mg.c: Perl_magic_get() dont call libc's getter func _errno() over and over

Ever since threads and SMP motherboards were invented, C's grammer token
errno has been a function call returning an int * and not a extern "C"
global data variable. Whether C grammer token is an inline asm intrinsic
macro, or a traditional linker symbol in the C lang's symbol table, doesn't
matter. On Windows errno is a macro to int * _errno(void) from ucrtbase.dll
The UCRT dll from its day 1 upto atleast the UCRT dllfrom Win 10 ~2019,
has a severe multi-eval bug in its C++/CPP getter method for its core
TLS fetcher function. The multi-eval problem involves GetProcAddress(),
and dynamic dispatching between TlsGetValue() and FlsGetValue(), and the
rule that non-inlined function calls may never be de-duped for any reason.
C++ operator overloading and CPP #define will never turn 10 method calls
in a .i file or C++ template, into 1 method call and 1 POD size_t/void *
variable. VC6, all msvcrt.dlls thru VC 2013, correctly only execute
TlsGetValue() once and cache the result. UCRT does not.

A pathologic reading of the POSIX spec, guarentees sv_setnv() UBed the
value inside errno, through some ISO C lexer token inside the sv.i file
made from sv.c that defines the body of sv_setnv(). If libc.so is chmod
to --x by root user, how can a user prove the machine code inside libc's
memcpy() does not call libc's sqrt() function? All behavior is unspecified
and valid until it is defined. A realistic, not pedantic, location where
errno was UBed could be these 2 lines:

SV_CHECK_THINKFIRST_COW_DROP(sv);
sv_upgrade(sv, SVt_PVNV);

This patch was written for Win32 perf reasons, and not POSIX/C compliance.
The later are an afterthought. To fix all of the above, cache the retval
of errno. The ticket for this patch has a before/after.

Author: bulk88

mg.c

@@ -1009,20 +1009,20 @@ Perl_magic_get(pTHX_ SV *sv, MAGIC *mg)
         {
             dSAVE_ERRNO;
 #ifdef VMS
-            sv_setnv(sv, (NV)((errno == EVMSERR) ? vaxc$errno : errno));
+            sv_setnv(sv, (NV)((saved_errno == EVMSERR) ? vaxc$errno : saved_errno));
 #else
-            sv_setnv(sv, (NV)errno);
+            sv_setnv(sv, (NV)saved_errno);
 #endif
 #ifdef OS2
-            if (errno == errno_isOS2 || errno == errno_isOS2_set)
+            if (saved_errno == errno_isOS2 || saved_errno == errno_isOS2_set)
                 sv_setpv(sv, os2error(Perl_rc));
             else
 #endif
-            if (! errno) {
+            if (! saved_errno) {
                 SvPVCLEAR(sv);
             }
             else {
-                sv_string_from_errnum(errno, sv);
+                sv_string_from_errnum(saved_errno, sv);
                 /* If no useful string is available, don't
                  * claim to have a string part.  The SvNOK_on()
                  * below will cause just the number part to be valid */