Fix GH-9068: Conditional jump or move depends on uninitialised value(s)

This patch preserves the scratch registers of the SysV x86-64 ABI by storing
them to the stack and restoring them later. We need to do this to prevent the
registers of the caller from being corrupted. The reason these get corrupted
is because the compiler is unaware of the Valgrind replacement function and
thus makes assumptions about the original function regarding registers which
are not true for the replacement function.

For implementation I used a GCC and Clang attribute. A more general
approach would be to use inline assembly but that's also less portable
and quite hacky. This attributes is supported since GCC 7.x.

Author: nielsdos

Zend/zend_string.c

@@ -372,11 +372,24 @@ ZEND_API void zend_interned_strings_switch_storage(bool request)
 # define I_REPLACE_SONAME_FNNAME_ZU(soname, fnname) _vgr00000ZU_ ## soname ## _ ## fnname
 #endif
 
-ZEND_API bool ZEND_FASTCALL I_REPLACE_SONAME_FNNAME_ZU(NONE,zend_string_equal_val)(zend_string *s1, zend_string *s2)
+/* See GH-9068 */
+#if defined(__GNUC__) && __has_attribute(no_caller_saved_registers)
+# define NO_CALLER_SAVED_REGISTERS __attribute__((no_caller_saved_registers))
+# pragma GCC push_options
+# pragma GCC target ("general-regs-only")
+#else
+# define NO_CALLER_SAVED_REGISTERS
+#endif
+
+ZEND_API bool ZEND_FASTCALL NO_CALLER_SAVED_REGISTERS I_REPLACE_SONAME_FNNAME_ZU(NONE,zend_string_equal_val)(zend_string *s1, zend_string *s2)
 {
 	return !memcmp(ZSTR_VAL(s1), ZSTR_VAL(s2), ZSTR_LEN(s1));
 }
 
+#if defined(__GNUC__) && __has_attribute(no_caller_saved_registers)
+# pragma GCC pop_options
+#endif
+
 #if defined(__GNUC__) && defined(__i386__)
 ZEND_API bool ZEND_FASTCALL zend_string_equal_val(zend_string *s1, zend_string *s2)
 {