena FPGA with a modexp with the perfopt changes.
Focus now is to complete operand size change to 64 bits.

- -- 
Med vänlig hälsning, Yours

Joachim Strömbergson - Alltid i harmonisk svängning.
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 Joachim Strömbergson          Secworks AB          joachim at secworks.se
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