Resolve MSVC C4146 Warning in php_random_uint128.h

[KaseyJenkins]

The following code in php_random_uint128.h gives out MSVC's C4146 warning (unary minus operator applied to unsigned type, result still unsigned):

static inline uint64_t php_random_pcgoneseq128xslrr64_rotr64(php_random_uint128_t num)
{
	const uint64_t
		v = ((uint64_t) (num >> 64U)) ^ (uint64_t) num,
		s = num >> 122U;

	return (v >> s) | (v << ((-s) & 63));
}

This PR resolves it with substituting (-s) with (~s + 1)

[TimWolla]

see comment on issue

[KaseyJenkins]

Thank you, @TimWolla!

Have I understood it correctly that the decision has been made and cannot be reversed? :)

Quoting @cmb69

Fixing this in only public headers doesn't make sense to me

The reasoning behind this particular change in this public header is that building a PHP extension with MSVC gives out this warning and usually requires additional measures to circumvent it (e.g. patching this particular file with #pragma warning(disable:4146)).

[cmb69]

Have I understood it correctly that the decision has been made and cannot be reversed? :)

No. It's certainly not up to me (or any other individual) to make final decisions regarding php-src. So this is up for debate.

However, AFAIK this is well-defined behavior, so I don't understand why MSVC raises a warning (level 2, with /sdl even level 1) for the code. And if the behavior is well-defined, I feel uncomfortable changing the code, or just adding a pragma to silence the warning.

[KaseyJenkins]

Thank you @cmb69 for your reply!

In terms of compiler optimization issues it's rather interesting that gcc seems to recognize (~x + 1) as (-x) (where x is uint), but MSVC might be 'struggling' a bit (with optimizations turned off).

    uint32_t s = 1;
    uint32_t t = -s;
    uint32_t u = ~s + 1;

GCC:

main:
        pushq   %rbp
        movq    %rsp, %rbp
        movl    $1, -4(%rbp)
       
        #  uint32_t t = -s;
        movl    -4(%rbp), %eax
        negl    %eax
        movl    %eax, -8(%rbp)
        
        # uint32_t u = ~s + 1;
        movl    -4(%rbp), %eax
        negl    %eax
        movl    %eax, -12(%rbp)
      
        movl    $0, %eax
        popq    %rbp
        ret

MSVC:

s$ = 0
t$ = 4
u$ = 8
main    PROC
$LN3:
        sub     rsp, 24
        mov     DWORD PTR s$[rsp], 1
        
        #  uint32_t t = -s;
        mov     eax, DWORD PTR s$[rsp]
        neg     eax
        mov     DWORD PTR t$[rsp], eax
        
        # uint32_t u = ~s + 1;
        mov     eax, DWORD PTR s$[rsp]
        not     eax
        inc     eax
        mov     DWORD PTR u$[rsp], eax
        
        xor     eax, eax
        add     rsp, 24
        ret     0
main    ENDP