[ktf]

AttachClustersLooperFollow scans all rows after each propagation step
and attaches clusters in the two to four rows within 1.5 cm of the
track. On a GPU the threads of a warp sit at different radii, so each
of them finds its rows at different iterations of the scan, and the
warp runs the attachment once per row that any of its threads is close
to, with one or two threads active each time.

Skip the rows out of reach in an inner loop without attachment, so that
all threads of a warp reach AttachClusters in the same iteration.

The inner loop only advances j over rows for which the condition of the
attachment is false, evaluated with the same expression and the same
track position (AttachClusters does not change it), and stops at the
last row, which the unchanged condition below still tests. Each thread
therefore calls AttachClusters for the same rows, in the same order and
with the same arguments as before.

GPUTPCGMMergerFollowLoopers on an M1 Max with Metal, 1/4 of a 30 kHz
Pb-Pb time frame: 217 ms -> 77 ms. The code is not specific to Metal.

[ktf]

@davidrohr this has an incredible effect on my laptop. It's probably worth checking if it improves CUDA / HIP as well.

[davidrohr]

This makes a lot of sense. There is actually a TODO for exactly this in the code :). But this function is anyway rewritten in the new tracking devlopment. Please leave the PR open, I'll see if I can merge it as is or if it needs changing.