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

[Hsiangkai Wang via llvm-dev]

Hi 韋任,

I will send my patches after I polish them in more detail. I am
looking forward to your review. Thanks a lot.

On Wed, Mar 28, 2018 at 9:12 PM, 陳韋任 <chenwj.cs97g at g2.nctu.edu.tw> wrote:
> LGTM. I think you can sent patch and let the ball rolling! I would
> like to review it. ;)
>
> 2018-03-28 10:41 GMT+08:00 Hsiangkai Wang via llvm-dev
> <llvm-dev at lists.llvm.org>:
>> 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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>> llvm-dev at lists.llvm.org
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>
>
>
> --
> Wei-Ren Chen (陳韋任)
> Homepage: https://people.cs.nctu.edu.tw/~chenwj