Self-host M4: Native gen-2 AOT emit without Zend fallback (post-#1498)

[PurHur]

Problem

#1498 closed the M4 scaffold: bootstrap-loop-probe.sh runs gen-1 link + Zend gen-2 emit and exits 2 until M3 strict is green. BOOTSTRAP_M4_GEN2_STRICT=1 in script/bootstrap-loop-gen1-link.sh still refuses Zend fallback, but there is no focused tracker for making gen-2 emit native end-to-end.

Docs still say M4 is “partial — gen-2 Zend emit” (docs/bootstrap-selfhost.md, docs/self-host-target.md).

Goal

BOOTSTRAP_M4_GEN2_STRICT=1 ./script/bootstrap-loop-gen1-link.sh
# emit_path=native; build/bootstrap-loop-gen2 runs without php bin/compile.php
./build/bootstrap-loop-gen2
# stdout contains compiler smoke marker

Full probe (non-dry-run) advances from exit 2 → 0 when M3 strict (#1937) is green.

Scope

Piece	Path	Notes
Gen-1 native compile	script/bootstrap-loop-gen1-link.sh	BOOTSTRAP_M4_LINK_COMPILE_DRIVER=1
Gen-2 emit	build/bootstrap-loop-gen2	Native bin/compile.php or src/cli.php (#1467)
Runtime compile	BOOTSTRAP_M4_RUNTIME_COMPILE=1	Pairs #1402 / #1937
Hashtable	#66 / #1761	Native emit may still fault without rehash
Probe	script/bootstrap-loop-probe.sh	Update exit-code docs when green
Presenter	#2379	make north-star4-verify bundles M4 commands when script lands
CI	#2112	Opt-in BOOTSTRAP_M4_GEN2_STRICT_GATE in ci-local after probe stable

Out of scope: vendor prelink (#1416), full-tree rebuild (#1521).

Implementation hints (ordered)

1. M3 strict emit — BOOTSTRAP_M3_COMPILE_SMOKE_STRICT_GATE=1 green (Self-host M3: Native emit for compiler compile-smoke fixture (post-#1493) #1937, CI: Default BOOTSTRAP_M3_COMPILE_SMOKE_STRICT_GATE=1 in ci-local after #1968 green #2165 flip)
2. Spine tail — Self-host M2 slice: native-link lib/AOT/Linker.php in spine smoke (1/2 deferred paths) #2267 lib/AOT/Linker.php + Self-host M2: BootstrapVmDriver native execute smoke (post-#2134 bin/vm.php link) #2201 bin/vm.php native link/execute
3. Gen-1 — bootstrap-loop-gen1-link.sh with BOOTSTRAP_M4_LINK_COMPILE_DRIVER=1
4. Gen-2 binary — run ./build/bootstrap-loop-gen2 -r 'echo smoke;' without Zend on PATH for compile step
5. Probe — ./script/bootstrap-loop-probe.sh (non --dry-run) exit 0

Debug when gen-2 still uses Zend

BOOTSTRAP_M4_GEN2_STRICT=1 PHP_COMPILER_DEBUG=1 ./script/bootstrap-loop-gen1-link.sh 2>&1 | tee /tmp/m4-gen2.log
grep -E 'emit_path|zend|native' /tmp/m4-gen2.log

Acceptance criteria

script/apply-patches.sh
BOOTSTRAP_M4_GEN2_STRICT=1 ./script/bootstrap-loop-gen1-link.sh
./script/bootstrap-loop-probe.sh   # exit 0 when M3 strict + gen-2 native green

Harness:

./script/docker-exec.sh -- bash -lc 'script/apply-patches.sh && BOOTSTRAP_M4_GEN2_STRICT=1 ./script/bootstrap-loop-gen1-link.sh'

• emit_path=native for gen-2 on php-compiler:22.04-dev
• docs/bootstrap-selfhost.md + docs/self-host-target.md M4 rows updated
• No GitHub Actions required (Disable github actions for now #394 ✅)

