For a given output position j on the time axis, only input positions
i such that i*s0 <= j < i*s0 + K contribute -- i.e.
i in [ceil((j - K + 1)/s0), floor(j/s0)] intersected with [0, IL-1].
That's at most ceil(K/s0) values (typically 2 for stride==K/2
transposed convs).
The current kernel iterates the full IL range and filters with an
`if`, amplifying per-thread work by IL/ceil(K/s0) (~160x for IL=320,
K=10, s0=5 -- a representative codec-decoder shape). On Apple M1
the wasted work trips the macOS GPU watchdog
(kIOGPUCommandBufferCallbackErrorImpactingInteractivity) on long
graphs.
Compute i_min, i_max analytically before the inner loop and iterate
only [i_min, i_max]. Output is bit-identical (same multiplies and
adds in the same order); loop bound shrinks by IL/ceil(K/s0).
Tested on M1 with a downstream consumer running a TTS codec at full
T_codec; end-to-end codec decode ~3-4x faster, zero watchdog hits
across long synthesis runs vs ~30% pre-patch.
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