D555 - Both interpolation halves answer the whole value

Status: assumed Date: 2026-09-04

v_interp_p1_f32 and v_interp_p2_f32 each translate to a read of the fragment Input variable, the fully interpolated attribute, and their barycentric operands are ignored. v_interp_mov_f32 is refused.

Why: the guest pair computes one interpolated value in two steps, and SPIR-V has no such pair: the host pipeline interpolates and the input variable is the result. Treating p2 as a no-op breaks as soon as the two halves do not share a destination. A shader that uses p1's intermediate for anything but feeding p2 gets a different number, which is the edge this assumes away. v_interp_mov_f32's selector has no citable encoding.

Rejected:

• p2 as a no-op on p1's result: wrong when the halves use different destinations.
• Reproducing the two-step arithmetic from the guest's I and J: the host does not expose its own barycentrics that way.
• Translating v_interp_mov_f32 as the attribute value: right for one selector of three and silently wrong for the others.