[LLVMdev] Advice for setting debug locations

[David Blaikie]

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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