gpgscm: Keep a history of calls for error messages.

* tests/gpgscm/init.scm (vm-history-print): New function.
* tests/gpgscm/opdefines.h: New opcodes 'CALLSTACK_POP', 'APPLY_CODE',
and 'VM_HISTORY'.
* tests/gpgscm/scheme-private.h (struct history): New definition.
(struct scheme): New field 'history'.
* tests/gpgscm/scheme.c (gc): Mark objects in the history.
(history_free): New function.
(history_init): Likewise.
(history_mark): Likewise.
(add_mod): New macro.
(sub_mod): Likewise.
(tailstack_clear): New function.
(callstack_pop): Likewise.
(callstack_push): Likewise.
(tailstack_push): Likewise.
(tailstack_flatten): Likewise.
(callstack_flatten): Likewise.
(history_flatten): Likewise.
(opexe_0): New variable 'callsite', keep track of the expression if it
is a call, implement the new opcodes, record function applications in
the history.
(opexe_6): Implement new opcode.
(scheme_init_custom_alloc): Initialize history.
(scheme_deinit): Free history.
* tests/gpgscm/scheme.h (USE_HISTORY): New macro.
--

This patch makes TinySCHEME keep a history of function calls.  This
history can be used to produce helpful error messages.  The history
data structure is inspired by MIT/GNU Scheme.

Signed-off-by: Justus Winter <justus@g10code.com>

fu history

tests/gpgscm/scheme-private.h

@@ -62,6 +62,34 @@ struct cell {
   } _object;
 };
 
+#if USE_HISTORY
+/* The history is a two-dimensional ring buffer.  A donut-shaped data
+ * structure.  This data structure is inspired by MIT/GNU Scheme.  */
+struct history {
+  /* Number of calls to store.  Must be a power of two.  */
+  size_t N;
+
+  /* Number of tail-calls to store in each call frame.  Must be a
+   * power of two.  */
+  size_t M;
+
+  /* Masks for fast index calculations.  */
+  size_t mask_N;
+  size_t mask_M;
+
+  /* A vector of size N containing calls.  */
+  pointer callstack;
+
+  /* A vector of size N containing vectors of size M containing tail
+   * calls.  */
+  pointer tailstacks;
+
+  /* Our current position.  */
+  size_t n;
+  size_t *m;
+};
+#endif
+
 struct scheme {
 /* arrays for segments */
 func_alloc malloc;
@@ -88,6 +116,11 @@ pointer envir;           /* stack register for current environment */
 pointer code;            /* register for current code */
 pointer dump;            /* stack register for next evaluation */
 
+#if USE_HISTORY
+struct history history;  /* we keep track of the call history for
+                          * error messages */
+#endif
+
 int interactive_repl;    /* are we in an interactive REPL? */
 
 struct cell _sink;