D900 - A computed descriptor is found by running the dispatch's waves

Status: decided Date: 2026-10-19

A compute dispatch's buffer access can take a descriptor or scalar offset that the program computes from lane data. A waterfall loop does this: v_readfirstlane of a lane's value, then a load from a descriptor table at that offset. The D753 prefix run, over user data alone, cannot know such a value. Instead, the host runs each wave of the dispatch through the program, as it does to find global loads (D883), and records the access's registers each time the access runs. A waterfall runs the access once for each distinct value across its wave's lanes, so the run yields every descriptor and offset the dispatch uses, and no others.

The run goes past the access, through the loop the access lies in, to the loop's closing backward branch; the program is read from guest memory at the dispatch's address. Every instruction is run. A buffer_load_dword* is evaluated in each running lane through its descriptor: the base, the scalar offset, the instruction offset, and the lane's vector offset (offen) or index times stride (idxen), with zero past the records, as the hardware reads there. The step bound is raised so each pass of a 64-lane waterfall fits.

Where the access still cannot be bound, its refusal names how many distinct descriptors the waves reach it with and where their runs ended, or why the waves could not be run. Amended by D951: above 256 waves, every wave of a lattice of the dispatch's groups runs instead. What the lattice reaches is bound as one buffer with a margin, and an access outside it is an escape.

Relaxed by D964: a dispatch whose waves the host cannot run to the access, refused here because the host cannot know what it reaches, runs device-addressed where the device maps guest memory. Its unresolved accesses then reach guest memory by device address, and nothing is bound for them. The rejected alternative below stands for every dispatch the host can resolve.

Why: Grand Theft Auto V's compute program at 0x74005b87ce00 picks each lane's store buffer from a 65,536-entry descriptor table through a waterfall. The 15,517 buffers the table names span the guest's whole address space, so no one range can be bound for them. The descriptors a dispatch actually reaches are the ones to bind.

Rejected: