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Brian Degenhardt
c8b51438cc
ipu_dither has had an SSE2 path and a scalar reference since forever, and
arm64 took the reference. The compiler closes half of that gap on its own —
with dithering off the loop is simple enough that clang vectorises it, and
measured here the scalar and NEON versions come out cycle-identical. With
dithering on it closes none of it: the clamp is written as std::max/std::min
around a table lookup, the destination is a 5/5/5/1 bitfield, and between
them the vectoriser gives up entirely. That arm ran at about 36 instructions
per pixel.
The NEON version is not a transliteration of the SSE2 one. x86 needs six
unpacks to split a row into channels because it has no deinterleaving load;
NEON has VLD4, so a whole 16-pixel row arrives already split one register per
channel and the shuffle chain simply does not exist. The dither tables are
the reference's coefficients with the sign folded into the choice of
operation, which lets saturating byte arithmetic supply the clamp for free —
the same trick the SSE2 path uses, and the reason both agree with the
reference bit for bit.
Measured on an M2 Max P-core, 2M macroblocks, two runs each:
dither on reference 18.76G instructions / 3.372G cycles
NEON 1.29G instructions / 0.293G cycles (11.5x)
dither off reference 1.08G instructions / 0.247G cycles
NEON 1.13G instructions / 0.247G cycles (even)
Function size drops from 476 to 208 bytes.
The tests are the point of the commit as much as the code is. Three
implementations of one function existed and nothing had ever compared them,
which is a bad shape here: a wrong result does not crash, it tints an FMV,
and nobody reports that. The transform depends on nothing but a pixel's four
bytes and its position modulo four in each axis, so the suite sweeps every
byte value through every one of the sixteen dither cells rather than
sampling. It holds whichever path the host selected to the reference, so it
gates the SSE2 arm on x86 exactly as it gates NEON here.
Proven to discriminate by mutation: transposing the r and b channels fails
three of four cases (correctly not the sweep that holds the channels equal),
perturbing one dither cell by one fails two, and dropping saturation fails
all four.
ipu_dither_reference loses its __ri so that a symbol survives into Release
for the tests to call.