1 files changed, 133 insertions, 0 deletions
diff --git a/Objects/codeobject.c b/Objects/codeobject.c
index f832911..8ae2399 100644
--- a/Objects/codeobject.c
+++ b/Objects/codeobject.c
@@ -451,3 +451,136 @@ PyCode_Addr2Line(PyCodeObject *co, int addrq)
 	}
 	return line;
 }
+
+/* 
+   Check whether the current instruction is at the start of a line.
+
+ */
+
+	/* The theory of SET_LINENO-less tracing.
+
+	   In a nutshell, we use the co_lnotab field of the code object
+	   to tell when execution has moved onto a different line.
+
+	   As mentioned above, the basic idea is so set things up so
+	   that
+
+	         *instr_lb <= frame->f_lasti < *instr_ub
+
+	   is true so long as execution does not change lines.
+
+	   This is all fairly simple.  Digging the information out of
+	   co_lnotab takes some work, but is conceptually clear.
+
+	   Somewhat harder to explain is why we don't *always* call the
+	   line trace function when the above test fails.
+
+	   Consider this code:
+
+	   1: def f(a):
+	   2:     if a:
+	   3:        print 1
+	   4:     else:
+	   5:        print 2
+
+	   which compiles to this:
+
+	   2           0 LOAD_FAST                0 (a)
+		       3 JUMP_IF_FALSE            9 (to 15)
+		       6 POP_TOP
+
+	   3           7 LOAD_CONST               1 (1)
+		      10 PRINT_ITEM
+		      11 PRINT_NEWLINE
+		      12 JUMP_FORWARD             6 (to 21)
+		 >>   15 POP_TOP
+
+	   5          16 LOAD_CONST               2 (2)
+		      19 PRINT_ITEM
+		      20 PRINT_NEWLINE
+		 >>   21 LOAD_CONST               0 (None)
+		      24 RETURN_VALUE
+
+	   If 'a' is false, execution will jump to instruction at offset
+	   15 and the co_lnotab will claim that execution has moved to
+	   line 3.  This is at best misleading.  In this case we could
+	   associate the POP_TOP with line 4, but that doesn't make
+	   sense in all cases (I think).
+
+	   What we do is only call the line trace function if the co_lnotab
+	   indicates we have jumped to the *start* of a line, i.e. if the
+	   current instruction offset matches the offset given for the
+	   start of a line by the co_lnotab.
+
+	   This also takes care of the situation where 'a' is true.
+	   Execution will jump from instruction offset 12 to offset 21.
+	   Then the co_lnotab would imply that execution has moved to line
+	   5, which is again misleading.
+
+	   Why do we set f_lineno when tracing?  Well, consider the code
+	   above when 'a' is true.  If stepping through this with 'n' in
+	   pdb, you would stop at line 1 with a "call" type event, then
+	   line events on lines 2 and 3, then a "return" type event -- but
+	   you would be shown line 5 during this event.  This is a change
+	   from the behaviour in 2.2 and before, and I've found it
+	   confusing in practice.  By setting and using f_lineno when
+	   tracing, one can report a line number different from that
+	   suggested by f_lasti on this one occasion where it's desirable.
+	*/
+
+
+int 
+PyCode_CheckLineNumber(PyCodeObject* co, int lasti, PyAddrPair *bounds)
+{
+        int size, addr, line;
+        unsigned char* p;
+
+        p = (unsigned char*)PyString_AS_STRING(co->co_lnotab);
+        size = PyString_GET_SIZE(co->co_lnotab) / 2;
+
+        addr = 0;
+        line = co->co_firstlineno;
+        assert(line > 0);
+
+        /* possible optimization: if f->f_lasti == instr_ub
+           (likely to be a common case) then we already know
+           instr_lb -- if we stored the matching value of p
+           somwhere we could skip the first while loop. */
+
+        /* see comments in compile.c for the description of
+           co_lnotab.  A point to remember: increments to p
+           should come in pairs -- although we don't care about
+           the line increments here, treating them as byte
+           increments gets confusing, to say the least. */
+
+        while (size > 0) {
+                if (addr + *p > lasti)
+                        break;
+                addr += *p++;
+                if (*p) 
+                        bounds->ap_lower = addr;
+                line += *p++;
+                --size;
+        }
+
+        /* If lasti and addr don't match exactly, we don't want to
+           change the lineno slot on the frame or execute a trace
+           function.  Return -1 instead.
+        */
+        if (addr != lasti)
+                line = -1;
+        
+        if (size > 0) {
+                while (--size >= 0) {
+                        addr += *p++;
+                        if (*p++)
+                                break;
+                }
+                bounds->ap_upper = addr;
+        }
+        else {
+                bounds->ap_upper = INT_MAX;
+        }
+
+        return line;
+}