traces: provide a means to get short m4 traces

Let --trace=m4-early dump all the logs from the start (as --trace=m4
used to do), and have --trace=m4 now start traces only when actually
working of the user's grammar.

Can make a big difference in the case of small inputs.  E.g.

    $ bison -S tests/testsuite.dir/001/input.m4 tests/testsuite.dir/001/input.y --trace=m4 |& wc
       3952   19446  251068
    $ bison -S tests/testsuite.dir/001/input.m4 tests/testsuite.dir/001/input.y --trace=m4-early |& wc
      19491  131904 1830495

* data/skeletons/traceon.m4: New.
* src/getargs.h, src/getargs.c: Introduce --trace=m4-early.
* src/output.c (output_skeleton): Adjust for --trace=m4 and --trace=m4-early.

src/getargs.c

@@ -257,7 +257,8 @@ static const argmatch_trace_doc argmatch_trace_docs[] =
   { "sets",       "grammar sets: firsts, nullable etc." },
   { "muscles",    "m4 definitions passed to the skeleton" },
   { "tools",      "m4 invocation" },
-  { "m4",         "m4 traces" },
+  { "m4-early",   "m4 traces starting from the start" },
+  { "m4",         "m4 traces starting from the skeleton evaluation" },
   { "skeleton",   "skeleton postprocessing" },
   { "time",       "time consumption" },
   { "ielr",       "IELR conversion" },
@@ -279,6 +280,7 @@ static const argmatch_trace_arg argmatch_trace_args[] =
   { "sets",      trace_sets },
   { "muscles",   trace_muscles },
   { "tools",     trace_tools },
+  { "m4-early",  trace_m4_early },
   { "m4",        trace_m4 },
   { "skeleton",  trace_skeleton },
   { "time",      trace_time },

src/getargs.h

@@ -100,11 +100,12 @@ enum trace
     trace_grammar   = 1 << 7,  /**< Reading, reducing the grammar. */
     trace_time      = 1 << 8,  /**< Time consumption. */
     trace_skeleton  = 1 << 9,  /**< Skeleton postprocessing. */
-    trace_m4        = 1 << 10, /**< M4 traces. */
-    trace_muscles   = 1 << 11, /**< M4 definitions of the muscles. */
-    trace_ielr      = 1 << 12, /**< IELR conversion. */
-    trace_closure   = 1 << 13, /**< Input/output of closure(). */
-    trace_locations = 1 << 14, /**< Full display of locations. */
+    trace_m4_early  = 1 << 10, /**< M4 early traces. */
+    trace_m4        = 1 << 11, /**< M4 traces. */
+    trace_muscles   = 1 << 12, /**< M4 definitions of the muscles. */
+    trace_ielr      = 1 << 13, /**< IELR conversion. */
+    trace_closure   = 1 << 14, /**< Input/output of closure(). */
+    trace_locations = 1 << 15, /**< Full display of locations. */
     trace_all       = ~0       /**< All of the above.  */
   };
 /** What debug items bison displays during its run.  */

src/output.c

@@ -687,6 +687,7 @@ output_skeleton (void)
   char *skeldir = xpath_join (datadir, "skeletons");
   char *m4sugar = xpath_join (datadir, "m4sugar/m4sugar.m4");
   char *m4bison = xpath_join (skeldir, "bison.m4");
+  char *traceon = xpath_join (skeldir, "traceon.m4");
   char *skel = (IS_PATH_WITH_DIR (skeleton)
                 ? xstrdup (skeleton)
                 : xpath_join (skeldir, skeleton));
@@ -701,7 +702,7 @@ output_skeleton (void)
   int filter_fd[2];
   pid_t pid;
   {
-    char const *argv[10];
+    char const *argv[11];
     int i = 0;
     argv[i++] = m4;
 
@@ -720,17 +721,16 @@ output_skeleton (void)
     argv[i++] = "-I";
     argv[i++] = datadir;
     /* Some future version of GNU M4 (most likely 1.6) may treat the
-       -dV in a position-dependent manner.  Keep it as the first
-       argument so that all files are traced.
-
-       See the thread starting at
+       -dV in a position-dependent manner.  See the thread starting at
        <http://lists.gnu.org/archive/html/bug-bison/2008-07/msg00000.html>
        for details.  */
-    if (trace_flag & trace_m4)
+    if (trace_flag & trace_m4_early)
       argv[i++] = "-dV";
     argv[i++] = m4sugar;
     argv[i++] = "-";
     argv[i++] = m4bison;
+    if (trace_flag & trace_m4)
+      argv[i++] = traceon;
     argv[i++] = skel;
     argv[i++] = NULL;
     aver (i <= ARRAY_CARDINALITY (argv));
@@ -751,6 +751,7 @@ output_skeleton (void)
   free (skeldir);
   free (m4sugar);
   free (m4bison);
+  free (traceon);
   free (skel);
 
   if (trace_flag & trace_muscles)