Inconsistent locations for conditional branches in `while` statements

[markshannon]

Bug report

Bug description:

Reported here: #122762 (comment)

The locations for branches in while statements are not consistent. The first iteration shows different locations from the second and later iterations.

I've not check earlier versions, but this could be an issue for 3.12 as well.

CPython versions tested on:

3.13, CPython main branch

Operating systems tested on:

No response

Linked PRs

• GH-122821: Simplify compilation of while statements to ensure consistency of offsets for sys.monitoring #122934

[markshannon]

Here's my hypothesis:

This is an artifact of compiling while cond: body as:

start:
    if not cond: goto end
    body
    if cond goto start
end:

and the assembler converting condition backwards jumps from if cond goto target to

    if not cond goto end
    goto target
end:

So we end up with:

start:
    if not cond: goto end
    body
    if not cond goto end
    goto start
end:

When cond is True, the first branch jumps from cond to body, but the second branch jumps from cond to cond.

Compiling while cond: body as:

start:
    if not cond: goto end
    body
    goto start
end:

should fix this.