Change vm->address_list from GSList to GQueue

The current emulator uses an unbounded linked list for tracking the
memory locations our program has traveled through. On a 64 bit system,
this requires 16 bytes of data for every instruction a MIX program
performs. For small programs that are light on computation cycles,
this does not cause a noticeable issue.

For programs that execute hundreds of millions of instructions, this
causes the memory footprint of the virtual machine to explode. I have
attached an example MIXAL program that will cause the VM to grow to
over 3 GB of memory usage when run.

To run the sample, compile coin-opt.mixal (attached), run it in mixvm,
and enter 499 at the prompt. Or use the following steps.

This patch changes all the appropriate references to GQueue references
and also caps the backtrace at 1000, which can be changed in the
mixlib/mix_vm.h header. I feel like 1000 is a reasonable limit for the
vast majority of debugging needs. Most people are looking back at the
most recent 100 instructions or so.

You can get the original behavior (unlimited tracing) back by setting
the MIX_MAXTRACE to -1, albeit with a slightly higher memory cost (24
bytes per instruction). Or you can turn it off entirely by setting it
to 0.

Using a queue doesn't change the logic of the program in any
significant way, and it allows programs to run for an extended period
of time without consuming all the memory on the machine and slowing
down to a crawl.

-Kevin Brunelle

7 files changed, 214 insertions, 20 deletions

diff --git a/doc/mdk_mixvm.texi b/doc/mdk_mixvm.texi
index 580f641..f0da1cf 100644
--- a/doc/mdk_mixvm.texi
+++ b/doc/mdk_mixvm.texi
@@ -417,6 +417,15 @@ MIX >
 @end example
 @end deffn
 
+@deffn {debug command} sbt [NUMBER]
+This command changes the limit for the backtrace of executed instructions.
+If the number is omitted, the command prints the current limit.  If you
+use a 0, backtraces are turned off. This can improve performance. If you
+wish for all the instructions to be logged, a -1 will enable that. The
+amount of memory required for unlimited backtraces can be substantial
+for long-running programs.
+@end deffn
+
 @deffn {debug command} pbt [INS_NUMBER]
 This command prints a backtrace of executed instructions. Its optional
 argument @var{ins_number} is the number of instructions to print. If it
diff --git a/mixlib/mix_vm.c b/mixlib/mix_vm.c
index d714bba..94eae1e 100644
--- a/mixlib/mix_vm.c
+++ b/mixlib/mix_vm.c
@@ -81,8 +81,8 @@ vm_reset_reload_ (mix_vm_t *vm, gboolean is_reload)
 
   if (vm->address_list)
     {
-      g_slist_free (vm->address_list);
-      vm->address_list = NULL;
+      g_queue_free (vm->address_list);
+      vm->address_list = g_queue_new ();
     }
 }
 
@@ -99,7 +99,8 @@ mix_vm_new (void)
   vm->symbol_table = NULL;
   vm->src_file = NULL;
   vm->pred_list = mix_predicate_list_new (vm);
-  vm->address_list = NULL;
+  vm->max_backtrace_amount = MIX_MAX_TRACE_AMOUNT;
+  vm->address_list = g_queue_new ();
   vm->last_error = MIX_VM_ERROR_NONE;
 
   for (i = 0; i < BD_NO_; ++i)
@@ -126,7 +127,7 @@ mix_vm_delete (mix_vm_t * vm)
   if (vm->symbol_table != NULL) mix_symbol_table_delete (vm->symbol_table);
   if (vm->src_file != NULL) mix_src_file_delete (vm->src_file);
   if (vm->pred_list != NULL) mix_predicate_list_delete (vm->pred_list);
-  if (vm->address_list != NULL) g_slist_free (vm->address_list);
+  if (vm->address_list != NULL) g_queue_free (vm->address_list);
   for (i = 0; i < BD_NO_; ++i)
     mix_device_delete (vm->devices[i]);
   mix_vm_clock_delete (vm->clock);
@@ -452,9 +453,14 @@ mix_vm_run (mix_vm_t *vm)
   while ( !is_halted_ (vm) )
     {
       mix_word_to_ins_uncheck (get_cell_ (vm, get_loc_ (vm)), ins);
-      vm->address_list =
-	g_slist_prepend (vm->address_list,
-			 GINT_TO_POINTER ((gint)get_loc_ (vm)));
+      if ( vm->max_backtrace_amount != 0 ) {
+	g_queue_push_head (vm->address_list,
+			   GINT_TO_POINTER ((gint)get_loc_ (vm)));
+	if ( vm->max_backtrace_amount > 0 &&
+	    g_queue_get_length (vm->address_list) > vm->max_backtrace_amount ) {
+	  g_queue_pop_tail (vm->address_list);
+	}
+      }
       if ( !(*ins_handlers_[ins.opcode]) (vm,&ins) )
 	return set_status_ (vm, MIX_VM_ERROR);
       else
@@ -476,9 +482,14 @@ mix_vm_exec_next (mix_vm_t *vm)
   g_return_val_if_fail (vm != NULL, MIX_VM_ERROR);
   if (get_loc_ (vm) >= MIX_VM_CELL_NO) halt_ (vm, TRUE);
   if (is_halted_ (vm)) return set_status_ (vm, MIX_VM_HALT);
-  vm->address_list =
-    g_slist_prepend (vm->address_list,
-		     GINT_TO_POINTER ((gint)get_loc_ (vm)));
+  if ( vm->max_backtrace_amount != 0 ) {
+    g_queue_push_head (vm->address_list,
+		       GINT_TO_POINTER ((gint)get_loc_ (vm)));
+    if ( vm->max_backtrace_amount > 0 &&
+	 g_queue_get_length (vm->address_list) > vm->max_backtrace_amount ) {
+      g_queue_pop_tail (vm->address_list);
+    }
+  }
   mix_word_to_ins_uncheck (get_cell_ (vm, get_loc_ (vm)), ins);
   if (!(*ins_handlers_[ins.opcode]) (vm, &ins))
     return set_status_ (vm, MIX_VM_ERROR);
@@ -698,9 +709,24 @@ mix_vm_get_uptime (const mix_vm_t *vm)
 }
 
