Perl_sv_setsv_flags: return after invlist_clone case

In the "first_ switch" statement in Perl_sv_setsv_flags, there's this case:
    case SVt_INVLIST:
        invlist_clone(ssv, dsv);
        break;

When the INVLIST code was added, it was unclear whether the logic following
that switch needed to apply to INVLISTs or not. Early returns were also seen
as risky from a future maintenance perspective. As a precaution, this case
was written to `break` rather than `return`.

With later analysis though, it seems like the code below this switch does
not apply to INVLISTs, apart from one unintended interaction.

* invlist_clone() - in regcomp.c - copies the PV buffer of ssv to dsv.
* However, whenever an invlist is initialized, SvPOK_on is called on its SV;
according to the nearby comment "/* This allows B to extract the PV */".
* Because the switch case does not return, the later `if (sflags & SVp_POK)`
branch will try to swipe/cow/copy the PV buffer of ssv to dsv. This is an
unnecessary double copy.

On that analysis, this commit changes the break into a return.

1 files changed, 4 insertions, 2 deletions

diff --git a/sv.c b/sv.c
index a7a004400d..538944dfb3 100644
--- a/sv.c
+++ b/sv.c
@@ -4156,7 +4156,9 @@ Perl_sv_setsv_flags(pTHX_ SV *dsv, SV* ssv, const I32 flags)
     SV_CHECK_THINKFIRST_COW_DROP(dsv);
     dtype = SvTYPE(dsv); /* THINKFIRST may have changed type */
 
-    /* There's a lot of redundancy below but we're going for speed here */
+    /* There's a lot of redundancy below but we're going for speed here
+     * Note: some of the cases below do return; rather than break; so the
+     * if-elseif-else logic below this switch does not see all cases. */
 
     switch (stype) {
     case SVt_NULL:
@@ -4244,7 +4246,7 @@ Perl_sv_setsv_flags(pTHX_ SV *dsv, SV* ssv, const I32 flags)
 
     case SVt_INVLIST:
         invlist_clone(ssv, dsv);
-        break;
+        return;
     default:
         {
         const char * const type = sv_reftype(ssv,0);