[llvm-dev] [RFC] Generate Debug Information for Labels in Function

[Hsiangkai Wang via llvm-dev]

Hi Reid,

Thanks for your suggestions. I dug in the implementation of
llvm.codeview.annotation. It seems to create a temp symbol as handling
Intrinsic::codeview_annotation and keep the mapping of this temp
symbol to some metadata. The mapping will be kept in MachineFunction.
Although it converts this intrinsic function to ANNOTATION_LABEL MI,
it seems no use of ANNOTATION_LABEL MI. All the debug information
generated from CodeViewDebug depends on mappings kept in
MachineFunction.

It is very different with the scenario to enable debugging with label.
Debugging with label needs to know file, line, and address of labels
in functions. It needs to keep these information from LLVM IR down to
MI and generate assembly through AsmPrinter finally. I will send
patches after I finish some clean up of these patches. Thanks a lot
anyway.

On Wed, Mar 28, 2018 at 11:08 AM, Reid Kleckner <rnk at google.com> wrote:
> Sounds good to me. You can also see llvm.codeview.annotation which does a
> similar thing, but it is modeled as having internal, invisible side effects.
> These should stay separate, but it may provide a guide for implementation.
>
> On Tue, Mar 27, 2018 at 7:41 PM Hsiangkai Wang via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
>>
>> Hello all,
>>
>> I would like to enhance LLVM debug info that supports setting
>> breakpoint on labels in function.
>>
>> Generally, if users use GDB as their debugger, they could set
>> breakpoints on labels in function. Following is an example.
>>
>> // C program
>> static int
>> myfunction (int arg)
>> {
>>   int i, j, r;
>>
>>   j = 0; /* myfunction location */
>>   r = arg;
>>
>>  top:
>>   ++j;  /* top location */
>>
>>   if (j == 10)
>>     goto done;
>>
>>   for (i = 0; i < 10; ++i)
>>     {
>>       r += i;
>>       if (j % 2)
>>         goto top;
>>     }
>>
>>  done:
>>   return r;
>> }
>>
>> int
>> main (void)
>> {
>>   int i, j;
>>
>>   for (i = 0, j = 0; i < 1000; ++i)
>>     j += myfunction (0);
>>
>>   return 0;
>> }
>>
>> Following is the GDB commands to illustrate how to set breakpoints on
>> labels.
>>
>> (gdb) b main
>> Breakpoint 1 at 0x10298: file explicit.c, line 50.
>> (gdb) r
>> Starting program: /home/users/kai/sandbox/gdbtest/explicit-gcc
>>
>> Breakpoint 1, main () at explicit.c:50
>> 50  for (i = 0, j = 0; i < 1000; ++i)
>> (gdb) b myfunction:top
>> Breakpoint 2 at 0x10214: file explicit.c, line 26.
>> (gdb) c
>> Continuing.
>>
>> Breakpoint 2, myfunction (arg=0) at explicit.c:27
>> 27  ++j;  /\* top location */
>> (gdb)
>>
>> However, LLVM does not generate debug information for labels. So, the
>> feature could not work for binaries generated by clang. I also found
>> that the problem is reported in PR35526 and PR36420. I propose an
>> implementation plan to support debug information for labels.
>>
>> Following are the steps I propose to implement the feature.
>>
>> 1. Define debug metadata and intrinsic functions for labels.
>>
>> First of all, we need to record debug information in LLVM IR. In LLVM
>> IR, LLVM uses metadata and intrinsic function to keep debug
>> information. So, I need to define new kind of metadata, DILabel, and
>> new intrinsic function, llvm.dbg.label, to associate DILabel with
>> label statement.
>>
>> DILabel will contain name of the label, file metadata, line number,
>> and scope metadata.
>>
>> Intrinsic function llvm.dbg.label uses DILabel metadata as its parameter.
>>
>> 2. Create MI instruction DBG_LABEL.
>>
>> I create new MI instruction DBG_LABEL to keep debug information after
>> LLVM IR converted to MI.
>>
>> DBG_LABEL uses DILabel metadata as its parameter.
>>
>> 3. Create data structure, SDDbgLabel, to store debug information of
>> labels in SelectionDAG.
>>
>> In SelectionDAG, we need a data structure to keep debug information of
>> label. It will keep DILabel metadata.
>>
>> 4. Convert SDDbgLabel to DBG_LABEL in SelectionDAG.
>>
>> After EmitSchedule(), SelectionDAG will be converted to a list of MI
>> instructions. In the function, we will generate DBG_LABEL MachineInstr
>> from SDDbgLabel.
>>
>> For FastISel and GlobalISel, we could convert llvm.dbg.label to
>> DBG_LABEL directly.
>>
>> 5. Collect debug information of labels from MI listing to
>> DebugHandlerBase.
>>
>> Before generating actual debug information in assembly format or
>> object format, we need to keep debug format-independent data in
>> DebugHandlerBase. Afterwards, we could convert these data to CodeView
>> format or DWARF format.
>>
>> 6. Create DWARF DIE specific data structure in DwarfDebug.
>>
>> In class DwarfDebug, we keep DWARF specific data structure for DILabel.
>>
>> 7. Generate DW_TAG_label and fill details of DW_TAG_label.
>>
>> Finally, generating DW_TAG_label DIE and its attributes into DIE
>> structure.
>>
>> I am looking forward to any thoughts & feedback!
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