SF bug 430991: wrong co_lnotab

Armin Rigo pointed out that the way the line-# table got built didn't work
for lines generating more than 255 bytes of bytecode.  Fixed as he
suggested, plus corresponding changes to pyassem.py, plus added some
long overdue docs about this subtle table to compile.c.

Bugfix candidate (line numbers may be off in tracebacks under -O).

1 files changed, 14 insertions, 17 deletions

diff --git a/Lib/compiler/pyassem.py b/Lib/compiler/pyassem.py
index e8810ae..0684623 100644
--- a/Lib/compiler/pyassem.py
+++ b/Lib/compiler/pyassem.py
@@ -613,16 +613,16 @@ def twobyte(val):
 class LineAddrTable:
     """lnotab
     
-    This class builds the lnotab, which is undocumented but described
-    by com_set_lineno in compile.c.  Here's an attempt at explanation:
+    This class builds the lnotab, which is documented in compile.c.
+    Here's a brief recap:
 
     For each SET_LINENO instruction after the first one, two bytes are
     added to lnotab.  (In some cases, multiple two-byte entries are
     added.)  The first byte is the distance in bytes between the
     instruction for the last SET_LINENO and the current SET_LINENO.
     The second byte is offset in line numbers.  If either offset is
-    greater than 255, multiple two-byte entries are added -- one entry
-    for each factor of 255.
+    greater than 255, multiple two-byte entries are added -- see
+    compile.c for the delicate details.
     """
 
     def __init__(self):
@@ -657,19 +657,16 @@ class LineAddrTable:
             # compiler because it only generates a SET_LINENO instruction
             # for the assignment.
             if line > 0:
-                while addr > 0 or line > 0:
-                    # write the values in 1-byte chunks that sum
-                    # to desired value
-                    trunc_addr = addr
-                    trunc_line = line
-                    if trunc_addr > 255:
-                        trunc_addr = 255
-                    if trunc_line > 255:
-                        trunc_line = 255
-                    self.lnotab.append(trunc_addr)
-                    self.lnotab.append(trunc_line)
-                    addr = addr - trunc_addr
-                    line = line - trunc_line
+                push = self.lnotab.append
+                while addr > 255:
+                    push(255); push(0)
+                    addr -= 255
+                while line > 255:
+                    push(addr); push(255)
+                    line -= 255
+                    addr = 0
+                if addr > 0 or line > 0:
+                    push(addr); push(line)
                 self.lastline = lineno
                 self.lastoff = self.codeOffset