manual-smoothstep
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/manual-smoothstep.md contains the canonical detection logic + GPU reasoning.
TL;DR
The cubic Hermite polynomial t*t*(3 - 2*t) is 5 FP32 operations for a scalar input: one multiply (for t*t), one multiply-by-2, one subtract, one multiply (for t*t * (3 - 2*t)). The preceding saturate((x - edge0) / (edge1 - edge0)) adds a subtract, a subtract, a division (or reciprocal-multiply), and a saturate — roughly 4 more operations, with the division typically being a 2-cycle v_rcp_f32 sequence. Total: approximately 10 scalar FP32 instructions.
What the rule fires on
A hand-rolled cubic Hermite interpolation that implements the body of the smoothstep intrinsic. Specifically, the rule matches the sequence:
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 manual-smoothstep.md -> Examples.
See also
- Rule page -- canonical reference + change log.
- math overview -- broader context.
- ADR 0018 -- v1.0 readiness plan.
This is a v1.0-ship stub. Full analysis pending; track issue link TBD.