D943 - A relative register move selects among the registers past its source

Status: decided Date: 2026-10-21 known_by: published

v_movrels_b32 d, s writes vector register s + m0 into d (AMD's published RDNA instruction set). It translates as a selection, per lane, among the 32 registers from s, keyed on m0. Every model reads a register it names this way, so none needs a register file it can index. An m0 of 32 or more reads s itself, a value no program here has been seen to form.

Why: Grand Theft Auto V's shadow-cascade lookup reads three registers at m0 of 0, 4, 8 or 12 past v0 (m0 four times the cascade, v_readfirstlane_b32 of it), and the translator had no translation for the instruction, so the draws that use it were refused.

Rejected:

• An access chain into the register array with m0 as its index: only the wavefront model keeps one, and each model would need its own form.