D759 - A primitive shader that exports its entry v0 runs passthrough

Status: decided Date: 2026-10-08 known_by: assumed

A primitive shader whose exp prim exports the v0 it was entered with is seeded as a passthrough primitive shader: v0 holds its primitive packed as the export takes it, three nine-bit vertex indices at bits 0, 10 and 20. This holds even when the stream never sets VGT_SHADER_STAGES_EN.PRIMGEN_PASSTHRU_EN. The program decides it. Nothing before the export may name v0 as its first operand, which counts as a write.

Why: Mesa's NGG lowering hands the export the packed primitive unchanged only in passthrough mode. Otherwise it packs the export from the vertex indices, which entry v0 then holds sixteen bits each (ac_nir_lower_ngg.c:127-131, :139-151). A shader exporting entry v0 was compiled for passthrough, because without it that value is not an export's primitive. PPSA28061's stream never writes VGT_SHADER_STAGES_EN. On hardware, the AGC record its shader goes through carries neither it nor GE_CNTL (REQ-20261008T0130Z-cn06, reports/hardware/20261008-111104-eboot.obs.log, rec-has-0x2d5 0x0). Seeded the other way, each triangle's indices were read from the wrong bit fields, and its text drew as wedges joining one glyph to another.

Rejected:

• Waiting for the register's source on hardware (REQ-20261008T1130Z-cn07 (4)): the program already says how it was compiled.
• Treating every primitive shader as passthrough: radeonsi's non-passthrough shaders pack their own export from the sixteen-bit indices.