> 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?
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
>>>
>>
>>
