reverse the order of POPBLOCK; POPFOO

Currently most pp_leavefoo subs have something along the lines of

    POPBLOCK(cx);
    POPFOO(cx);

where POPBLOCK does cxstack_ix-- and sets cx to point to the top CX stack
entry. It then restores a bunch of PL_ vars saved in the CX struct.

Then POPFOO does any type-specific restoration, e.g. POPSUB decrements the
ref count of the cv that was just executed.

However, this is logically the wrong order. When we *enter* a scope, we do

    PUSHBLOCK;
    PUSHFOO;

so undoing the PUSHBLOCK should be the last thing we do.  As it happens,
it doesn't really make any difference to the running, which is why we've
never fixed it before.

Reordering it has two advantages.

First, it allows the steps for scope exit to be the exact logical reverse
of scope exit, which makes understanding what's going on and debugging
easier.

It allows us to make the code cleaner.

This commit also removes the cxstack_ix-- and setting cx steps from
POPBLOCK; now we already expect cx to be set (which it usually already is)
and we do the cxstack_ix-- ourselves. This also means we can remove a
whole bunch of cxstack_ix++'s that were added immediately after the
POPBLOCK in order to prevent the context being inadvertently overwritten
before we've finished using it.

So in full,

    POPBLOCK(cx);
    POPFOO(cx);

is now implemented as:

    cx = &cxstack[cxstack_ix];
    ... other stuff done with cx ...
    POPFOO(cx);
    POPBLOCK(cx);
    cxstack_ix--;

Finally, this commit also tweaks PL_curcop in pp_leaveeval, since
otherwise PL_curcop could temporarily be NULL when debugging code is
called in the presence of 'use re Debug'. It also stops the debugging code
crashing if PL_curcop is still NULL.

1 files changed, 1 insertions, 1 deletions

diff --git a/regcomp.h b/regcomp.h
index b9c0613946..44c2c1c8de 100644
--- a/regcomp.h
+++ b/regcomp.h
@@ -1055,7 +1055,7 @@ re.pm, especially to the documentation.
 /* get_sv() can return NULL during global destruction. */
 #define GET_RE_DEBUG_FLAGS DEBUG_r({ \
         SV * re_debug_flags_sv = NULL; \
-        re_debug_flags_sv = get_sv(RE_DEBUG_FLAGS, 1); \
+        re_debug_flags_sv = PL_curcop ? get_sv(RE_DEBUG_FLAGS, 1) : NULL; \
         if (re_debug_flags_sv) { \
             if (!SvIOK(re_debug_flags_sv)) \
                 sv_setuv(re_debug_flags_sv, RE_DEBUG_COMPILE_DUMP | RE_DEBUG_EXECUTE_MASK ); \