> On 3 Nov 2025, at 05:53, Iain Sandoe <[email protected]> wrote:
>> On 3 Nov 2025, at 04:18, Jason Merrill via Gcc <[email protected]> wrote:
>> On Mon, Nov 3, 2025 at 2:26 PM Adrian Vogelsgesang via Gcc <[email protected]>
>> wrote:
>>
>>> And I just realized that I inadvertently dropped the ball by replying
>>> only to Sandoe, and forgot to keep the list in CC.
>>>
>>> Thanks for the quick response, Iain!
>>> Inline below my (original, by now pretty late) replies
>>>
>>>>> There are two reasons the script doesn't work for g++:
>>>>> 1. g++ does not emit a `__promise` variable inside the destroy
>>>>> function - that can be worked around by changing the script, though
>>>>
>>>> g++ accesses via the frame pointer - the following entries should be
>>>> available to the debugger.
>>>>
>>>> frame_pointer->_Coro_promise
>>>> frame_pointer->_Coro_resume_index
>>>
>>> Yes, that can be easily fixed in the script - I just didn't implement it,
>>> yet.
>>>
>>>>> 2. g++ provides no way to map the compiler-assigned suspension point
>>>>> IDs back to line numbers - i.e., the topic of this email
>>>>
>>>> g++ retains the location information for user-authored code (so that
>>> setting
>>>> breakpoints on line nunber etc. should work.)
>>>
>>> yes, and that works great! However, that is besides my point / solving a
>>> different problem.
>>>
>>> In my case, I have a suspended `std::coroutine_handle` which I want to
>>> pretty-print. From that coroutine_handle, I can get
>>> 1. a pointer to the destroy function's entry point (e.g., 0x7f1345)
>>> 2. the resume index (e.g., 15)
>>>
>>> I now need a way to map `0x7f1345` and `idx 15` back to its location.
>>> I.e., I need to answer "the suspension point with id 15 within the
>>> coroutine whose destroy function starts at address 0x7f1345 is located
>>> in foobar.cpp at line 65".
>>>
>>> Note that I cannot simply lookup 0x7f1345 in the line table, since that
>>> would give me the start of the coroutine function, not the position of
>>> suspension point 15 inside the function's body.
>>>
>>> In clang, I solved this by:
>>> 1. lookup the scope of the destroy function:
>>> destroy_func = gdb.block_for_pc(int(self.destroy_ptr))
>>> 2. lookup the label for suspension point 15 within that function
>>> label_name = f"__coro_resume_{suspension_point_index}"
>>> resume_label = gdb.lookup_symbol(label_name, self.resume_func,
>>> gdb.SYMBOL_LABEL_DOMAIN)[0]
>>> 3. look at that label's line/column
>>> print(f"suspended at line {resume_label.line}")
>>>
>>> For gcc-compiled code
>>> 1. step 1 also works
>>> 2. step 2 works with a small adjustment to the label name
>>> 3. step 3 does not work, since the labels produced by gcc have
>>> neither location information nor a DW_AT_low_pc which I could look
>>> up in the line table.
>>>
>>>> However, for synthetic code (e.g. the ramp and the expansion of the
>>>> co_await expressions) so far, we have intentionally generated the code
>>>> with “unknown” locations. This (absent the kind of process you are
>>>> mentioning) tends to impove the debug experience - because it avoids
>>>> the apparent location jumping around.
>>>
>>> That's great! clang unfortunately does emit debug info for this syntetic
>>> code, and hence single stepping into / out of a clang-compiled coroutine
>>> is a bit clunky
>>> On Fri, Oct 10, 2025 at 8:19 AM Iain Sandoe <[email protected]>
>>> wrote:
>>>>
>>>>
>>>>
>>>>> On 10 Oct 2025, at 03:16, Adrian Vogelsgesang via Gcc <[email protected]>
>>> wrote:
>>>>>
>>>>> Hi gcc-devs!
>>>>>
>>>>> TLDR: For debugging C++ coroutines, I patched clang to emit artificial
>>>>> DW_TAG_labels, mapping suspension point ids to the corresponding
>>>>> source location. Looking for alignment re debugging info between clang
>>>>> and gcc.
>>>>>
>>>>> (Finally following up on Iain Sandoe's request to send this email)
>>>>
>>>> Thanks, there are others here much better able to comment on debug info
>>>> than I am. I’ve added a couple of notes below but hope that others will
>>>> chime in with opinions on how to proceed.
>>>>
>>>>> ~~~~~~~~~
>>>>> Background
>>>>> ~~~~~~~~~
>>>>>
>>>>> When using coroutines for asynchronous programming, the physical stack
>>>>> only tells part of the truth. One also wants to see the chain of
>>>>> "awaiting" coroutines, i.e. the coroutines which initiated the current
>>>>> coroutine frame and are waiting for its completion.
>>>>>
>>>>> I published a gdb debugger script which provides a FrameFilter to
>>>>> inject those async stack frames. With this script, the `bt` command
>>>>> now returns
>>>>>
>>>>>> #0 write_output(...) at ...
>>>>>> [async] greet() at ...
>>>>>> [async] [noop_coroutine] at ...
>>>>>> #1 coroutine_handle<task::promise_type>::resume() const at ...
>>>>>> #2 task::syncStart() at ...
