2026-08-19 - Per-opcode operands, solved rather than written

Done. The mechanism D096 said was needed.

• orbistoun-gen operands: assembles probes with llvm-mc, solves each operand's bit field by correlation over the samples, emits data/opcode-operands.toml.
• EncodingTable loads and prefers the per-opcode layouts; SlotKind::Immediate added so offsets decode as values rather than being skipped.
• Probe assembly under tools/shader-fixtures/probes/.

Verified. Gate green. 10 opcodes solved, 142 operands checked against the reference (was 99), covering VOP3, SMEM and DS - three families that had no layout at all. One opcode reports as unsolved rather than approximated.

Surprises. Four distinct ambiguities, each found by the differential test rather than by thinking, and each producing output that looked entirely reasonable:

• Scalar registers stop at 101, so a seven-bit field explains every register sample a real eight-bit field does. The solver picked the narrow one and read exec_lo for the literal marker. Only inline constants and special registers reach high enough to pin the width.
• Consecutive registers are the worst possible samples. With v5, v6, v7, v8 in adjacent fields, a field shifted slightly reads values differing by the same constant and looks consistent across every sample. Spread and disorder are what kill that.
• A position that only ever holds a register cannot be told from the shared numbering. Code 242 is register 242 or the constant 1.0, and it decoded v242 where a shader meant 1.0. Now refused as ambiguous rather than picked - the fix is a better probe, not a tie-break.
• An operand that is not a register is still an operand. Refusing to solve a whole opcode because one position held an offset lost three of eleven; immediates are fields too, and solvable the same way once the probes vary them.

The pattern across all four: a solver is only as honest as its samples, and the failure mode is never an error - it is a confident answer that happens to be wrong.

Not done. Ten opcodes is a start against a few hundred. The probe set covers what the fixtures already exercise; widening it is mechanical and unbounded. Nothing translates.