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EFM decoder performance improvements #801

Merged
merged 3 commits into from
Oct 2, 2022
Merged

EFM decoder performance improvements #801

merged 3 commits into from
Oct 2, 2022

Commits on Oct 2, 2022

  1. Use const pointers in F3Frame.

    @atsampson
    atsampson committed Oct 2, 2022
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  2. Rewrite the frame-unpacking code in F3Frame::setTValues. 

    This code converted T-values to a representation of the frame, then
extracted EFM values from it. Both phases looped through every bit of
the frame, which was very slow.

Rewrite it to go directly from T-values to EFM values, with a single
loop that executes once per T-value rather than once per bit. Profiling
shows it now takes 5% of ld-process-efm's runtime rather than 50%.

Comparing the output of the two versions also revealed that the original
code would sometimes write the very last bit in the wrong place, usually
hitting a merging bit but very occasionally corrupting the last EFM
value in the frame (1-3 times an hour). So the new code produces very
slightly better results.
    @atsampson
    atsampson committed Oct 2, 2022
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  3. Use a hash table to decode EFM symbols. 

    The existing code did a linear search across a 256-entry array. Since
this fits in L1 cache it wasn't as bad as it looked, but doing it with a
hash table instead reduces the number of memory reads by a factor of
100 (at most 2 reads per lookup).

The hash function here was designed by an exhaustive search using a
Python program.
    @atsampson
    atsampson committed Oct 2, 2022
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