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Tracing JIT: segfault on $f($arr[k]) when $f alternates between by-value and by-reference functions

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@aasul arbeitet bereits daran.

Seit 25.10.2026.

Bewertung

Schwierigkeit
4/5
Geschätzter Aufwand
1-2 Tage
Anfängerfreundlichkeit
48/100
Issue-Typ
Bug
Klarheit
Größtenteils klar
Aktivitätsstatus
Aktiv
Tech-Stack
c, php
Bereich
backend, performance

Rechercherichtung

Start with zend_jit_trace.c and zend_jit_ir.c, following the reported INIT_DYNAMIC_CALL, FETCH_DIM_FUNC_ARG, and ZEND_SEND_FUNC_ARG paths; compare the DIM handling with the object fetch guard mentioned in the issue. Run the supplied reproducer with tracing JIT enabled and with JIT off to confirm the failure. Done means the JIT run exits normally while preserving the expected warnings.

Vom Indexierungsmodell aus dem Issue-Text verfasst.

Beschreibung

With the tracing JIT, a dynamic call $f($arr[k]) where $f alternates between a by-value and a by-reference function crashes.

<?php
function byval($a) { return (int)$a; }
function byref(&$a) { return (int)$a; }
$fns = ["byval", "byref"]; $arr = [];
for ($i = 0; $i < 120; $i++) { $f = $fns[$i % 2]; $f($arr[1]); $f($arr[0]); }

php -n -d opcache.enable=1 -d opcache.enable_cli=1 -d opcache.file_update_protection=0 -d opcache.jit_buffer_size=64M -d opcache.jit=tracing -d opcache.jit_hot_loop=1 -d opcache.jit_hot_func=1 -d opcache.jit_hot_return=1 -d opcache.jit_hot_side_exit=1 -d opcache.jit_max_polymorphic_calls=0 reduced.php

Debug build:

Warning: Undefined array key 1 in reduced.php on line 5

Warning: Undefined array key 0 in reduced.php on line 5
php: Zend/zend_operators.c:1016: zval_get_long_func: Assertion `0' failed.

Release build: the same two warnings, then SIGSEGV (exit 139).

With opcache.jit=off the script only prints the two warnings and exits normally, so I expected the same under the JIT. jit_max_polymorphic_calls=0 is not required: with the default value, a variant with five callees (four by-value, one by-ref) and a defined $arr = [5, 6] crashes the same way. The hot counters of 1 are needed to form the traces in a short script.

It reproduces at 9253b03bef3f7cb96ac2594c23382b0ba5ab6e19 (latest upstream when checked): the debug build hits the assertion above, the release build segfaults (rc 139).

Looks like the megamorphic INIT_DYNAMIC_CALL (call->func unknown) makes zend_jit_trace.c skip the specialised FETCH_DIM_FUNC_ARG, but zend_jit_send_var (zend_jit_ir.c, ZEND_SEND_FUNC_ARG) still compiles the by-ref branch using the recorded type of the fetched value, which does not include MAY_BE_INDIRECT. ZEND_FETCH_OBJ_FUNC_ARG has a zend_jit_func_arg_by_ref_guard for this case and the DIM variant does not. This is from reading jit_debug trace output, not confirmed in a debugger. Open PR #18392 (reworks EX(call) handling in the call/send path) may be related; I did not test it, its diff does not apply on the latest commit.

I can send a patch if that helps.

Found with an LLM-based testing tool; I used Claude to reduce it and look for the cause.

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