JIT: Optimize array checks in comparisons

[ndossche]

There are two optimizations:

1. This patch implements the JIT counterpart of the VM change 2d6b869 to optimize comparisons against empty arrays.
2. This patch ensures that if a type can be array, the element count check happens inline rather than going to a slow path.

PHP demo code for #1:

function f(array $a) {
	if ($a) return 0;
	return 1;
}

Before #1 (for func JIT in this case):

.L1:
	movq $0x418d88c0, (%r14)        ; EX(opline) = opline
	leaq 0x50(%r14), %rdi           ; arg = &CV0($a)
	movabsq $zend_is_true, %rax
	callq *%rax
	testb %al, %al
	je .L2

After #1:

.L1:
	movq 0x50(%r14), %rax           ; Z_ARRVAL($a)
	cmpl $0, 0x1c(%rax)             ; compare nNumOfElements
	je .L2

PHP demo code for #2:

function is_empty(array $a) {
	if ($a === []) return 1;
	return 2;
}

Before #2:

.L1:
	leaq 0x50(%r14), %rdi           ; arg1 = &CV0($a)
	movl $0x418d8ad8, %esi          ; arg2 = &literal []
	movabsq $zend_is_identical, %rax
	callq *%rax
	testb %al, %al
	je .L2

After #2:

.L1:
	movq 0x50(%r14), %rax           ; Z_ARRVAL($a)
	cmpl $0, 0x1c(%rax)             ; compare nNumOfElements
	jne .L2

It ain't much, but it should help.
I think many small frontend improvements can make a nice change in total together.

[arnaud-lb]

Looks good to me!

[arnaud-lb]

I think many small frontend improvements can make a nice change in total together.

I agree, especially when it reduces code size like this