Backport GH-16348

Agreed by RM: #16168 (comment)

The inline assembly uses labels with the prefix `.L`. On Linux systems
this is the local label prefix. It appears that macOS uses `L` as a
local prefix, which means that the prefix used in the inline assembly is not
local for macOS systems [1].
When combined with inlining, this causes the compiler to get confused
and merge a part of the inline assembly between different functions,
causing control flow to jump from one function to another function.
This is avoided on PHP 8.2 and up by the fact that it
uses `zend_never_inline NOIPA`, but nothing guarantees that compiler
changes won't affect this as well.

To solve this issue, we instead use local labels. These will make the
compiler pick the correct prefix, preventing the issue.

Additionally, while here, we also change the computation of `delta`.
It is undefined behaviour to compute the pointer difference between
two different objects. To circumvent this, we cast first to `uintptr_t`.

This change is cleanly backportable to 8.1 for vendors to pick up.

[1] #16168 (comment)

With the help of investigation and testing of @ryandesign.

Closes GH-16348.

Author: nielsdos

Zend/zend_string.c

@@ -397,33 +397,33 @@ ZEND_API bool ZEND_FASTCALL NO_CALLER_SAVED_REGISTERS I_REPLACE_SONAME_FNNAME_ZU
 #if defined(__GNUC__) && defined(__i386__)
 ZEND_API bool ZEND_FASTCALL zend_string_equal_val(zend_string *s1, zend_string *s2)
 {
-	char *ptr = ZSTR_VAL(s1);
-	size_t delta = (char*)s2 - (char*)s1;
+	const char *ptr = ZSTR_VAL(s1);
+	uintptr_t delta = (uintptr_t) s2 - (uintptr_t) s1;
 	size_t len = ZSTR_LEN(s1);
 	zend_ulong ret;
 
 	__asm__ (
-		".LL0%=:\n\t"
+		"0:\n\t"
 		"movl (%2,%3), %0\n\t"
 		"xorl (%2), %0\n\t"
-		"jne .LL1%=\n\t"
+		"jne 1f\n\t"
 		"addl $0x4, %2\n\t"
 		"subl $0x4, %1\n\t"
-		"ja .LL0%=\n\t"
+		"ja 0b\n\t"
 		"movl $0x1, %0\n\t"
-		"jmp .LL3%=\n\t"
-		".LL1%=:\n\t"
+		"jmp 3f\n\t"
+		"1:\n\t"
 		"cmpl $0x4,%1\n\t"
-		"jb .LL2%=\n\t"
+		"jb 2f\n\t"
 		"xorl %0, %0\n\t"
-		"jmp .LL3%=\n\t"
-		".LL2%=:\n\t"
+		"jmp 3f\n\t"
+		"2:\n\t"
 		"negl %1\n\t"
 		"lea 0x20(,%1,8), %1\n\t"
 		"shll %b1, %0\n\t"
 		"sete %b0\n\t"
 		"movzbl %b0, %0\n\t"
-		".LL3%=:\n"
+		"3:\n"
 		: "=&a"(ret),
 		  "+c"(len),
 		  "+r"(ptr)
@@ -435,33 +435,33 @@ ZEND_API bool ZEND_FASTCALL zend_string_equal_val(zend_string *s1, zend_string *
 #elif defined(__GNUC__) && defined(__x86_64__) && !defined(__ILP32__)
 ZEND_API bool ZEND_FASTCALL zend_string_equal_val(zend_string *s1, zend_string *s2)
 {
-	char *ptr = ZSTR_VAL(s1);
-	size_t delta = (char*)s2 - (char*)s1;
+	const char *ptr = ZSTR_VAL(s1);
+	uintptr_t delta = (uintptr_t) s2 - (uintptr_t) s1;
 	size_t len = ZSTR_LEN(s1);
 	zend_ulong ret;
 
 	__asm__ (
-		".LL0%=:\n\t"
+		"0:\n\t"
 		"movq (%2,%3), %0\n\t"
 		"xorq (%2), %0\n\t"
-		"jne .LL1%=\n\t"
+		"jne 1f\n\t"
 		"addq $0x8, %2\n\t"
 		"subq $0x8, %1\n\t"
-		"ja .LL0%=\n\t"
+		"ja 0b\n\t"
 		"movq $0x1, %0\n\t"
-		"jmp .LL3%=\n\t"
-		".LL1%=:\n\t"
+		"jmp 3f\n\t"
+		"1:\n\t"
 		"cmpq $0x8,%1\n\t"
-		"jb .LL2%=\n\t"
+		"jb 2f\n\t"
 		"xorq %0, %0\n\t"
-		"jmp .LL3%=\n\t"
-		".LL2%=:\n\t"
+		"jmp 3f\n\t"
+		"2:\n\t"
 		"negq %1\n\t"
 		"lea 0x40(,%1,8), %1\n\t"
 		"shlq %b1, %0\n\t"
 		"sete %b0\n\t"
 		"movzbq %b0, %0\n\t"
-		".LL3%=:\n"
+		"3:\n"
 		: "=&a"(ret),
 		  "+c"(len),
 		  "+r"(ptr)