+ yy_stack_frame *ps = parser->ps;
+ int i = 0;
+
+ if (!parser->stack || ps == parser->stack)
+ return;
+
+ YYDPRINTF ((Perl_debug_log, "clearing the parse stack\n"));
+
+ /* Freeing ops on the stack, and the op_latefree / op_latefreed /
+ * op_attached flags:
+ *
+ * When we pop tokens off the stack during error recovery, or when
+ * we pop all the tokens off the stack after a die during a shift or
+ * reduce (i.e. Perl_croak somewhere in yylex() or in one of the
+ * newFOO() functions), then it's possible that some of these tokens are
+ * of type opval, pointing to an OP. All these ops are orphans; each is
+ * its own miniature subtree that has not yet been attached to a
+ * larger tree. In this case, we should clearly free the op (making
+ * sure, for each op we free that we have PL_comppad pointing to the
+ * right place for freeing any SVs attached to the op in threaded
+ * builds.
+ *
+ * However, there is a particular problem if we die in newFOO() called
+ * by a reducing action; e.g.
+ *
+ * foo : bar baz boz
+ * { $$ = newFOO($1,$2,$3) }
+ *
+ * where
+ * OP *newFOO { ....; if (...) croak; .... }
+ *
+ * In this case, when we come to clean bar baz and boz off the stack,
+ * we don't know whether newFOO() has already:
+ * * freed them
+ * * left them as is
+ * * attached them to part of a larger tree
+ * * attached them to PL_compcv
+ * * attached them to PL_compcv then freed it (as in BEGIN {die } )
+ *
+ * To get round this problem, we set the flag op_latefree on every op
+ * that gets pushed onto the parser stack. If op_free() sees this
+ * flag, it clears the op and frees any children,, but *doesn't* free
+ * the op itself; instead it sets the op_latefreed flag. This means
+ * that we can safely call op_free() multiple times on each stack op.
+ * So, when clearing the stack, we first, for each op that was being
+ * reduced, call op_free with op_latefree=1. This ensures that all ops
+ * hanging off these op are freed, but the reducing ops themselces are
+ * just undefed. Then we set op_latefreed=0 on *all* ops on the stack
+ * and free them. A little thought should convince you that this
+ * two-part approach to the reducing ops should handle the first three
+ * cases above safely.
+ *
+ * In the case of attaching to PL_compcv (currently just newATTRSUB
+ * does this), then we set the op_attached flag on the op that has
+ * been so attached, then avoid doing the final op_free during
+ * cleanup, on the assumption that it will happen (or has already
+ * happened) when PL_compcv is freed.
+ *
+ * Note this is fairly fragile mechanism. A more robust approach
+ * would be to use two of these flag bits as 2-bit reference count
+ * field for each op, indicating whether it is pointed to from:
+ * * a parent op
+ * * the parser stack
+ * * a CV
+ * but this would involve reworking all code (core and external) that
+ * manipulate op trees.
+ *
+ * XXX DAPM 17/1/07 I've decided its too fragile for now, and so have
+ * disabled it */
+
+#define DISABLE_STACK_FREE
+
+
+#ifdef DISABLE_STACK_FREE
+ ps -= parser->yylen;
+ PERL_UNUSED_VAR(i);
+#else
+ /* clear any reducing ops (1st pass) */
+
+ for (i=0; i< parser->yylen; i++) {
+ LEAVE_SCOPE(ps[-i].savestack_ix);
+ if (yy_type_tab[yystos[ps[-i].state]] == toketype_opval
+ && ps[-i].val.opval) {
+ if ( ! (ps[-i].val.opval->op_attached
+ && !ps[-i].val.opval->op_latefreed))
+ {
+ if (ps[-i].comppad != PL_comppad) {
+ PAD_RESTORE_LOCAL(ps[-i].comppad);
+ }
+ op_free(ps[-i].val.opval);
+ }
+ }
+ }
+#endif