Both loops indexed the delay line as (in_idx + j + delay) & mask, one
masked index per tap, which made the address non-affine and stopped the
vectorizer. The window wraps at most once, so split it at the wrap and
walk two contiguous runs instead.
As in resample.c, the y/sigma reduction cannot be reassociated without
-ffast-math (which this library must not enable, see linux_port.h), so
carry four independent accumulator pairs to break the FMA dependency
chain and let the vectorizer in.
in_buff and out_buff alias in RXA -- both are midbuff -- so only the
private d/w arrays are marked restrict.
Measured in situ on an Apple M1 Pro, 512-sample buffers, cost of turning
the block on, best of 5:
anr 59303 ns -> 20253 ns 2.93x
anf 55073 ns -> 20511 ns 2.68x
Summation order changes, so output is not bit-identical: over 300 buffers
of the full RX chain the worst deviation is 2.4e-07, an SNR of 153 dB.
An LMS filter is an adaptive feedback loop, so its trajectory is
chaotic. As a control, perturbing a single input sample of the unmodified
code by one ulp diverges it from itself by 6.6e-07, an SNR of 144.6 dB --
i.e. this change disturbs the filter less than the last bit of the input
does.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
The tap loop wrapped the ring with a test on every tap:
if ((idx_out = idx_in + j) >= ringsize) idx_out -= ringsize;
which made the address non-affine and stopped the vectorizer. Split the
walk at the wrap point instead, so both halves are unit-stride, and split
the complex ring into separate I/Q arrays so the taps load contiguously
rather than through a de-interleaving ld2.
The dot product also could not be vectorized as written: reassociating an
fp reduction needs -ffast-math, which this library must not enable (it
relies on IEEE semantics for 0/0 = NaN and x/0 = Inf, see linux_port.h).
Carry four independent accumulator pairs instead, which both breaks the
FMA dependency chain and lets the vectorizer in on any compiler.
Measured on an Apple M1 Pro, 512-sample DSP buffers, best of 5:
xresample, decimation to 48 kHz before after speedup
192k -> 48k (561 taps) 342.0 us 84.2 us 4.06x
384k -> 48k (1121 taps) 708.9 us 175.1 us 4.05x
576k -> 48k (1681 taps) 1074.4 us 266.9 us 4.03x
768k -> 48k (2241 taps) 1438.9 us 360.0 us 4.00x
full xrxa() chain, 576k input 1148.2 us 337.6 us 3.40x
10.76% 3.16% of one core
xresampleF (float, host audio) 2.6x - 3.4x
Summation order changes, so the double path is not bit-identical: over
400 buffers of the full RX chain the worst deviation is 1.1e-12, an SNR
of 251 dB. The float path is bit-identical, as the cast to float absorbs
the difference. With the resampler bypassed (48k in, 48k out) the chain
is unchanged bit-for-bit.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Uladzimir Karpenka