regmatch(): improve savestack ix debugging output

Under

    use re Debug => 'STATE';

S_regmatch() produces various bits of debugging output regarding
savestack manipulation; e.g. when the current index is saved; or when
the savestack is popped back to that index on success or failure etc.

Unfortunately that output is patchy and inconsistent. For example it
often talks confusingly about "Setting an EVAL scope" even when an EVAL
hasn't been executed. And it doesn't log all savestack manipulation.

After this commit there is a standard set of debugging messages, such
as:

    savestack: snapping ix=14
    savestack: regcppush filled in range [14..18)
    savestack: regcppop freed in range [14..18)
    savestack: freeing in range [10..14)

which are consistently applied.

There should be no change in functionality unless that specific
debugging is enabled.

Author: iabyn

regexec.c

@@ -298,29 +298,40 @@ S_regcppush(pTHX_ const regexp *rex, I32 parenfloor, U32 maxopenparen comma_pDEP
                 depth, retval, PL_savestack_ix);
     });
 
+    DEBUG_STATE_r(
+        Perl_re_exec_indentf(aTHX_
+            "savestack: regcppush filled in range [%" IVdf "..%" IVdf ")\n",
+                depth, (IV)retval, (IV)PL_savestack_ix
+        )
+    );
+
     return retval;
 }
 
 /* These are needed since we do not localize EVAL nodes: */
 #define REGCP_SET(cp)                                           \
     DEBUG_STATE_r(                                              \
         Perl_re_exec_indentf( aTHX_                             \
-            "Setting an EVAL scope, savestack = %" IVdf ",\n",  \
+            "savestack: snapping ix=%" IVdf "\n",               \
             depth, (IV)PL_savestack_ix                          \
         )                                                       \
     );                                                          \
     cp = PL_savestack_ix
 
+/* REGCP_UNWIND(cp): unwind savestack back to marked index,
+ * but without restoring regcppush()ed data (leave_scope() treats
+ * SAVEt_REGCONTEXT as a NOOP)
+ */
+
 #define REGCP_UNWIND(cp)                                        \
     DEBUG_STATE_r(                                              \
-        if (cp != PL_savestack_ix)                              \
-            Perl_re_exec_indentf( aTHX_                         \
-                "Clearing an EVAL scope, savestack = %"         \
-                IVdf "..%" IVdf "\n",                           \
-                depth, (IV)(cp), (IV)PL_savestack_ix            \
-            )                                                   \
+        Perl_re_exec_indentf( aTHX_                             \
+            "savestack: freeing in range [%"                    \
+            IVdf "..%" IVdf ")\n",                              \
+            depth, (IV)(cp), (IV)PL_savestack_ix                \
+        )                                                       \
     );                                                          \
-    regcpblow(cp)
+    LEAVE_SCOPE(cp)
 
 /* set the start and end positions of capture ix */
 #define CLOSE_ANY_CAPTURE(rex, ix, s, e)                                    \
@@ -403,6 +414,10 @@ S_regcppop(pTHX_ regexp *rex, U32 *maxopenparen_p comma_pDEPTH)
 {
     UV i;
     U32 paren;
+#ifdef DEBUGGING
+    I32 orig_ix = PL_savestack_ix;
+#endif
+
     DECLARE_AND_GET_RE_DEBUG_FLAGS;
 
     PERL_ARGS_ASSERT_REGCPPOP;
@@ -491,6 +506,14 @@ S_regcppop(pTHX_ regexp *rex, U32 *maxopenparen_p comma_pDEPTH)
                 "finished regcppop at %" IVdf "\n",
                 depth, PL_savestack_ix);
     });
+
+    DEBUG_STATE_r(
+        Perl_re_exec_indentf(aTHX_
+            "savestack: regcppop freed in range [%" IVdf "..%" IVdf ")\n",
+                depth, (IV)(PL_savestack_ix), (IV)orig_ix
+        )
+    );
+
 }
 
 /* restore the parens and associated vars at savestack position ix,
@@ -499,15 +522,25 @@ S_regcppop(pTHX_ regexp *rex, U32 *maxopenparen_p comma_pDEPTH)
 STATIC void
 S_regcp_restore(pTHX_ regexp *rex, I32 ix, U32 *maxopenparen_p comma_pDEPTH)
 {
+    DECLARE_AND_GET_RE_DEBUG_FLAGS;
     I32 tmpix = PL_savestack_ix;
     PERL_ARGS_ASSERT_REGCP_RESTORE;
 
+    DEBUG_STATE_r(
+        Perl_re_exec_indentf( aTHX_
+            "savestack: regcp_restore: ix was %" IVdf "\n",
+            depth, (IV)PL_savestack_ix);
+    );
     PL_savestack_ix = ix;
     regcppop(rex, maxopenparen_p);
     PL_savestack_ix = tmpix;
+    DEBUG_STATE_r(
+        Perl_re_exec_indentf( aTHX_
+            "savestack: regcp_restore: reset ix=%" IVdf "\n",
+            depth, (IV)PL_savestack_ix);
+    );
 }
 
-#define regcpblow(cp) LEAVE_SCOPE(cp)	/* Ignores regcppush()ed data. */
 
 STATIC bool
 S_isFOO_lc(pTHX_ const U8 classnum, const U8 character)
@@ -8723,7 +8756,7 @@ S_regmatch(pTHX_ regmatch_info *reginfo, char *startpos, regnode *prog)
                 SV *save_sv= GvSV(PL_replgv);
                 SV *replsv;
                 SvREFCNT_inc(save_sv);
-                regcpblow(ST.cp); /* LEAVE in disguise */
+                REGCP_UNWIND(ST.cp); /* LEAVE in disguise */
                 /* don't move this initialization up */
                 replsv = GvSV(PL_replgv);
                 sv_setsv(replsv, save_sv);
@@ -8990,7 +9023,7 @@ NULL
 
             ST.prev_curlyx= cur_curlyx;
             cur_curlyx = st;
-            ST.cp = PL_savestack_ix;
+            REGCP_SET(ST.cp);
 
             /* these fields contain the state of the current curly.
              * they are accessed by subsequent WHILEMs */
@@ -9013,7 +9046,7 @@ NULL
             NOT_REACHED; /* NOTREACHED */
 
         case CURLYX_end_fail: /* just failed to match all of A*B */
-            regcpblow(ST.cp);
+            REGCP_UNWIND(ST.cp); /* LEAVE in disguise */
             cur_curlyx = ST.prev_curlyx;
             sayNO;
             NOT_REACHED; /* NOTREACHED */