Speed up variable access with SIMD scan and slot inline caches

[hadashiA]

Summary

Two layered optimizations for Symbol-keyed variable lookup in the interpreter, worth +26% on optcarrot combined (18.9 → 23.8–24.0 fps, video checksum unchanged).

1. SIMD scan for VariableTable

Symbol is a single uint, so the key array can be searched with the vectorized MemoryExtensions.IndexOf over MemoryMarshal.Cast<Symbol, uint> instead of a scalar loop. optcarrot's PPU has 79 distinct ivars (CPU: 36), so the linear scan was a real cost: this alone is +10% on optcarrot.

2. Slot inline caches for variable-access opcodes

Irep.VariableSlotCache stores a ushort slot guess per symbol-pool index, used by GetIV/SetIV/GetGV/SetGV and the GetConst fast path; the trivial-getter fast path in Send gets the same treatment via MethodCacheEntry.TrivialGetterSlot.

Cached slots are verified guesses: every use re-checks keys[slot] == symbol (TryGetAt/TrySetAt), so stale slots (after remove_instance_variable, table growth, or a call site alternating between different layouts) can never read or write the wrong variable — they just fall back to the scan and re-learn the slot.

Miss paths are deliberately [MethodImpl(NoInlining)]: an earlier version with the miss path inlined into the dispatch-switch case bodies lost the entire SIMD win on optcarrot due to code-layout effects, despite winning on a microbenchmark.

Numbers

benchmark	base	SIMD only	SIMD + IC
optcarrot	18.5–19.1 fps	21.5–21.7	23.7–24.0
bm_ao_render	~2900 ms	neutral	−5% ~ neutral
bm_ivar (60 ivars)	~112 ms	~58	~37 (3.0x)

Interleaved runs, alternating order, compared within round; optcarrot checksum 59662 identical in every configuration.

Tests

• New tests/ChibiRuby.Tests/ruby/test/variables.rb: slot shift after remove_instance_variable, one call site alternating between different ivar layouts, reads through a warmed site on an empty table, global table growth mid-loop.
• Full suite: 155 passed. All TFMs (net8.0/9.0/10.0 + netstandard2.1) build.

Also adds a --quick-script <file.rb> [iters] [warmups] wall-clock runner and ruby/bm_ivar.rb to ChibiRuby.Benchmark.

🤖 Generated with Claude Code