Hi Skip,
I modified co_lnotab format when I worked on the FAT Python
optimization project which modified the AST to emit more efficient
bytecode. My optimizer moved instructions, the line number could
become non-monotonic (..., line 2, line 3, line 2, line 4, ...), and
so lnotab (..., +1, -1, +2, ...) could be negative.
Example with loop unrolling:
def func1():
for i in ("a", "b"):
print(i)
is optimized:
def func2():
i = "a" # line 1
print(i) # line 2 (+1)
i = "b" # line 1 (-1)
print(i) # line 2 (+1)
I'm happy to see that Python 3.10 now also implements faster bytecode
which rely on this change ;-)
It seems like loop unrolling is not implemented in Python 3.10 yet. I
guess that Python/ast_opt.c has no strategy yet to decide if an
optimization should be applied or not depending on the code size. Loop
unrolling can make code way bigger.
Victor
On Thu, Mar 18, 2021 at 12:12 AM Ned Batchelder <n...@nedbatchelder.com> wrote:
>
> On 3/17/21 6:41 PM, MRAB wrote:
> > On 2021-03-17 22:10, Skip Montanaro wrote:
> >
> >> I stumbled on this while trying to generate a line number table in my
> >> side project register VM. As I understand it, the line number delta
> >> in the output table is supposed to always be >= 0. In my code I'm
> >> using dis.findlinestarts() to determine the line numbers for each
> >> block. Perhaps I should be modifying its results. OTOH, maybe it's a
> >> bug. (If that's the consensus, I will create an issue.)
>
> co_lnotab has had negative deltas since 3.6.
>
> --Ned.
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