Eliminate is_gv_magical_sv

This resolves perl bug #97978.

Many built-in variables, like $], are actually created on the fly
when first accessed.  Perl likes to pretend that these variables have
always existed, so it autovivifies the *] glob even in rvalue context
(e.g., defined *{"]"}, close "]").

The list of variables that were autovivified was maintained separ-
ately (in is_gv_magical_sv) from the code that actually creates
them (gv_fetchpvn_flags).  ‘Maintained’ is not actually precise: it
*wasn’t* being maintained, and there were new variables that never
got added to is_gv_magical_sv and one deleted variable that was
never removed.

There are only two pieces of code that call is_gv_magical_sv, both in
pp.c: S_rv2gv (called by *{} and also the implicit *{} that functions
like close() provide) and Perl_softrefxv (called by ${}, @{}, %{}).

In both cases, the glob is immediately autovivified if
is_gv_magical_sv returns true.

So this commit eliminates the extra maintenance burden by extirpat-
ing is_gv_magical_sv altogether, and replacing it with a new flag to
gv_fetchpvn_flags, GvADDMG, which will autovivify a glob *if* it’s a
magical one.

It does make defined(*{"frobbly"}) slightly slower, in that it creates
a temporary glob and then frees it when it sees nothing magical has
been done with it.  But this case is rare enough it should not matter.
At least I got rid of the bugginess.

1 files changed, 6 insertions, 2 deletions

diff --git a/gv.h b/gv.h
index a70a9060af..b9d04e6b58 100644
--- a/gv.h
+++ b/gv.h
@@ -213,13 +213,17 @@ Return the SV from the GV.
 				   package (so skip checks for :: and ')  */
 #define GV_AUTOLOAD	0x100	/* gv_fetchmethod_flags() should AUTOLOAD  */
 #define GV_CROAK	0x200	/* gv_fetchmethod_flags() should croak  */
+#define GV_ADDMG	0x400	/* add if magical */
 
 /*      SVf_UTF8 (more accurately the return value from SvUTF8) is also valid
 	as a flag to gv_fetch_pvn_flags, so ensure it lies outside this range.
 */
 
-#define GV_NOADD_MASK	(SVf_UTF8|GV_NOADD_NOINIT|GV_NOEXPAND|GV_NOTQUAL)
-/* The bit flags that don't cause gv_fetchpv() to add a symbol if not found */
+#define GV_NOADD_MASK \
+    (SVf_UTF8|GV_NOADD_NOINIT|GV_NOEXPAND|GV_NOTQUAL|GV_ADDMG)
+/* The bit flags that don't cause gv_fetchpv() to add a symbol if not
+   found (with the exception GV_ADDMG, which *might* cause the symbol
+   to be added) */
 
 #define gv_fullname3(sv,gv,prefix) gv_fullname4(sv,gv,prefix,TRUE)
 #define gv_efullname3(sv,gv,prefix) gv_efullname4(sv,gv,prefix,TRUE)