radix_scan

Stage 2 of one pass of the shared GPU radix sort. Exactly one workgroup, once per radix pass.

Turns the per-block digit populations radix_hist wrote into their exclusive prefix sum, in place. The histogram is laid out digit-major, so the scan gives every (digit, block) pair the first order slot its items occupy: every item of a lower digit comes first, and within one digit every item from an earlier block comes first.

The scan is blocked: each thread sums a contiguous stripe of entries, the stripe sums are scanned in workgroup memory, then each thread rewalks its stripe applying the running base. One workgroup keeps the whole scan inside a single barrier domain, which is what makes it correct without a second dispatch, and the two barriers keep each thread's read pass clear of every other thread's write pass.

Bindings

Slot	Access	Contents
hist	read_write	Digit populations, scanned in place.
sort	read	[1] = block count; the scan covers block_count * 256 entries.

Dispatch { 1, 1, 1 }.