jit: delete the exc_edge_bridge_enabled gate and the call-site exception prologue it guarded

[youknowone]

Follow-up to #1038, which left this behind.

Opening the exception-edge bridge on every backend made exc_edge_bridge_enabled() return a constant true, which made call_jit.rs's if !exc_edge_bridge_enabled() prologue unreachable. That block read the backend exception cells and emitted a snapshot-less GUARD_EXCEPTION through emit_exception_bridge_prologue.

The bridge walker emits the whole SAVE_EXC_CLASS / SAVE_EXCEPTION / RESTORE_EXCEPTION / GUARD_EXCEPTION sequence itself, at the bridge-entry frame state where the guard carries a snapshot. So the dead block goes, and with it the gate and its two routing conjuncts (call_jit.rs's route_exc_edge, bridge_subwalk.rs's exc_edge_precondition).

emit_exception_bridge_prologue stays. It is the _prepare_exception_resumption (pyjitpl.py:3101) + prepare_resume_from_failure (pyjitpl.py:3132) counterpart, and its doc already describes the two-phase split it needs once pyre gains resume-data replay; deleting it would drop an upstream counterpart. Its doc now says it is off the live path and why.

The gate's doc comment carried the diagnosis that closed #1038, so its substance moves to where the read that caused it lives — bridge_subwalk's exc_edge_class, next to the pointer-width read itself. A future backend that skips the exception triple instead of lowering it hits the same wall one step later, so that is recorded there too.

Verification

Rebuilt from re-extracted LLBC at this base: check.py dynasm 378/378, wasm 374/374, cranelift 378/378.

One caveat worth stating: an earlier cranelift run reported fib_recursive at 8.1x against an 8x gate. Re-run on the same binary it measured 6.2x (exec 2.30s → 1.49s, pypy oracle 0.28s → 0.25s). That gate sits close enough to the boundary that a loaded box can trip it; the diff here is a dead-code deletion and cannot reach a recursion benchmark.

— authored by Claude

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📥 Commits

Reviewing files that changed from the base of the PR and between 2f61280 and 84a2161.

📒 Files selected for processing (4)

• majit/majit-metainterp/src/pyjitpl.rs
• pyre/pyre-jit-trace/src/jitcode_dispatch/bridge_subwalk.rs
• pyre/pyre-jit-trace/src/jitcode_dispatch/mod.rs
• pyre/pyre-jit/src/call_jit.rs

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🤖 Codex parity review

Static analysis of this diff vs the local RPython/PyPy sources (commit 84a2161).
Updated: 2026-08-05T04:30:18.968Z

Files in the reviewed diff

majit/majit-metainterp/src/pyjitpl.rs
pyre/pyre-jit-trace/src/jitcode_dispatch/bridge_subwalk.rs
pyre/pyre-jit-trace/src/jitcode_dispatch/mod.rs
pyre/pyre-jit/src/call_jit.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)

• pyre/pyre-jit/src/call_jit.rs:3450 ↔ rpython/jit/metainterp/pyjitpl.py:3156: “pending exception with no routable live-frame handler” aborts bridge tracing and resumes the blackhole; upstream reconstructs the resumed frame stack, restores exception state, and continues through handle_possible_exception().

• pyre/pyre-jit/src/call_jit.rs:3391 ↔ rpython/jit/metainterp/pyjitpl.py:2530: a catch in an inlined callee is deliberately declined (“still needs the carrier subwalk”), whereas upstream’s finishframe_exception() walks every reconstructed frame and enters the first matching callee handler. The carrier implementation is also default-off via PYRE_CARRIER_EXC_RESUME.

• majit/majit-metainterp/src/pyjitpl.rs:12870 ↔ rpython/jit/metainterp/pyjitpl.py:3149: dormant emit_exception_bridge_prologue() records SaveExcClass/SaveException with baked constant operands, while upstream uses zero-argument record0 operations that read the runtime exception state. The new comment correctly says this path is not live, but the dormant implementation remains non-equivalent.

4. Structural adaptations

• pyre/pyre-jit/src/call_jit.rs:3374 ↔ rpython/jit/metainterp/blackhole.py:400: Pyre locates handlers through CPython-style exception-table lookup; PyPy’s JitCode directly inspects live followed by catch_exception. This is a necessary CPython-compatible compiler/opcode adaptation, not a parity defect.