save old PL_comppad in CXt_SUB/FORMAT block

Currently when we call a sub, the old value of PL_comppad is
saved on the save stack using SAVECOMPPAD(). Instead, save it in
a new field in the context struct, called prevcomppad. This is simpler
and more efficient.

Note that there is already a confusingly-named field in the CXt_SUB
context struct called oldcomppad, which holds the value of PL_comppad for
the *current* sub, not for its caller. So the new field had to be called
something else.

One side effect of this is that an existing bug  - which causes too much
to be popped off the savestack when dieing while leaving a sub scope - is
now more noticeable, since PL_curpad and SAVEt_CLEARSV are now out of
sync: formerly, the unwinding of the save stack restored PL_curpad in
lockstep. The fix for this will come later in this branch, when the whole
issue of context stack popping order and reentrancy is addressed; for
now, a TODO test has been added.

1 files changed, 7 insertions, 0 deletions

diff --git a/regexec.c b/regexec.c
index 8e1c1f68f2..9aec6c3ca5 100644
--- a/regexec.c
+++ b/regexec.c
@@ -6590,6 +6590,13 @@ S_regmatch(pTHX_ regmatch_info *reginfo, char *startpos, regnode *prog)
                     U8 flags = (CXp_SUB_RE |
                                 ((newcv == caller_cv) ? CXp_SUB_RE_FAKE : 0));
 		    if (last_pushed_cv) {
+                        /* PUSH/POP_MULTICALL save and restore the
+                         * caller's PL_comppad; if we call multiple subs
+                         * using the same CX block, we have to save and
+                         * unwind the varying PL_comppad's ourselves,
+                         * especially restoring the right PL_comppad on
+                         * backtrack - so save it on the save stack */
+                        SAVECOMPPAD();
 			CHANGE_MULTICALL_FLAGS(newcv, flags);
 		    }
 		    else {