 /* Get the list of addresses for executed instructions */
-const GSList *
+GQueue *
 mix_vm_get_backtrace (const mix_vm_t *vm)
 {
   g_return_val_if_fail (vm != NULL, NULL);
-  return get_address_list_ (vm);
+  return vm->address_list;
+}
+
+gint
+mix_vm_get_max_trace (const mix_vm_t *vm)
+{
+  g_return_val_if_fail (vm != NULL, -2);
+  return vm->max_backtrace_amount;
+}
+
+void
+mix_vm_set_max_trace (mix_vm_t *vm, gint val)
+{
+  g_return_if_fail (vm != NULL);
+  g_return_if_fail (val >= -1);
+  vm->max_backtrace_amount = val;
 }
diff --git a/mixlib/mix_vm.h b/mixlib/mix_vm.h
index 0f4e690..4b07164 100644
--- a/mixlib/mix_vm.h
+++ b/mixlib/mix_vm.h
@@ -34,6 +34,9 @@
 /* Comparison flag */
 typedef enum { mix_LESS, mix_EQ, mix_GREAT } mix_cmpflag_t;
 
+/* Maximum number of addresses for backtraces: -1 = unlimited, 0 = off */
+enum { MIX_MAX_TRACE_AMOUNT = 500 };
+
 /* Number of memory cells in the virtual machine */
 enum { MIX_VM_CELL_NO = 4000 };
 
@@ -247,9 +250,16 @@ extern mix_time_t
 mix_vm_get_uptime (const mix_vm_t *vm);
 
 /* Get the list of addresses for executed instructions */
-extern const GSList *
+extern GQueue *
 mix_vm_get_backtrace (const mix_vm_t *vm);
 
+/* Get the maximum number of instructions we will trace */
+gint
+mix_vm_get_max_trace (const mix_vm_t *vm);
+
+/* Set the maximum number of instructions we will trace */
+void
+mix_vm_set_max_trace (mix_vm_t *vm, gint val);
 
 #endif /* MIX_VM_H */
 
diff --git a/mixlib/xmix_vm.h b/mixlib/xmix_vm.h
index 0d6605f..34dedd7 100644
--- a/mixlib/xmix_vm.h
+++ b/mixlib/xmix_vm.h
@@ -50,6 +50,7 @@ struct mix_vm_t
   mix_vm_status_t status;
   mix_vm_error_t last_error;
   mix_device_t * devices[BD_NO_];
+  gint max_backtrace_amount;    /* the limit of backtraces */
   mix_address_t start_addr;	/* start address of loaded file */
   GTree *line_table;		/* source line no -> address  */
   GTree *address_table;		/* adress -> source line no */
@@ -59,7 +60,7 @@ struct mix_vm_t
   mix_src_file_t *src_file;	/* source of last loaded code file */
   mix_device_factory_t factory; /* the factory for new devices */
   mix_predicate_list_t *pred_list; /* predicates for conditional bps */
-  GSList *address_list;		/* list of executed addresses */
+  GQueue *address_list;		/* list of executed addresses */
 };
 
