maszyna/shaders/instance_cull_offsets.comp

// turns the counts of instances of the entries, written by stage 0 of instance_cull.comp, into the first slots of the
// entries in the output (cursors, which stage 1 advances as it writes), and puts the total in the header.
// a single work group: each invocation sums a run of the entries, then the sums are scanned in shared memory

#include <common>

layout(local_size_x = INSTANCE_OFFSETS_GROUP_SIZE) in;

layout(std430, binding = INSTANCE_COUNTER_SSBO_BINDING) buffer instance_counter_ssbo
{
	uint counters[];
};

// x: entries
layout(location = 0) uniform uvec4 counts;

shared uint sums[INSTANCE_OFFSETS_GROUP_SIZE];

void main()
{
	uint entries = counts.x;
	uint invocation = gl_LocalInvocationIndex;
	uint run = (entries + INSTANCE_OFFSETS_GROUP_SIZE - 1u) / INSTANCE_OFFSETS_GROUP_SIZE;
	uint begin = min(invocation * run, entries);
	uint end = min(begin + run, entries);

	uint sum = 0u;
	for (uint entry = begin; entry < end; ++entry)
		sum += counters[INSTANCE_COUNTER_HEADER + entry];
	sums[invocation] = sum;
	memoryBarrierShared();
	barrier();

	// inclusive scan of the sums
	for (uint step = 1u; step < INSTANCE_OFFSETS_GROUP_SIZE; step *= 2u)
	{
		uint added = (invocation >= step ? sums[invocation - step] : 0u);
		memoryBarrierShared();
		barrier();
		sums[invocation] += added;
		memoryBarrierShared();
		barrier();
	}

	uint offset = sums[invocation] - sum;
	for (uint entry = begin; entry < end; ++entry)
	{
		counters[INSTANCE_COUNTER_HEADER + entries + entry] = offset;
		offset += counters[INSTANCE_COUNTER_HEADER + entry];
	}
	if (invocation == INSTANCE_OFFSETS_GROUP_SIZE - 1u)
		counters[1] = sums[invocation];
}