Dependencies

• Self-host M3: Native emit for compiler compile-smoke fixture (post-#1493) #1937 / CI: Default BOOTSTRAP_M3_COMPILE_SMOKE_STRICT_GATE=1 in ci-local after #1968 green #2165 — M3 compile-smoke strict native emit
• M4: Bootstrap loop — native binary rebuilds next compiler revision #1498 ✅ — scaffold scripts
• M2 spine: bundle src/cli.php entry shim (#1432 follow-up) #1467 — optional src/cli.php spine entry
• M2/M5: Retire PHP_COMPILER_SELFHOST_AOT stubs on Compiler compile spine #1520 — shrink self-host stubs on compile spine
• Arrays: Full associative array support (string keys, holes, rehash) #66 / Self-host M3: HashTable rehash for native AOT runtime (#66 → #1514) #1761 — hashtable rehash (native runtime)

Links

• Epic Epic: Self-host critical path — compiler compiles itself (M3→M5) #1492 · ROADMAP 🗺️ ROADMAP: Self-host + web-capable PHP subset (living) #78 Phase 4 Polish / M4 · CI: BOOTSTRAP_M4_LOOP_PROBE opt-in in ci-local after #1498 probe green #2058 ci-local hook · Testing: make north-star4-verify — M4 bootstrap-loop strict presenter bundle #2379 north-star4 presenter

[PurHur]

Depends on M2 spine compile green (#2499) and M3 emit-TU execute (#2442) before gen-2 strict emit can be meaningful.

[PurHur]

Status update (2026-05-26 harness)

Native gen-2 emit without Zend is green when M4 runtime + strict flags are set:

./script/docker-exec.sh -- bash -lc '
  BOOTSTRAP_M4_LINK_COMPILE_DRIVER=1 BOOTSTRAP_M4_RUNTIME_COMPILE=1 BOOTSTRAP_M4_GEN2_STRICT=1 \
  ./script/bootstrap-loop-gen1-link.sh
'
# OK emit_path=native … gen-2 stdout: compiler smoke

make bootstrap-loop-probe   # exit 0, M3 strict + gen-2 native slice OK

Remaining for this issue to close:

1. Self-host M4: Runtime emit-TU init without BOOTSTRAP_M4_RUNTIME_COMPILE env gate #2599 — drop requirement for BOOTSTRAP_M4_RUNTIME_COMPILE=1 (real lib/ init, not env gate)
2. Self-host M4: north-star4-verify --strict must export BOOTSTRAP_M4_RUNTIME_COMPILE=1 #2598 — north-star4-verify --strict must pass without manual env exports
3. Update docs/bootstrap-selfhost.md / docs/self-host-target.md M4 rows from 🚧 partial → ✅ slice green, full tree still Self-host M5: gen-1 binary compiles bin/compile.php spine (full driver, not loop smoke slice) #2600 / M5: Bootstrap driver — run compiled bin/compile.php instead of Zend php #1521

Repro that still fails (presenter):

./script/docker-exec.sh -- bash -lc './script/north-star4-verify.sh --strict'
# step 4: BOOTSTRAP_M4_RUNTIME_COMPILE=1 not set

Do not close until default probes (no extra env) report emit_path=native.

[PurHur]

Harness verification (2026-05-26 backlog-architect)

Repro on master via ./script/docker-exec.sh (php-compiler:22.04-dev):

make bootstrap-loop-probe          # exit 0 — M3 strict + M4 gen-2 native slice
make bootstrap-loop-gen1-link      # emit_path=native; gen-2 stdout: compiler smoke
BOOTSTRAP_M4_GEN2_STRICT=1 make bootstrap-loop-gen1-link  # native, no Zend fallback
./script/north-star4-verify.sh --strict  # OK

Closing as fixed on master — remaining gaps tracked in #2611 (script defaults without Makefile env) and #2600 (gen-1 compiles bin/compile.php spine, not loop smoke slice only).

Parent #1492 updated.

[PurHur]

Verified green on master 2026-05-26; follow #2611 / #2600 for remaining M4/M5 scope.