D533 - The differential compares only specified results

Status: decided Date: 2026-09-04

The libc differential compares a function against the reference C library only where a published specification fixes the answer, and compares floating-point results as raw bit patterns. Transcendental math, random generators, error message text, NaN payloads and returned addresses are out of it.

Why: a bit-for-bit comparison states the contract only when the contract has one answer. Where the specification permits a range, agreeing cases would pin orbistoun to one implementation's choices while looking like verification. Bit patterns keep last-place differences and the sign of zero visible, which a decimal rendering hides.

Rejected:

• Comparing every implemented function: forty agreeing transcendental cases would assert an implementation detail nobody promised.
• Comparing decimal renderings: hides -0.0 against +0.0 and last-place errors.