mirror of
https://github.com/python/cpython.git
synced 2024-11-21 21:09:37 +01:00
229 lines
8.7 KiB
Plaintext
229 lines
8.7 KiB
Plaintext
Description of the internal format of the line number table in Python 3.10
|
|
and earlier.
|
|
|
|
(For 3.11 onwards, see Objects/locations.md)
|
|
|
|
Conceptually, the line number table consists of a sequence of triples:
|
|
start-offset (inclusive), end-offset (exclusive), line-number.
|
|
|
|
Note that not all byte codes have a line number so we need handle `None` for the line-number.
|
|
|
|
However, storing the above sequence directly would be very inefficient as we would need 12 bytes per entry.
|
|
|
|
First, note that the end of one entry is the same as the start of the next, so we can overlap entries.
|
|
Second, we don't really need arbitrary access to the sequence, so we can store deltas.
|
|
|
|
We just need to store (end - start, line delta) pairs. The start offset of the first entry is always zero.
|
|
|
|
Third, most deltas are small, so we can use a single byte for each value, as long we allow several entries for the same line.
|
|
|
|
Consider the following table
|
|
Start End Line
|
|
0 6 1
|
|
6 50 2
|
|
50 350 7
|
|
350 360 No line number
|
|
360 376 8
|
|
376 380 208
|
|
|
|
Stripping the redundant ends gives:
|
|
|
|
End-Start Line-delta
|
|
6 +1
|
|
44 +1
|
|
300 +5
|
|
10 No line number
|
|
16 +1
|
|
4 +200
|
|
|
|
|
|
Note that the end - start value is always positive.
|
|
|
|
Finally, in order to fit into a single byte we need to convert start deltas to the range 0 <= delta <= 254,
|
|
and line deltas to the range -127 <= delta <= 127.
|
|
A line delta of -128 is used to indicate no line number.
|
|
Also note that a delta of zero indicates that there are no bytecodes in the given range,
|
|
which means we can use an invalid line number for that range.
|
|
|
|
Final form:
|
|
|
|
Start delta Line delta
|
|
6 +1
|
|
44 +1
|
|
254 +5
|
|
46 0
|
|
10 -128 (No line number, treated as a delta of zero)
|
|
16 +1
|
|
0 +127 (line 135, but the range is empty as no bytecodes are at line 135)
|
|
4 +73
|
|
|
|
Iterating over the table.
|
|
-------------------------
|
|
|
|
For the `co_lines` method we want to emit the full form, omitting the (350, 360, No line number) and empty entries.
|
|
|
|
The code is as follows:
|
|
|
|
def co_lines(code):
|
|
line = code.co_firstlineno
|
|
end = 0
|
|
table_iter = iter(code.internal_line_table):
|
|
for sdelta, ldelta in table_iter:
|
|
if ldelta == 0: # No change to line number, just accumulate changes to end
|
|
end += sdelta
|
|
continue
|
|
start = end
|
|
end = start + sdelta
|
|
if ldelta == -128: # No valid line number -- skip entry
|
|
continue
|
|
line += ldelta
|
|
if end == start: # Empty range, omit.
|
|
continue
|
|
yield start, end, line
|
|
|
|
|
|
|
|
|
|
The historical co_lnotab format
|
|
-------------------------------
|
|
|
|
prior to 3.10 code objects stored a field named co_lnotab.
|
|
This was an array of unsigned bytes disguised as a Python bytes object.
|
|
|
|
The old co_lnotab did not account for the presence of bytecodes without a line number,
|
|
nor was it well suited to tracing as a number of workarounds were required.
|
|
|
|
The old format can still be accessed via `code.co_lnotab`, which is lazily computed from the new format.
|
|
|
|
Below is the description of the old co_lnotab format:
|
|
|
|
|
|
The array is conceptually a compressed list of
|
|
(bytecode offset increment, line number increment)
|
|
pairs. The details are important and delicate, best illustrated by example:
|
|
|
|
byte code offset source code line number
|
|
0 1
|
|
6 2
|
|
50 7
|
|
350 207
|
|
361 208
|
|
|
|
Instead of storing these numbers literally, we compress the list by storing only
|
|
the difference from one row to the next. Conceptually, the stored list might
|
|
look like:
|
|
|
|
0, 1, 6, 1, 44, 5, 300, 200, 11, 1
|
|
|
|
The above doesn't really work, but it's a start. An unsigned byte (byte code
|
|
offset) can't hold negative values, or values larger than 255, a signed byte
|
|
(line number) can't hold values larger than 127 or less than -128, and the
|
|
above example contains two such values. (Note that before 3.6, line number
|
|
was also encoded by an unsigned byte.) So we make two tweaks:
|
|
|
|
(a) there's a deep assumption that byte code offsets increase monotonically,
|
|
and
|
|
(b) if byte code offset jumps by more than 255 from one row to the next, or if
|
|
source code line number jumps by more than 127 or less than -128 from one row
|
|
to the next, more than one pair is written to the table. In case #b,
|
|
there's no way to know from looking at the table later how many were written.