>>>>>
>>>>> However, this script currently doesn't work for gcc-compiled binaries,
>>>>> yet, due to missing debug information.
>>>>>
>>>>> ~~~~~~~~~
>>>>> Current state of gcc-generated debug info
>>>>> ~~~~~~~~~
>>>>>
>>>>> There are two reasons the script doesn't work for g++:
>>>>> 1. g++ does not emit a `__promise` variable inside the destroy
>>>>> function - that can be worked around by changing the script, though
>>>>
>>>> g++ accesses via the frame pointer - the following entries should be
>>>> available to the debugger.
>>>>
>>>> frame_pointer->_Coro_promise
>>>> frame_pointer->_Coro_resume_index
>>>>
>>>> The resume index is updated as we pass the test for the awaiter being
>>>> ready - so that it should be correct whether the coroutine suspends or
>>>> continues.
>>>>
>>>>> 2. g++ provides no way to map the compiler-assigned suspension point
>>>>> IDs back to line numbers - i.e., the topic of this email
>>>>
>>>> g++ retains the location information for user-authored code (so that
>>> setting
>>>> breakpoints on line nunber etc. should work.)
>>>>
>>>> However, for synthetic code (e.g. the ramp and the expansion of the
>>>> co_await expressions) so far, we have intentionally generated the code
>>>> with “unknown” locations. This (absent the kind of process you are
>>>> mentioning) tends to impove the debug experience - because it avoids
>>>> the apparent location jumping around.
>>>>
>>>>> In clang, I solved this issue by emitting DW_TAG labels like
>>>>>
>>>>>> 0x00000f71: DW_TAG_label
>>>>>> DW_AT_name ("__coro_resume_17")
>>>>>> DW_AT_decl_file ("generator-example.cpp")
>>>>>> DW_AT_decl_line (5)
>>>>>> DW_AT_decl_column (3)
>>>>>> DW_AT_artificial (true)
>>>>>> DW_AT_LLVM_coro_suspend_idx (0x11)
>>>>>> DW_AT_low_pc (0x00000000000019be)
>>>>>
>>>>> The debugging script can lookup the DW_TAG_label for a given
>>>>> suspension point either by name or via DW_AT_LLVM_coro_suspend_idx and
>>>>> retrieve the line, column and address (for setting breakpoints) from
>>>>> that label.
>>>>>
>>>>> gcc emits similar information:
>>>>>
>>>>>> 0x0000297c: DW_TAG_label
>>>>>> DW_AT_name ("resume.17")
>>>>>>
>>>>>> 0x00002981: DW_TAG_label
>>>>>> DW_AT_name ("destroy.17")
>>>>>
>>>>> Unfortunately, this information is not useful because it lacks file,
>>>>> line, column and address information. It would be great if g++ could
>>>>> also emit file, line, column and address information for those labels.
>>>
>>
>> It looks like those labels internally already have the location
>> information, but the DWARF writer decides not to represent it because the
>> labels are marked DECL_ARTIFICIAL. We might just remove that flag from
>> create_named_label_with_ctx (even though they are indeed artificial)?
>
> I can certainly experiment with doing that, I’m just not aware it there are
> other
> consequences of removing the “artificial” state?
I’ve made this change (temporarily) here:
https://github.com/iains/gcc-cxx-coroutines/commits/c%2B%2B-coroutines/
This is also the branch that appears as “x86-64 gcc (coroutines)” on compiler
explorer.
(but I guess it will take 24h or so to percolate through)
Right now, very pressed for time - but hopefully that might help your
exploration?
Iain
>
> Iain
>
>>>>> ~~~~~~~~~
>>>>> Can gcc also emit useful DW_TAG debug information for coroutines?
>>>>> ~~~~~~~~~
>>>>>
>>>>> What do you think about the approach of using DW_TAG_label for
>>>>> debugging coroutines? Would you be willing to adopt the same approach
>>>>> also for g++? (I would also be happy to adjust clang, in case we come
>>>>> to a different alignment between both compilers).
>>>>>
>>>>> Adding the file, line, column and address would probably be pretty
>>>>> fundamental for a good debugging experience. Also it would be nice
>>>>> (although completely optional) if we could use the same naming
>>>>> convention (`__coro_resume_x`) and you might want to set the
>>>>> DW_AT_artificial tag. I chose `__coro_resume_x` for clang, because
>>>>> this is a reserved name which is still easily writeable in debugger
>>>>> commands. Using the DW_AT_artificial for those labels also seems to
>>>>> make semantically sense (although it is strictly speaking not blessed
>>>>> by the DWARF standard).
>>>>>
>>>>> ~~~~~~~~~
>>>>> Further Reading
>>>>> ~~~~~~~~~
>>>>>
>>>>> RFC for LLVM/clang:
>>>>>
>>> https://discourse.llvm.org/t/rfc-debug-info-for-coroutine-suspension-locations-take-2/86606
>>>>>
>>>>> Corresponding clang commit:
>>> https://github.com/llvm/llvm-project/pull/141937
>>>>>
>>>>> Background on debugging of coroutines, both from the user's point of
>>>>> view and toolchain implementation details, such as the approach for
>>>>> devirtualizing the coroutine frame's state:
>>>>> https://clang.llvm.org/docs/DebuggingCoroutines.html
>>>>>
>>>>> Best,
>>>>> Adrian
>>>>
>>>
>>>
>
