[LLVMdev] Advice for setting debug locations

[Keno Fischer]

Using the 3 field DW_TAG_lexical_block worked just fine. Thanks!

On Wed, Aug 13, 2014 at 7:21 PM, David Blaikie <dblaikie at gmail.com> wrote:
> On Wed, Aug 13, 2014 at 4:06 PM, Keno Fischer
> <kfischer at college.harvard.edu> wrote:
>> Oh, I see. Sorry I misunderstood. I'll try to come up with some
>> minimal IR. The assertion stems from the fact that getCompileUnitDIE()
>> returns null and then crashes at DWARFUnit.cpp:301. I admit I don't
>> know if this problem is on the parsing or the generation side.
>>
>> While I come up with the IR, basically what I was doing was using a
>> DebugLoc with scope being a DIFile rather than a DISubprogram as
>> before.
>
> Yes, I imagine that would end poorly. Wouldn't be hard to construct
> such an example and demonstrate the failure.
>
> Instead, using the DW_TAG_lexical_block (the one with 3 fields) you
> nest that inside - so your DebugLoc references the lexical block and
> the lexical block references the subprogram.
>
> All the DebugLocs should reference scopes that eventually reference
> the (non-inlined) subprogram that contains the instruction.
>
>>
>> On Wed, Aug 13, 2014 at 7:00 PM, David Blaikie <dblaikie at gmail.com> wrote:
>>> On Wed, Aug 13, 2014 at 3:47 PM, Keno Fischer
>>> <kfischer at college.harvard.edu> wrote:
>>>> Sorry, I didn't have a small IR example and I was sure I was just
>>>> doing something stupid. Thanks for the help, I'll try it out and
>>>> report back. Maybe it would be good to add an assertion or something
>>>> that tells people what's wrong in this case, since the generated DWARF
>>>> seems to be invalid?
>>>
>>> I'm still not sure exactly where things went wrong for you, so unsure
>>> as to where to add the assertion - a small IR example would be helpful
>>> for that. The suggestion to use the DW_TAG_lexical_block 3-field-form
>>> was just based on your description of your goals, it sounded like that
>>> was the tool you needed & weren't using yet.
>>>
>>>>
>>>> On Wed, Aug 13, 2014 at 5:53 PM, David Blaikie <dblaikie at gmail.com> wrote:
>>>>> Use the second form of DW_TAG_lexical_block metadata (the one with 3
>>>>> fields - http://llvm.org/docs/SourceLevelDebugging.html#block-descriptors
>>>>> ) to describe file changes within a sequence of LLVM IR functions.
>>>>>
>>>>> A small example of IR would be easier to follow if you want help
>>>>> debugging why your IR is causing problems.
>>>>>
>>>>> On Wed, Aug 13, 2014 at 2:25 PM, Keno Fischer
>>>>> <kfischer at college.harvard.edu> wrote:
>>>>>> Hello,
>>>>>>
>>>>>> I'm trying to fix a long standing issue we are having in Julia where
>>>>>> when the file information switched, we weren't recording that
>>>>>> correctly, so line information showed up in the wrong file. Basically
>>>>>> we would always create a scope with the DISubprogram and the given
>>>>>> line number. What I tried was to change the scope to be a DIFile
>>>>>> instead so we'd get the correct file information. Unfortunately I had
>>>>>> to revert this [1], because it crashes llvm's dwarf reader (which we
>>>>>> use to get line information for backtraces when exceptions are thrown)
>>>>>> with the following backtrace:
>>>>>>
>>>>>> isNULL at /Users/kfischer/Documents/julia-debug/deps/llvm-svn/lib/DebugInfo/DWARFDebugInfoEntry.h:52
>>>>>> getAddressRanges at
>>>>>> /Users/kfischer/Documents/julia-debug/deps/llvm-svn/lib/DebugInfo/DWARFDebugInfoEntry.cpp:242
>>>>>> collectAddressRanges at
>>>>>> /Users/kfischer/Documents/julia-debug/deps/llvm-svn/lib/DebugInfo/DWARFUnit.cpp:302
>>>>>> generate at /Users/kfischer/Documents/julia-debug/deps/llvm-svn/lib/DebugInfo/DWARFDebugAranges.cpp:54
>>>>>> getDebugAranges at
>>>>>> /Users/kfischer/Documents/julia-debug/deps/llvm-svn/lib/DebugInfo/DWARFContext.cpp:270
>>>>>> getCompileUnitForAddress at
>>>>>> /Users/kfischer/Documents/julia-debug/deps/llvm-svn/lib/DebugInfo/DWARFContext.cpp:423
>>>>>>
>>>>>> Can somebody tell me what the correct way to achieve this is?
>>>>>>
>>>>>> [1] https://github.com/JuliaLang/julia/commit/e494cb6dff193ecb0e254981be0f1122ae294d04
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