D951 - A computed descriptor too large to run wave by wave binds from a sample of its groups

Status: decided Date: 2026-10-24

A dispatch's computed buffer access that the group-id replay (D914) cannot answer is found by running its waves on the host (D900). Above 256 waves, the host no longer runs every wave. It runs every wave of a lattice of groups instead, chosen as D950 chooses them: the first and last group along each dimension, as many evenly spaced between as 256 waves allow, and every combination. The corners of the grid are always in the lattice. A group's waves all run, because a waterfall's descriptor follows its lanes, and another wave of the same group can reach another descriptor.

A lattice that reaches the access with nothing that binds a byte places nothing. That means no descriptor, or only null ones. An access only some threads take can lie wholly between the sampled groups, as a waterfall over a tree's leaves of one kind does. The host then runs every wave, as without a sample, where the dispatch has at most 4096 waves, the bound D883 sets. Above that the access is refused, saying the sample reached it in no wave. An empty range is never bound from a sample, because every access through it would escape.

The host replay is bounded in steps as well as in waves. One wave's run stops at its own bound, at most 2^20 steps. All the waves one preparation runs share 2^22 steps, about two seconds of the host interpreter. The lattice and any full run share that same budget, and a dispatch's global loads (D883, D950) get the same budget across their waves. A replay that runs past either bound before every wave is done refuses the access. The refusal names the bound and the access's byte, and says how many waves ran. A refused dispatch costs the frame its result, while an unbounded replay costs the whole run.

What the lattice reaches is bound as one buffer, however many descriptors it holds. The range runs from the lowest descriptor's base to the highest descriptor's end, each through its offset and extent as D900 binds one. It is widened by D950's margin: an eighth of its bytes each side, at most a megabyte, in whole pages. The margin is kept only where guest memory is mapped, and it gives way where it would overlap a buffer the dispatch already binds. A range larger than one draw buffer binds is refused. A range that crosses a four-gigabyte span is refused too, because the module orders addresses by their low halves. A scalar load's base pair is not sampled. It is still refused unless every wave reaches it with one pair.

The bound buffer starts at its own guest base, not at a descriptor's. The dispatch's push block carries that base for each buffer placed this way, after the global loads' base, at most eight buffers. An access through such a buffer is addressed from the descriptor it computes, less the buffer's base. The module checks every access the descriptor's own bounds admit. The access is an escape when its first or last byte falls outside the bound buffer's words, when its address carried past the span's low half, or when the descriptor sits in another four-gigabyte span. A descriptor whose addressing is not modelled is also an escape. The dispatch runs, and its result is refused after it, as D746 refuses any escape. An access the descriptor's bounds refuse reads zero on the hardware too, so it is not an escape. Robust buffer access never stands in for a word the sample missed.

The run report notes once per program that a computed buffer was bound from a sample: how many groups ran of how many, what they reached, and the range bound.

Amended by D982: where the backend runs device-addressed dispatches, no sample is run, of a computed descriptor or of global loads (D950). The dispatch is prepared device-addressed (D964). The sample stays for a program device addressing refuses, and for a backend without it. Grand Theft Auto V's sample at 0x74005b87e400 cost 0.6 to 2.4 s a dispatch, twice per dispatch.

Why: Grand Theft Auto V's in-world frame spent more than ten minutes preparing dispatches of the program at 0x400003e1f500 and never got past it. Its computed descriptor depends on its lanes, so the D914 replay cannot answer it, and D900 ran every wave on the host interpreter, refusing only above 4096 waves. A dispatch just under that limit ran thousands of full program runs, each with its own loops, on every frame. The first run with the lattice bound an empty range for the programs at 0x400003ce6000 (452 groups) and 0x74005b87e400 (632 groups). Their tree walks enter the waterfall only for one kind of entry, and no sampled group's thread held one.

Rejected: