nointerpolation-mismatch
Status: stub. The full-length analysis is queued for a v1.0.x patch release per ADR 0018, section 5, criterion #6. The companion rule page at docs/rules/nointerpolation-mismatch.md contains the canonical detection logic + GPU reasoning.
TL;DR
The rasteriser's default interpolation mode is barycentric: for each pixel sample, the hardware computes attr_pixel = w0*attr_v0 + w1*attr_v1 + w2*attr_v2, where w0..w2 are the perspective-correct barycentric weights. On AMD RDNA 2 and RDNA 3, this is performed by the parameter cache and the per-pixel v_interp_* instructions, each of which consumes a VALU slot. On NVIDIA Turing and Ada Lovelace, the SM's attribute interpolator unit issues an IPA instruction per attribute component per pixel — Turing's whitepaper documents IPA as a half-rate operation against general FP32 throughput, and Ada inherits the same scheduling. For a 4-component attribute that is then immediately cast to int and used as an index, every one of those interpolator cycles is wasted: the fractional output is discarded the instant the cast happens.
What the rule fires on
A vertex-output / pixel-input attribute that the pixel shader uses in a flat-only context — read as an integer (asint, (uint)), used as an array index, used as a switch selector, used as a buffer/texture index, or compared with == against a per-primitive integer constant — but whose declaration on the vertex-output side does not carry the nointerpolation modifier. The rule cross-references the matched VS-output struct member (or geometry/mesh-shader output) with the PS-input usage and fires on the VS-side declaration, suggesting that nointerpolation be added so the rasteriser broadcasts the provoking-vertex value unchanged instead of computing a barycentric-weighted blend.
See the What it detects section of the rule page for the full pattern definition.
Why it matters
The full GPU-mechanism analysis lives in the Why it matters on a GPU section of the companion rule page.
Examples
The bad / good code snippets are kept canonical on the rule page; see nointerpolation-mismatch.md -> Examples.
See also
- Rule page -- canonical reference + change log.
- bindings overview -- broader context.
- ADR 0018 -- v1.0 readiness plan.
This is a v1.0-ship stub. Full analysis pending; track issue link TBD.