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138 lines
5.9 KiB
138 lines
5.9 KiB
All about co_lnotab, the line number table.
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Code objects store a field named co_lnotab. This is an array of unsigned bytes
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disguised as a Python bytes object. It is used to map bytecode offsets to
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source code line #s for tracebacks and to identify line number boundaries for
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line tracing. Because of internals of the peephole optimizer, it's possible
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for lnotab to contain bytecode offsets that are no longer valid (for example
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if the optimizer removed the last line in a function).
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The array is conceptually a compressed list of
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(bytecode offset increment, line number increment)
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pairs. The details are important and delicate, best illustrated by example:
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byte code offset source code line number
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0 1
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6 2
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50 7
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350 207
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361 208
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Instead of storing these numbers literally, we compress the list by storing only
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the difference from one row to the next. Conceptually, the stored list might
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look like:
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0, 1, 6, 1, 44, 5, 300, 200, 11, 1
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The above doesn't really work, but it's a start. An unsigned byte (byte code
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offset) can't hold negative values, or values larger than 255, a signed byte
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(line number) can't hold values larger than 127 or less than -128, and the
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above example contains two such values. (Note that before 3.6, line number
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was also encoded by an unsigned byte.) So we make two tweaks:
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(a) there's a deep assumption that byte code offsets increase monotonically,
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and
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(b) if byte code offset jumps by more than 255 from one row to the next, or if
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source code line number jumps by more than 127 or less than -128 from one row
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to the next, more than one pair is written to the table. In case #b,
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there's no way to know from looking at the table later how many were written.
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That's the delicate part. A user of co_lnotab desiring to find the source
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line number corresponding to a bytecode address A should do something like
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this:
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lineno = addr = 0
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for addr_incr, line_incr in co_lnotab:
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addr += addr_incr
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if addr > A:
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return lineno
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if line_incr >= 0x80:
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line_incr -= 0x100
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lineno += line_incr
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(In C, this is implemented by PyCode_Addr2Line().) In order for this to work,
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when the addr field increments by more than 255, the line # increment in each
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pair generated must be 0 until the remaining addr increment is < 256. So, in
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the example above, assemble_lnotab in compile.c should not (as was actually done
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until 2.2) expand 300, 200 to
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255, 255, 45, 45,
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but to
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255, 0, 45, 127, 0, 73.
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The above is sufficient to reconstruct line numbers for tracebacks, but not for
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line tracing. Tracing is handled by PyCode_CheckLineNumber() in codeobject.c
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and maybe_call_line_trace() in ceval.c.
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*** Tracing ***
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To a first approximation, we want to call the tracing function when the line
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number of the current instruction changes. Re-computing the current line for
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every instruction is a little slow, though, so each time we compute the line
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number we save the bytecode indices where it's valid:
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*instr_lb <= frame->f_lasti < *instr_ub
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is true so long as execution does not change lines. That is, *instr_lb holds
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the first bytecode index of the current line, and *instr_ub holds the first
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bytecode index of the next line. As long as the above expression is true,
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maybe_call_line_trace() does not need to call PyCode_CheckLineNumber(). Note
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that the same line may appear multiple times in the lnotab, either because the
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bytecode jumped more than 255 indices between line number changes or because
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the compiler inserted the same line twice. Even in that case, *instr_ub holds
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the first index of the next line.
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However, we don't *always* want to call the line trace function when the above
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test fails.
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Consider this code:
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1: def f(a):
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2: while a:
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3: print(1)
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4: break
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5: else:
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6: print(2)
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which compiles to this:
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2 0 SETUP_LOOP 26 (to 28)
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>> 2 LOAD_FAST 0 (a)
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4 POP_JUMP_IF_FALSE 18
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3 6 LOAD_GLOBAL 0 (print)
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8 LOAD_CONST 1 (1)
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10 CALL_FUNCTION 1
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12 POP_TOP
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4 14 BREAK_LOOP
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16 JUMP_ABSOLUTE 2
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>> 18 POP_BLOCK
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6 20 LOAD_GLOBAL 0 (print)
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22 LOAD_CONST 2 (2)
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24 CALL_FUNCTION 1
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26 POP_TOP
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>> 28 LOAD_CONST 0 (None)
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30 RETURN_VALUE
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If 'a' is false, execution will jump to the POP_BLOCK instruction at offset 18
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and the co_lnotab will claim that execution has moved to line 4, which is wrong.
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In this case, we could instead associate the POP_BLOCK with line 5, but that
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would break jumps around loops without else clauses.
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We fix this by only calling the line trace function for a forward jump if the
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co_lnotab indicates we have jumped to the *start* of a line, i.e. if the current
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instruction offset matches the offset given for the start of a line by the
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co_lnotab. For backward jumps, however, we always call the line trace function,
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which lets a debugger stop on every evaluation of a loop guard (which usually
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won't be the first opcode in a line).
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Why do we set f_lineno when tracing, and only just before calling the trace
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function? Well, consider the code above when 'a' is true. If stepping through
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this with 'n' in pdb, you would stop at line 1 with a "call" type event, then
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line events on lines 2, 3, and 4, then a "return" type event -- but because the
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code for the return actually falls in the range of the "line 6" opcodes, you
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would be shown line 6 during this event. This is a change from the behaviour in
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2.2 and before, and I've found it confusing in practice. By setting and using
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f_lineno when tracing, one can report a line number different from that
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suggested by f_lasti on this one occasion where it's desirable.
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