 /* Macros for accessing/modifying the above structure.
diff --git a/mixlib/xmix_vm_handlers.c b/mixlib/xmix_vm_handlers.c
index c964116..f356607 100644
--- a/mixlib/xmix_vm_handlers.c
+++ b/mixlib/xmix_vm_handlers.c
@@ -86,6 +86,8 @@ mix_vm_command_info_t commands_[] = {
     "strace on|off"},
   { "pbt", cmd_pbt_, N_("Print backtrace of executed instructions"),
     "pbt [INS_NO] (e.g pbt 5)"},
+  { "sbt", cmd_sbt_, N_("Set limit on backtrace of executed instructions"),
+    "sbt [LIMIT] (e.g sbt 100)"},
   { "stime", cmd_stime_, N_("Turn on/off timing statistics"),
     "stime on|off"},
   { "ptime", cmd_ptime_, N_("Print current time statistics"), "ptime"},
@@ -1306,6 +1308,7 @@ gboolean
 cmd_pbt_ (mix_vm_cmd_dispatcher_t *dis, const gchar *arg)
 {
   enum {SIZE = 256};
+  static const gchar *NULLSTR = "(null)";
   static gchar BUFFER[SIZE];
   gint no = atoi (arg);
   gint k = 0, address;
@@ -1314,29 +1317,59 @@ cmd_pbt_ (mix_vm_cmd_dispatcher_t *dis, const gchar *arg)
   char *name =
     file ? g_path_get_basename (mix_src_file_get_path (file)) : NULL;
 
-  const GSList *add = mix_vm_get_backtrace (dis->vm);
-  while (add && (no == 0 || k < no))
+  GQueue *add = mix_vm_get_backtrace (dis->vm);
+  while (g_queue_get_length (add) > k && (no == 0 || k < no))
     {
       BUFFER[0] = '\0';
-      address = GPOINTER_TO_INT (add->data);
+      address = GPOINTER_TO_INT (g_queue_peek_nth (add, k));
       line = mix_vm_get_address_lineno (dis->vm, address);
       if (line && file)
 	{
 	  int j = 0;
 	  g_snprintf (BUFFER, SIZE, "%s", mix_src_file_get_line (file, line));
-	  while (!isspace (BUFFER[j])) j++;
+	  while ( (j < SIZE - 1) && (BUFFER[j] != '\n') && (BUFFER[j] != '\r')
+		  && (BUFFER[j] != '\f') && (BUFFER[j] != '\0') ) j++;
 	  BUFFER[j] = '\0';
 	}
-      if (strlen (BUFFER) == 0)	g_snprintf (BUFFER, SIZE, "%d", address);
+      if ( (strlen (BUFFER) == 0) || !(strcmp(NULLSTR,BUFFER)) )
+	g_snprintf (BUFFER, SIZE, "%d", address);
       fprintf (dis->out, "#%d\t%s\tin %s%s:%d\n", k, BUFFER, name,
 	       MIX_SRC_DEFEXT, line);
       ++k;
-      add = add->next;
     }
   return TRUE;
 }
 
