Zend/zend_operators: fix casting underflowed unsigned to signed

[MaxKellermann]

Casting a huge unsigned value to signed is undefined behavior in C.
We need to cast to signed before subtracting.

[nikic]

Casting a huge unsigned value to signed is undefined behavior in C.

This is incorrect. It's implementation-defined behavior, and all compiler implementations supported by PHP agree on the behavior. In fact, it is your new implementation that has undefined behavior in case the subtraction overflows.

That being said, the current code is fishy in that size_t may be larger than int, which means that we might truncate the subtraction to a zero value, reporting equality of strings that are not equal in degenerate cases. As this code is only interested in returning a three-way comparison result, the absolute value of which should not matter, I would recommend adding a helper macro that compares two values and returns 0, 1 or -1. (Along the lines of ZEND_NORMALIZE_BOOL, but that one works on a subtraction result.)

[MaxKellermann]

I rewrote the commit adding the macro ZEND_THREEWAY_COMPARE() as suggested, which uses two comparisons like ZEND_NORMALIZE_BOOL(), instead of explicit subtraction + cast.

[cmb69]

That changes requires to adjust some test expectations.

[MaxKellermann]

That changes requires to adjust some test expectations.

Will do.

[MaxKellermann]

Another admendment: I had the parantheses around macro parameters wrong. (I hate macros, because they force me to write obscure code like that.)

[cmb69]

Thanks for the PR! I'm fine with this, but it might make sense to add some info about the change to UPGRADING(.INTERNALS).

[ramsey]

@MaxKellermann This is waiting on you to add info to UPGRADING.INTERNALS. Let me know if you need any help with that.