dis: FOR_ITER reports wrong exhausted-path jump target (END_FOR instead of POP_ITER)

Bug Report

Bug description:

In Python 3.15, dis reports the wrong jump target for FOR_ITER when the iterator is exhausted. The reported target lands on END_FOR, but the CPython runtime actually jumps one instruction past it to POP_ITER.

Root cause: In bytecodes.c, FOR_ITER uses JUMPBY(oparg + 1) to skip past END_FOR. However, dis computes the jump target as NIP + oparg * 2 bytes (without the +1), landing on END_FOR rather than POP_ITER.

import dis

def f():
    for x in [1, 2, 3]:
        pass

instrs = {i.offset: i for i in dis.get_instructions(f.__code__, show_caches=True)}

for i in dis.get_instructions(f.__code__):
    if i.opname == "FOR_ITER":
        for_iter = i

# dis reports this as the jump target:
reported_target = for_iter.jump_target
reported_name = instrs[reported_target].opname  # "END_FOR"

# CPython runtime actually jumps here (oparg+1 instructions past NIP):
nip = for_iter.offset + 4  # 4 bytes = FOR_ITER word + CACHE word
actual_target = nip + (for_iter.arg + 1) * 2
actual_name = instrs[actual_target].opname  # "POP_ITER"

print(f"dis reports:    offset {reported_target} = {reported_name}")   # END_FOR
print(f"runtime jumps:  offset {actual_target}   = {actual_name}")     # POP_ITER

Output:

dis reports:    offset 20 = END_FOR
runtime jumps:  offset 22 = POP_ITER

The stack-effect model remains self-consistent (FOR_ITER +1, END_FOR -1, POP_ITER -2), but the reported jump target is misleading to any tool that builds a control-flow graph from dis output. The exhausted-iterator edge should point to POP_ITER, not END_FOR.

CPython versions tested on:

CPython main branch (3.15)

Operating systems tested on:

macOS

Linked PRs

• gh-149503

Bắt đầu với việc triển khai FOR_ITER trong bytecodes.c và điểm vào dis tính toán các đích nhảy. So sánh đích của iterator đã cạn được mô tả trong báo cáo với hành vi khi chạy, sau đó bổ sung độ bao phủ hồi quy cho thấy đích là POP_ITER chứ không phải END_FOR.