Phase 15.9 — LLVM Optimization (Wasm stage)

Goal

Validate the user hypothesis: “inner loop header is too heavy”, and identify simple, structural fixes that reduce loop overhead without changing semantics.

Context (v4 vs v2)

v2 inner header (bb12):
- 5× PHI
- Timer check + compare (extern; non-optimizable by LICM)
v4 inner header (bb11) observed (simplified):
- 5× PHI
- const 100 (loop-invariant but rebuilt every iteration)
- copy ×2 (operands normalization)
- icmp + copy + br

Problem (confirmed)

A loop-invariant constant is emitted inside the loop header:
- %63 = const 100 appears in the header block each iteration.
- This is invariant across the loop and should be hoisted to a preheader or function scope.
Redundant copies around compare (copy %52 and copy %63) add extra instructions.

Root cause candidates

The MIR/JSON v0 → LLVM lowering currently materializes small constants near use-sites (per-block), not at a preheader.
Minimal canonicalization in the loop header leaves copy and const in place; LLVM may not fully clean due to block structure or missing canonical preheader.

Proposed fixes (small, structural)

LICM at our level (preferred minimal impact)
- Add a tiny MIR optimizer pass that hoists loop-invariant const and simple pure ops to the loop preheader.
- Scope: integer/float constants and read-only intrinsics; skip any extern calls or stateful ops.
- Acceptance: no const <imm> inside loop headers after pass; PHI remain grouped at the block top.
LLVM builder placement policy
- When lowering ICmp with an immediate operand, materialize the constant in the preheader (or re-use a function-scope constant) instead of in the header.
- Re-use strategy: map literals (e.g., i64 100) to a single LLVM const Value per function.
Copy-folding in headers
- Remove trivial copies feeding ICmp; wire ICmp directly from PHI and the hoisted constant.
- Prefer: icmp lt %inner, 100 without intermediate copy.
Ensure loop preheader exists (if not, create)
- Loop canonicalization: PHI nodes grouped at header top, and a unique preheader to host invariants.
- Keep invariants in preheader; leave only PHI + compare + branch in the header.

Where to implement (order of preference)

MIR optimizer (licm_simplify.rs): simplest, backend-agnostic、可逆（既定OFF→段階ONでも可）。
LLVM builder (ny-llvmc): constant placement and copy-folding in codegen (fast win even without a general LICM).
JSON v0 bridge: avoid introducing header-local const where not necessary.

Validation plan

IR dump: NYASH_LLVM_USE_HARNESS=1 NYASH_LLVM_DUMP_IR=tmp/phase159.ll ... で IR を出力。
Gate tests:
- Loop header must contain: PHI×N + icmp + br（Timer系は除外）。
- Disallow = addi/= const in the header unless used by PHI init.
Performance smoke:
- micro-bench loop (100 iters) before/after：ヘッダ命令数削減を確認（静的）＋実行時間（相対）

Acceptance criteria

v4 inner headerから const 100 が除去（preheaderへ移動）。
copy×2 が 0〜1 まで削減（オペランドを直接使用）。
PHI は先頭に並び、ヘッダ終端は icmp + br のみ（Timer チェックあるケースは例外）。
代表スモーク（quick）グリーン維持。IR 検査は dev/gated に限定。

Implementation sketch

Pass: mir/optimizer/licm_simplify.rs（新規）
- 入力: CFG + ブロック毎の命令列
- 手順:
  1. ループ自然ヘッダ検出（back-edgeからの簡易検出でOK）
  2. 入力ブロック集合の指す値の使用を解析し、const かつ side‑effect なし・ループ内で値が変わらないものを抽出
  3. preheader がなければ作成、対象命令を preheader に移動／再生成
  4. ヘッダの copy をオペランド直接使用に書き換え（SSA更新は局所）
- 出力: MIR（構造不変）
Builder/LLVM 補助
- ICmp 生成時に即値を優先（copy 不要）。
- 定数値は function-scope のコンスタントテーブルから再利用（i64:HashMap）。

Risk/Notes

LICM の対象は const のみに限定（第1段階）。
Timer 系の extern 呼び出しは不変とみなさない（移動禁止）。
preheader 生成で IR が分岐しても PHI 不変条件（先頭グループ化）は崩さない。

Next steps

MIR: licm_simplify（const hoist）試験的に実装（既定OFFで dev のみ）
LLVM builder: ICmp 即値最適化 + const 再利用テーブル
IR 検査（dev）: ループヘッダに const が存在しないことを assert
micro-bench を quick 任意ジョブに追加（CI 既定は維持）

Appendix — Evidence (v4 sample)

bb11:
  %61 = phi [%31, bb9], [%61, bb12]
  %60 = phi [%30, bb9], [%82, bb12]
  %57 = phi [%27, bb9], [%79, bb12]
  %53 = phi [%23, bb9], [%53, bb12]
  %52 = phi [%46, bb9], [%85, bb12]
  %63 = const 100          ; loop-invariant, should be hoisted
  %64 = icmp lt %52, %63
  br %64, label bb12, label bb13

README.md Unescape Escape

Phase 15.9 — LLVM Optimization (Wasm stage)

README.md