tangentstorm/coinops · GitHub

coinops is a repo for experimenting with optimization of freepascal code via x86-64 assembly language, using an iterated sha256 program (the hashing algorithm used in bitcoin) as a subject.

This junkops branch contains an experiment that shows how adding useless junk operations (moving the contents of an arbitrary register into ram) can increase the speed of the executed code.

A compiler flag is used to toggle the inclusion of several lines in sha256_mjw.pas, starting on line 64.

avg time(ms) with -dJUNKOPS: 1822.84ms
avg time(ms) without: 1836.44ms

Why would adding extra useless operations to the code speed it up?

I have no idea. Hopefully someone out there can explain:

http://stackoverflow.com/questions/17896714/why-would-introducing-useless-mov-instructions-speed-up-a-tight-loop-in-x86-64-a

To run the experiment for yourself, you need free pascal for x86_64 from http://freepascal.org/ as well as python to run the experiment.