D089 - Verify the encoding table against a reference disassembler

decided · 2026-08-19

orbistoun-gen fixtures compiles shaders from source this project wrote, and crates/orbistoun-shader/tests/differential.rs checks our decode against what LLVM's disassembler says is in them.

D085 shipped a table that was transcribed and unverified, with no way to check it short of waiting for real shaders from a real title - which is hundreds of functions away. This closes that, today, with no console and no title involved.

The assertion is instruction offsets, not counts. An offset is the sum of every length before it, so agreement across a hundred instructions means the lengths are right, and the first offset where the two disagree names the instruction whose encoding is wrong. Counts can coincidentally match while every boundary is wrong.

Result: 126 instructions across 10 fixtures, every boundary matching. All seventeen families are covered - fourteen by compiled fixtures, and SOPK, MTBUF and VINTRP by hand-written assembly, since nothing the generator can persuade the compiler to emit produces those three (D105).

The provenance line, drawn deliberately. The disassembler is used to detect that an entry is wrong; correcting one is done from the published AMD document. Differential testing against another implementation is ordinary engineering; reading its tables to source the right value is deriving from it. Worth stating because the temptation is strongest exactly where the answer is hardest to look up.

Fixtures are committed, so LLVM is not a test dependency. The binaries were compiled from source in tools/shader-fixtures/ - generated, never extracted, the same rule every other fixture here follows.