Word-1 of every ALU triple holds three 8-bit component-relative
swizzles (`src1_swiz`/`src2_swiz`/`src3_swiz` at bits 16-23/8-15/0-7
per canary ucode.h:2064-2066) and three per-operand negate flags
(bits 24/25/26). Pre-fix, both the WGSL interpreter and the AOT
translator discarded word-1 entirely with `_ = w1;` — every ALU
result was missing its swizzle (broadcast/permute patterns like
`.zyxw`, `.xxxx`) and any negated operand was used positive instead.
Component-relative semantics (canary's
`AluInstruction::GetSwizzledComponentIndex`, ucode.h:1996): for output
component i, the source component is `((swizzle >> (2*i)) + i) & 3`.
Identity swizzle is 0x00, NOT 0xE4 — the original `apply_swizzle` in
the interpreter shader treated it as absolute, also incorrect.
Mechanical changes:
- crates/xenia-gpu/src/ucode/alu.rs: extend AluInstruction with
src_X_swiz (u8) and src_X_negate (bool) fields. decode_alu unpacks
them from word 1.
- crates/xenia-gpu/src/shaders/xenos_interp.wgsl: apply_swizzle uses
component-relative semantics. interpret_alu decodes the modifiers
and applies via apply_swizzle + apply_modifiers (with abs=false).
- crates/xenia-gpu/src/translator.rs: src_operand emits the
precomputed swizzle inline as `vec4<f32>(base.x, base.y, ...)`,
then wraps in `(-…)` when negated. Identity swizzle (0x00) emits a
bare base expression so it round-trips with the trivial-shader
fixture.
Abs is omitted in this commit — the abs flag is dual-meaning (for
temps it lives at bit 7 of the src byte; for constants at word-2 bit
7 `abs_constants`). Wiring it up correctly requires more careful
case-split logic; deferred to Phase G.
Verification at -n 100M lockstep:
swaps: 2 → 2 (gated by Phase E for draws)
draws: 0 → 0
packets: ~58M (within noise)
Tests: 554 → 555 (+1 swizzle/negate test, no count change otherwise
because identity swizzle test merged into D1's parameterised test).
WGSL still validates via naga (combined_module_parses_as_wgsl).
Closes GPUBUG-100 (P0). Abs deferred to Phase G.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
c5c6713419