Don't copy defunct buffer contents when undoing OOK.

pod/perldelta.pod

@@ -348,6 +348,19 @@ well.
 
 =item *
 
+C<Perl_sv_backoff>, which undoes the OOK string offset mechanism now only
+moves the string buffer contents to the start of the buffer if `SvOK(sv)`
+is true. Historically, the buffer contents were always moved, but if the
+buffer contents are defunct and no longer required, doing that incurs an
+unnecessary cost proportional to the size of the shifted contents.
+
+(The assumption in this change is that `SvOK(sv)` is a valid indicator of
+whether the string buffer contents are "live" or not.)
+
+See GH#23967 for an example of where such a copy was noticeable.
+
+=item *
+
 XXX
 
 =back

scope.c

@@ -1440,8 +1440,20 @@ Perl_leave_scope(pTHX_ I32 base)
 
                         if (SvTYPE(sv) == SVt_PVHV && HvHasAUX(sv))
                             Perl_hv_kill_backrefs(aTHX_ MUTABLE_HV(sv));
-                        else if(SvOOK(sv))
-                            sv_backoff(sv);
+                        else if(SvOOK(sv)) {
+                            /* Inlined sv_backoff() - the buffer contents are
+                             * defunct and there's no need to copy them. All that
+                             * is needed is resetting SvLEN and the SvPVX pointer. */
+                            assert(SvTYPE(sv) != SVt_PVHV); /* the branch above */
+                            assert(SvTYPE(sv) != SVt_PVAV);
+
+                            STRLEN delta;
+                            SvOOK_offset(sv, delta);
+
+                            SvLEN_set(sv, SvLEN(sv) + delta);
+                            SvPV_set(sv, SvPVX(sv) - delta);
+                            SvFLAGS(sv) &= ~SVf_OOK;
+                        }
 
                         if (SvMAGICAL(sv)) {
                             /* note that backrefs (either in HvAUX or magic)

sv.c

@@ -1336,6 +1336,8 @@ wrapper instead.
 
 /* prior to 5.000 stable, this function returned the new OOK-less SvFLAGS
    prior to 5.23.4 this function always returned 0
+   prior to 5.43.x, the contents of the string buffer were always copied
+      down to the start of the buffer. Now, this only happens if SvOK(sv)
 */
 
 void
@@ -1355,7 +1357,11 @@ Perl_sv_backoff(SV *const sv)
     SvLEN_set(sv, SvLEN(sv) + delta);
     SvPV_set(sv, SvPVX(sv) - delta);
     SvFLAGS(sv) &= ~SVf_OOK;
-    Move(s, SvPVX(sv), SvCUR(sv)+1, char);
+
+    /* Don't copy a buffer if the contents are already defunct. */
+    if (SvOK(sv))
+        Move(s, SvPVX(sv), SvCUR(sv)+1, char);
+
     return;
 }