JIT: implement empty($cv) fast path in function & tracing JIT

[ndossche]

We can simply defer to zend_jit_bool_jmpznz() which already generates optimized paths for many types.

Example PHP:

function f(int $i) {
    if (empty($i)) return 1;
    return 2;
}

Before (function JIT):

.L1:
	movl $0x414d88a0, %r15d          ; opline = ISSET_ISEMPTY_CV
	movabsq $0x55a80140fe00, %rax
	callq *%rax                      ; Call VM handler
	cmpl $0x414d88e0, %r15d          ; check if the handler branched
	jne .L2
	movq 0x10(%r14), %rax            ; return 1
	...
.L2:                                     ; return 2
	...

After (function JIT):

.L1:
	cmpq $0, 0x50(%r14)              ; empty($i) check
	je .L3
	movq 0x10(%r14), %rax            ; return 2
	...
.L3:                                     ; return 1
	...

[arnaud-lb]

Looks good to me otherwise

[arnaud-lb]

I feel that this could kick back later. Can we add a "isempty" argument to zend_jit_bool_jmpznz() instead? I think that we only need to change the condition before the zend_jit_type_check_undef() call?

[ndossche]

Okay I agree that's maybe cleaner. Although I'd call it warn_on_undef or something like that. I believe isempty leaks details.
There are 2 places where zend_jit_type_check_undef is done, so both need to be guarded by an if.