|
|
That's the delicate part. A user of co_lnotab desiring to find the source
|
|
line number corresponding to a bytecode address A should do something like
|
|
this:
|
|
|
|
lineno = addr = 0
|
|
for addr_incr, line_incr in co_lnotab:
|
|
addr += addr_incr
|
|
if addr > A:
|
|
return lineno
|
|
if line_incr >= 0x80:
|
|
line_incr -= 0x100
|
|
lineno += line_incr
|
|
|
|
(In C, this is implemented by PyCode_Addr2Line().) In order for this to work,
|
|
when the addr field increments by more than 255, the line # increment in each
|
|
pair generated must be 0 until the remaining addr increment is < 256. So, in
|
|
the example above, assemble_lnotab in compile.c should not (as was actually done
|
|
until 2.2) expand 300, 200 to
|
|
255, 255, 45, 45,
|
|
but to
|
|
255, 0, 45, 127, 0, 73.
|
|
|
|
The above is sufficient to reconstruct line numbers for tracebacks, but not for
|
|
line tracing. Tracing is handled by PyCode_CheckLineNumber() in codeobject.c
|
|
and maybe_call_line_trace() in ceval.c.
|
|
|
|
*** Tracing ***
|
|
|
|
To a first approximation, we want to call the tracing function when the line
|
|
number of the current instruction changes. Re-computing the current line for
|
|
every instruction is a little slow, though, so each time we compute the line
|
|
number we save the bytecode indices where it's valid:
|
|
|
|
*instr_lb <= frame->f_lasti < *instr_ub
|
|
|
|
is true so long as execution does not change lines. That is, *instr_lb holds
|
|
the first bytecode index of the current line, and *instr_ub holds the first
|
|
bytecode index of the next line. As long as the above expression is true,
|
|
maybe_call_line_trace() does not need to call PyCode_CheckLineNumber(). Note
|
|
that the same line may appear multiple times in the lnotab, either because the
|
|
bytecode jumped more than 255 indices between line number changes or because
|
|
the compiler inserted the same line twice. Even in that case, *instr_ub holds
|
|
the first index of the next line.
|
|
|
|
However, we don't *always* want to call the line trace function when the above
|
|
test fails.
|
|
|
|
Consider this code:
|
|
|
|
1: def f(a):
|
|
2: while a:
|
|
3: print(1)
|
|
4: break
|
|
5: else:
|
|
6: print(2)
|
|
|
|
which compiles to this:
|
|
|
|
2 0 SETUP_LOOP 26 (to 28)
|
|
>> 2 LOAD_FAST 0 (a)
|
|
4 POP_JUMP_IF_FALSE 18
|
|
|
|
3 6 LOAD_GLOBAL 0 (print)
|
|
8 LOAD_CONST 1 (1)
|
|
10 CALL_NO_KW 1
|
|
12 POP_TOP
|
|
|
|
4 14 BREAK_LOOP
|
|
16 JUMP_ABSOLUTE 2
|
|
>> 18 POP_BLOCK
|
|
|
|
6 20 LOAD_GLOBAL 0 (print)
|
|
22 LOAD_CONST 2 (2)
|
|
24 CALL_NO_KW 1
|
|
26 POP_TOP
|
|
>> 28 LOAD_CONST 0 (None)
|
|
30 RETURN_VALUE
|
|
|
|
If 'a' is false, execution will jump to the POP_BLOCK instruction at offset 18
|
|
and the co_lnotab will claim that execution has moved to line 4, which is wrong.
|
|
In this case, we could instead associate the POP_BLOCK with line 5, but that
|
|
would break jumps around loops without else clauses.
|
|
|
|
We fix this by only calling the line trace function for a forward jump 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. For backward jumps, however, we always call the line trace function,
|
|
which lets a debugger stop on every evaluation of a loop guard (which usually
|
|
won't be the first opcode in a line).
|
|
|
|
Why do we set f_lineno when tracing, and only just before calling the trace
|
|
function? 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, 3, and 4, then a "return" type event -- but because the
|
|
code for the return actually falls in the range of the "line 6" opcodes, you
|
|
would be shown line 6 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.
|