wasm: streamline hot JIT paths

[youknowone]

Summary

• move Wasm guard recovery spills out of the hot trace into a shared cold dispatcher
• fast-path signed-32 IntMulOvf while retaining the full-width overflow fallback
• remove redundant recursive CALL_ASSEMBLER nursery-frame clearing
• invert fused integer guard comparisons directly and direct-call five exact uniform-i64 blackhole helpers in the guest

Why

Profiling showed software 64x64 overflow checks in nested_loop, duplicate 1064-byte frame clears in recursive calls, inline cold fail-arg spill blocks in fannkuch, and 44,678 exact-ABI blackhole helper calls reflecting guest→host→guest. The changes preserve PyPy/backend structure and retain generic fallbacks for ambiguous ABIs.

Results

• nested_loop: ~0.66s → ~0.45s
• fib_recursive: ~2.39s → ~1.98s
• fannkuch host jit_calls: 44,692 → 14
• full benchmark suite: dynasm 17/17, cranelift 17/17, wasm 14/14

Validation

• cargo check --features dynasm
• cargo test --workspace --exclude pyrex --features dynasm
• cargo test -p majit-backend-wasm --test codegen_test
• cargo test -p majit-backend-wasm --test codegen_test -- --ignored
• python3 pyre/check.py --no-synthetic --no-cpython-suite

Note: the unexcluded workspace test reaches a pre-existing user-only PYRE_WASM_CALL_HIST worktree diagnostic and fails its gate-triage completeness check; that unrelated file is not included in this PR.

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Review details

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📥 Commits

Reviewing files that changed from the base of the PR and between 0d83827 and c3234c3.

📒 Files selected for processing (3)

• majit/majit-backend-wasm/src/codegen.rs
• majit/majit-backend-wasm/tests/codegen_test.rs
• pyre/pyre-wasm/src/lib.rs

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[github-actions]

🤖 Codex parity review

Static analysis of this diff vs the local RPython/PyPy sources (commit c3234c3).
Updated: 2026-08-13T04:59:56.780Z

Files in the reviewed diff

majit/majit-backend-wasm/src/codegen.rs
majit/majit-backend-wasm/tests/codegen_test.rs
pyre/pyre-wasm/src/lib.rs

1. Regressions to PyPy parity introduced by this patch

None.

2. Other mismatches introduced by this patch

None.

3. Pre-existing mismatches (already present before this patch)

None.

4. Structural adaptations

• majit/majit-backend-wasm/src/codegen.rs:2078 ↔ rpython/jit/backend/x86/assembler.py:835 — Wasm uses a structured hot-exit block and br_table cold dispatcher; x86 appends per-guard recovery stubs to linear machine code. The patch preserves positional fail-argument recovery while adapting to structured Wasm control flow.

• majit/majit-backend-wasm/src/codegen.rs:5531 ↔ rpython/jit/backend/x86/assembler.py:1869 — Wasm lacks x86 overflow flags, so INT_MUL_OVF uses an exact signed-32 fast-path proof plus a software full-width high-word fallback, rather than imul plus NO condition flags.

• majit/majit-backend-wasm/src/codegen.rs:3591 ↔ rpython/memory/gc/incminimark.py:1345 — The removed per-frame memory.fill relies on the wasm nursery’s target-specific full-arena zeroing on reset. This is a wasm GC-layout adaptation; frame metadata and Ref homes are still explicitly initialized.

• pyre/pyre-wasm/src/lib.rs:181 ↔ rpython/jit/backend/x86/assembler.py:2272 — The exact-function wasm ABI allow-list directly invokes known uniform-i64 helpers instead of reflecting through the host trampoline. This replaces a wasm guest→host→guest call with an in-module call while retaining the generic path for all other signatures.