GCC global-register warning suppression does not restore caller diagnostic state

Description

The GCC diagnostic pragmas around PHP's required global-register declarations do not restore the warning policy selected by the build. They replace it with PHP's own policy for the remainder of each translation unit.

The affected sites are:

• Zend/zend_execute.c, around the execute_data declaration;
• Zend/zend_execute.c, around the opline declaration;
• ext/opcache/jit/zend_jit_vm_helpers.c, around its global-register declaration block.

Each site currently uses this shape:

#pragma GCC diagnostic ignored "-Wvolatile-register-var"
/* required global register declaration */
#pragma GCC diagnostic warning "-Wvolatile-register-var"

The final warning is not the inverse of ignored. GCC documents diagnostic pragmas as overriding command-line options; specifically, warning makes the diagnostic a warning even when -Werror is in effect. Only push / pop saves and restores the previous state.

This creates a policy-ownership conflict. PHP legitimately needs a narrow exception for these declarations, while the build system or downstream packager owns the diagnostic policy for the translation unit. The current code lets the narrow exception overwrite that caller policy after the exception has ended:

Caller-selected policy	State after the current PHP block	Consequence
-Wno-volatile-register-var	forced to warning	a warning the caller explicitly disabled is re-enabled
-Werror=volatile-register-var	forced to warning	an error the caller explicitly requested is downgraded, so compilation can succeed

The affected blocks occur near the start of files containing 5,733 and 1,327 lines respectively, so the unintended state covers most of both translation units. The problem is not that PHP suppresses its own deliberate declarations; the problem is that the suppression is not scoped to those declarations.

Steps to reproduce

This is a compiler-state issue rather than PHP runtime behavior, so it cannot be demonstrated with a PHP snippet or 3v4l.

On x86-64 Linux with GCC, the essential behavior can be seen with:

#pragma GCC diagnostic ignored "-Wvolatile-register-var"
register void *volatile php_required_register __asm__("%r14");
#pragma GCC diagnostic warning "-Wvolatile-register-var"

register void *volatile post_php_oracle __asm__("%r13");

Compile it once with -Wno-volatile-register-var and once with -Werror=volatile-register-var.

I also tested the actual PHP sources rather than relying only on this reduced example:

1. Configure PHP-8.4 with --disable-all --enable-opcache --enable-opcache-jit; configure detects HAVE_GCC_GLOBAL_REGS=1.
2. Compile the real Zend/zend_execute.lo and ext/opcache/jit/zend_jit_vm_helpers.lo objects.
3. Compile wrappers containing each complete source file followed by the valid %r13 global-register oracle shown above, using the owning object's generated include paths and defines.
4. Run both wrappers with -Wno-volatile-register-var and -Werror=volatile-register-var.

Actual behavior

With the current source:

Caller policy	Result after the complete PHP source
-Wno-volatile-register-var	exit 0, but the post-source oracle warns
-Werror=volatile-register-var	exit 0; the post-source oracle is only a warning

Thus the first policy is unexpectedly re-enabled and the second is silently downgraded.

Expected behavior

PHP's suppression should apply only to its required global-register declarations. The exact caller state—disabled, warning, or error—should resume immediately afterward.

Using #pragma GCC diagnostic push before ignored and #pragma GCC diagnostic pop after the declarations produces the expected result in both complete translation units:

Caller policy	Result with scoped suppression
-Wno-volatile-register-var	exit 0, no diagnostic
-Werror=volatile-register-var	exit 1; the post-source oracle is an error

This change does not alter the treatment of PHP's own global-register declarations.

Impact and scope

This is a bounded compiler-policy bug. I found no declaration later in the current owning files that presently triggers -Wvolatile-register-var, so the unmodified configured objects build successfully. I am not reporting a current default-build failure, runtime miscompile, or security impact.

The demonstrated defect is loss of caller policy. A later PHP change, a platform/configuration-specific branch, instrumentation, or a downstream patch can consequently emit a warning that the build disabled or, more importantly, be silently downgraded from an explicitly requested error.

The same source pattern is present in the inspected PHP-8.4, PHP-8.5, and master (8.6.0-dev) branch tips.

LLM disclosure: The wording of this report was prepared with LLM assistance. The source relations, supported branches, compiler behavior, full-translation-unit results, and proposed patch were independently checked against the repository and disposable GCC builds.

PHP Version

`PHP-8.4` development branch at commit `2bca7364790124b75fbfc9e10b08277e3744cafd`. The same pattern was also confirmed in the inspected `PHP-8.5` and master branch tips.

Operating System

Ubuntu 22.04 (x86-64), GCC 11.4.0 (Ubuntu 11.4.0-1ubuntu1~22.04.3).

Inspectez les blocs de diagnostic GCC autour des global-register declarations dans Zend/zend_execute.c et ext/opcache/jit/zend_jit_vm_helpers.c. Commencez par comparer les transitions d’état actuelles de pragma avec le comportement de diagnostic push/pop de GCC. Le travail est terminé lorsque chaque suppression est limitée à son declaration block et que la politique d’avertissement ou d’erreur du caller reste effective ensuite ; validez cela avec les commandes de reproduction décrites dans l’issue.