From e5734bef9074fa8b1c80c35aa9bf528e31d966a4 Mon Sep 17 00:00:00 2001
From: rocky
Date: Mon, 27 Apr 2020 15:14:12 -0400
Subject: Experiment giving bytecode in traceback...

This commit only changes the behavior when `(cdr)` when it is not
given a `cons` node, in order to give some quick idea of how adding
more traceback information might work.

Here's how to see/use. Build this code.

Byte-compile this buggy function in `/tmp/foo.el`
with (byte-compile-file)
```lisp
(defun foo()
  (setq x 5)
  (cdr 'b)
  )
```

```
(load-file "/tmp/foo.elc")
(foo)
```

You should see:

```
Debugger entered--Lisp error: (wrong-type-argument listp b 3)
                                        this is the offset ^
  foo()
  eval((foo) nil)
  elisp--eval-last-sexp(nil)
  eval-last-sexp(nil)
  funcall-interactively(eval-last-sexp nil)
  call-interactively(eval-last-sexp nil nil)
  command-execute(eval-last-sexp)
```

Compare against disassembly:

```
byte code for foo:
  args: nil
0	constant  5
1	varset	  x
2	constant  b
3	cdr
^^^ offset from above
4	return
```

You can try with other offsets such as by removing the `(setq x 5)`
and you'll see offset 1 instead.

Right now, we just pass to `signal` bytecode offset. More elaborate would be
to pass the code object and its offset. Even more elaborate schemes
could be imagined.
---
 src/bytecode.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

(limited to 'src/bytecode.c')

diff --git a/src/bytecode.c b/src/bytecode.c
index 3c90544f3f2..8ef84682035 100644
--- a/src/bytecode.c
+++ b/src/bytecode.c
@@ -286,12 +286,13 @@ enum byte_code_op
 
 /* Fetch the next byte from the bytecode stream.  */
 
-#define FETCH (*pc++)
+#define FETCH (last_pc = pc, *pc++)
+#define FETCH_NORECORD (*pc++)
 
 /* Fetch two bytes from the bytecode stream and make a 16-bit number
    out of them.  */
 
-#define FETCH2 (op = FETCH, op + (FETCH << 8))
+#define FETCH2 (op = FETCH, op + (FETCH_NORECORD << 8))
 
 /* Push X onto the execution stack.  The expression X should not
    contain TOP, to avoid competing side effects.  */
@@ -375,6 +376,7 @@ exec_byte_code (Lisp_Object bytestr, Lisp_Object vector, Lisp_Object maxdepth,
   bytestr_data = ptr_bounds_clip (bytestr_data + item_bytes, bytestr_length);
   memcpy (bytestr_data, SDATA (bytestr), bytestr_length);
   unsigned char const *pc = bytestr_data;
+  unsigned char const *last_pc = pc;
   ptrdiff_t count = SPECPDL_INDEX ();
 
   if (!NILP (args_template))
@@ -535,7 +537,7 @@ exec_byte_code (Lisp_Object bytestr, Lisp_Object vector, Lisp_Object maxdepth,
 	    if (CONSP (TOP))
 	      TOP = XCDR (TOP);
 	    else if (!NILP (TOP))
-	      wrong_type_argument (Qlistp, TOP);
+	      wrong_type_argument_new (Qlistp, TOP, last_pc - bytestr_data);
 	    NEXT;
 	  }
 
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