Add function exposing HAVE_GCC_GLOBAL_REGS

[joerowell]

What's the problem?

Understanding what's happening during the PHP JIT compilation process is a bit tricky. Some of this complexity is inherent, but the Zend Engine (at present) doesn't make it that easy to gauge what's going on via a debugger like GDB.

For example, consider the following snippet from the JIT compiler:

php-src/ext/opcache/jit/zend_jit_x86.dasc

Lines 1742 to 1766 in c345b6e

	if (zend_jit_vm_kind == ZEND_VM_KIND_HYBRID) {
	const void *handler = zend_get_opcode_handler_func(EG(exception_op));
	| ADD_HYBRID_SPAD
	| EXT_CALL handler, r0
	| JMP_IP
	} else {
	const void *handler = EG(exception_op)->handler;
	if (GCC_GLOBAL_REGS) {
	| add r4, SPAD // stack alignment
	| EXT_JMP handler, r0
	} else {
	| mov FCARG1a, FP
	| EXT_CALL handler, r0
	| mov FP, aword T2 // restore FP
	| mov RX, aword T3 // restore IP
	| add r4, NR_SPAD // stack alignment
	| test eax, eax
	| jl >1
	| mov r0, 1 // ZEND_VM_ENTER
	|1:
	| ret
	}
	}

To understand how this impacts the generated code you need to know at minimum:

• The VM mode that has been used, and
• Whether GCC was used to compile the Zend Engine or not.

You can deduce the first of these at runtime using zend_vm_kind:

php-src/Zend/zend_vm.h

Line 31 in 5b01c48

ZEND_API int zend_vm_kind(void);

But there's no way to know the second of these programatically at present. You can deduce this by looking at the object code in GDB, but that's tedious and not suited to any sort of automated checking.

This PR

This PR just adds a public function called zend_gcc_global_regs that returns 1 if HAVE_GCC_GLOBAL_REGS is set and 0 otherwise. This makes it far simpler to detect what calling convention to expect.

[joerowell]

Is this PR waiting for something, or can it be merged?

[bwoebi]

Can you please export that as part of zend_execute.c instead of the VM? We try to leave everything out of the VM which doesn't strictly need to be there.

[ramsey]

Is there a reason this targets the PHP-8.0 branch?

[joerowell]

Only because I read this:

If you are fixing a bug, then please submit your PR against the lowest actively supported branch of PHP 
that the bug affects...

And since my primary use case is for JIT code I figured I'd put it against the first version that had a JIT (e.g PHP8).
I'm happy to re-aim it at any branch that you'd prefer: I'd actually forgotten that the sentence above applied only to bugs.

[ramsey]

If this isn't fixing a bug in PHP-8.0, please change the base to master. Thanks!

[joerowell]

Is there something I need to do for this? I.e is there part of the PR process for this feature that I've missed? I had assumed I didn't need to do an RFC for it.