 gboolean
+cmd_sbt_ (mix_vm_cmd_dispatcher_t *dis, const gchar *arg)
+{
+  gint no = atoi (arg);
+  GQueue *add = mix_vm_get_backtrace (dis->vm);
+
+  if ( strlen(arg) == 0 ) {
+    log_message_ (dis, "Backtrace limit is %d instructions",
+		  mix_vm_get_max_trace(dis->vm));
+  } else {
+    if ( no >= -1 ) {
+      log_message_ (dis, "Backtrace limit changed from %d to %d instructions",
+		    mix_vm_get_max_trace(dis->vm), no);
+      mix_vm_set_max_trace (dis->vm, no);
+
+      /* We might have shrunk the queue and need to resize it */
+      if ( no >= 0 ) {
+	while ( g_queue_get_length (add) > no ) {
+	  g_queue_pop_tail(add);
+	}
+      }
+
+    } else {
+      log_error_ (dis, "Error: %d is not a valid number of instructions.", no);
+    }
+  }
+  return TRUE;
+}
+
+gboolean
 cmd_slog_ (mix_vm_cmd_dispatcher_t *dis, const gchar *arg)
 {
   static const gchar *ON = "on";
diff --git a/mixlib/xmix_vm_handlers.h b/mixlib/xmix_vm_handlers.h
index decdd24..be276d4 100644
--- a/mixlib/xmix_vm_handlers.h
+++ b/mixlib/xmix_vm_handlers.h
@@ -71,6 +71,7 @@ DEC_FUN (cbpm_);
 DEC_FUN (cbpc_);
 DEC_FUN (cbpo_);
 DEC_FUN (pbt_);
+DEC_FUN (sbt_);
 DEC_FUN (slog_);
 DEC_FUN (pprog_);
 DEC_FUN (psrc_);
diff --git a/samples/coin-opt.mixal b/samples/coin-opt.mixal
new file mode 100644
index 0000000..6d86d45
--- /dev/null
+++ b/samples/coin-opt.mixal
@@ -0,0 +1,114 @@
+* Count the number of coin combinations
+
+TERM	EQU	19		Terminal uses 14 words or 70 char
+VAL	EQU	4:5		The value of a coin
+AMT	EQU	1:3		The current amount of those coins
+
+* Start of the program.
+	ORIG	1000
+PROG	OUT	PRMPT(TERM)
+* Get the value we want combos for
+        IN	INPT1(TERM)
+	LDA	INPT1		THE NUMBER IS PROBABLY IN THIS WORD
+	LDX	INPT2		WE LOAD 10 CHARS OF INPUT
+	JAP	1F		MAKE SURE THEY ENTERED SOMETHING
+	JXZ	PROG		MAYBE THEY HAD SPACES FIRST?
+1H	CMPX	PRMPT(5:5)	IS THE LAST CHAR A SPACE
+	JNE	3F		IF NOT THEN WE CAN MOVE ON
+	SRAX	1		IF IT IS, SHIFT RIGHT 1 SPOT AND
+	JMP	1B		LOOP BACK TO MAKE SURE
+3H	NUM
+	CMPA	MAX
+	JG	PROG
+	STZ	MAX
+	STA	WANTED
+
+* Get a current value of the coins we have
+CURRVAL	LD6	TYPES		How many kinds of coins
+	LDA	COINS,6(AMT)	Load current amount of first coin
+2H	DEC6	1		Move to next coin
+	ADD	COINS,6(AMT)	Add the amount of the next coin
+	J6NZ	2B		Keep going for all combos
+
+* If less than goal, add a coin
+        CMPA    WANTED		Compare the current value to wanted value
+	JGE	1F		If it's < wanted
+	LDA	COINS(AMT)	{
+	ADD	COINS(VAL)	  add another of the first coin
+	STA	COINS(AMT)        and go back to CURRVAL
+	JMP	CURRVAL		}
+1H	JG	3F		If it's = wanted
+
+* It was equal, print, and the fall through
+	LDX	MAX
+	INCX	1      	        We use this for counting the hits
+	LD6	TYPES		We iterate through the types
+	ENT5	0		This points to the word in PBUFF
+	STX	MAX
+	LDA	MAX
+	CHAR
+	STX	PBUF,5		Print the current number of hits
+1H	INC5	2		Move to the next column
+	ENTA	0
+	LDX	COINS,6(AMT)	Get the number of coins by dividing the amount
+	DIV	COINS,6(VAL)	by the value of 1 coin
+6H	CHAR
+	STX	PBUF,5(3:5)	Print the current amount
+	DEC6	1		Go to the next coin type
+	J6NN	1B		If we go negative, we're done with coins
+	OUT	PBUF(TERM)	Output the line to the console
+
+* Equal or Greater, find first non-zero spot, add one to the next
+* spot up, and then zero it out.
+3H	ENT5	0
+5H	LDA	COINS,5(AMT)	Load current coin value
+	INC5	1		Go to next type of coin
+	CMP5	TYPES		Compare to number of coin types
+	JG	DONE		If greater, we're out of coins
+	JAZ	5B		If current coin value is 0, go to next coin
+	LDA	COINS,5(AMT)	Load next coin up
+	ADD	COINS,5(VAL)	Add it's value to the amount
+	STA	COINS,5(AMT)	Save it
+	DEC5	1		Go back to previous type
+	STZ	COINS,5(AMT)	Zero it out
+	JMP	CURRVAL		Go back to the main loop
+DONE	HLT
+* Tables to keep the values and counts
+* The below can be used to test for 1000 (1:2), 100 (3:4), and 10 (5:5)
+MAX	CON	499	   	MAX AMOUNT OF CENTS BEFORE COUNTER RESETS IS 221
+WANTED	CON	62		THE COMBINATION WE WANT
+TYPES	CON	5		NUMBER OF TYPES - 1
+COINS	CON	1		PENNIES
+	CON	5		NICKLES
+	CON	10		DIMES
+	CON	25		QUARTERS
+	CON	50		HALF-DOLLARS
+	CON	100		DOLLARS
+PRMPT	ALF	"WHAT "
+	ALF	"AMOUN"
+	ALF	"T <IN"
+	ALF	" CENT"
+	ALF	"S> DO"
+	ALF	" YOU "
+	ALF	"WANT "
+	ALF	"MAX 2"
+	ALF	"20:  "
+	ORIG	PRMPT+14
+INPT1	CON	0
+INPT2	CON	0
+* This will be our printline
+	ORIG	PRMPT+28
+PBUF	CON	0		COUNT
+	ALF	"  DLR"
+	ALF	":    "
+	ALF	"  HLF"
+	ALF	":    "
+	ALF	"  QTR"
+	ALF	":    "
+	ALF	"  DMS"
+	ALF	":    "
+	ALF	" NCKL"
+	ALF	":    "
+	ALF	"  PNY"
+	ALF	":    "
+	END	PROG