[llvm] r264316 - [debuginfo] Include dwo_name in the split unit to improve dwp diagnostics

Mon Mar 28 14:41:31 PDT 2016

On Mon, Mar 28, 2016 at 1:57 PM, Chaoren Lin <chaorenl at google.com> wrote:

> Hi David,
>
> This change caused clang to produce inferior binaries that causes LLDB to
> go into an infinite loop while parsing their debug info.
>
> I don't know enough about the debug information format to tell if it's
> clang now generating incorrect debug info due to this change, or if it's
> lldb incorrectly parsing the debug info (which just happen to have been
> working fine until now).
> Could you please take a look?
>

This is an "off label" use, as it were (the Split DWARF feature says that
GNU_dwo_name will appear on the skeleton CU, I don't think it implies that
it can't be used elsewhere - but perhaps a more careful reading than mine
might imply such)

Essentially LLDB shouldn't be following GNU_dwo_name when it's already
reading a CU from a dwo/dwp file already. I would consider doing this maybe
one day because I have fancy ideas about how to support modules debug info
using Split DWARF without needing to expand the DWARF spec* or debugger
functionality - but it might need to be able to put skeleton DIEs in .dwo
files & have them recursively expanded. I haven't thought about it too
carefully/all the way through, so can't be sure at this point

* OK, maybe a little bit of extra spec wording... *shrug*

We could create a new name for the attribute if necessary - at the moment
this use is just between Clang and llvm-dwp to improve diagnostics as
described in the commit message.

>
> For LLDB folks, it's looping through these 4 functions:
>
>
> lldb/source/Plugins/SymbolFile/DWARF/DWARFCompileUnit.h:GetCompileUnitDIEPtrOnly()
>
> lldb/source/Plugins/SymbolFile/DWARF/DWARFCompileUnit.h:GetCompileUnitDIEOnly()
>
> lldb/source/Plugins/SymbolFile/DWARF/DWARFCompileUnit.cpp:AddCompileUnitDIE()
>
> lldb/source/Plugins/SymbolFile/DWARF/DWARFCompileUnit.cpp:ExtractDIEsIfNeeded()
>
>
>
> On Thu, Mar 24, 2016 at 11:37 AM, David Blaikie via llvm-commits <
> llvm-commits at lists.llvm.org> wrote:
>
>> Author: dblaikie
>> Date: Thu Mar 24 13:37:08 2016
>> New Revision: 264316
>>
>> URL: http://llvm.org/viewvc/llvm-project?rev=264316&view=rev
>> Log:
>> [debuginfo] Include dwo_name in the split unit to improve dwp diagnostics
>>
>> When multiple DWP files are merged together and duplicate DWO IDs are
>> found it's currently difficult to give an actionable error message - the
>> DW_AT_name of the CU could be provided, but might be identical (if the
>> same source file is built into two different configurations), which
>> doesn't help the user identify the problem.
>>
>> When no intermediate DWP files are generated, the path to the two DWO
>> files could be provided - but is lost once the DWOs are merged into a
>> DWP.
>>
>> So, include the name of the DWO (dwo_name) in the split file so that
>> collissions involving a source CU from a DWP can be better diagnosed.
>>
>> (improvements to llvm-dwp using this to come shortly)
>>
>> Modified:
>>     llvm/trunk/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
>>     llvm/trunk/test/DebugInfo/X86/fission-cu.ll
>>
>> Modified: llvm/trunk/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
>> URL:
>> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/AsmPrinter/DwarfDebug.cpp?rev=264316&r1=264315&r2=264316&view=diff
>>
>> ==============================================================================
>> --- llvm/trunk/lib/CodeGen/AsmPrinter/DwarfDebug.cpp (original)
>> +++ llvm/trunk/lib/CodeGen/AsmPrinter/DwarfDebug.cpp Thu Mar 24 13:37:08
>> 2016
>> @@ -391,8 +391,11 @@ DwarfDebug::constructDwarfCompileUnit(co
>>    DwarfCompileUnit &NewCU = *OwnedUnit;
>>    DIE &Die = NewCU.getUnitDie();
>>    InfoHolder.addUnit(std::move(OwnedUnit));
>> -  if (useSplitDwarf())
>> +  if (useSplitDwarf()) {
>>      NewCU.setSkeleton(constructSkeletonCU(NewCU));
>> +    NewCU.addString(Die, dwarf::DW_AT_GNU_dwo_name,
>> +                    DIUnit->getSplitDebugFilename());
>> +  }
>>
>>    // LTO with assembly output shares a single line table amongst
>> multiple CUs.
>>    // To avoid the compilation directory being ambiguous, let the line
>> table
>>
>> Modified: llvm/trunk/test/DebugInfo/X86/fission-cu.ll
>> URL:
>> http://llvm.org/viewvc/llvm-project/llvm/trunk/test/DebugInfo/X86/fission-cu.ll?rev=264316&r1=264315&r2=264316&view=diff
>>
>> ==============================================================================
>> --- llvm/trunk/test/DebugInfo/X86/fission-cu.ll (original)
>> +++ llvm/trunk/test/DebugInfo/X86/fission-cu.ll Thu Mar 24 13:37:08 2016
>> @@ -33,6 +33,7 @@
>>  ; CHECK: .debug_abbrev.dwo contents:
>>  ; CHECK: Abbrev table for offset: 0x00000000
>>  ; CHECK: [1] DW_TAG_compile_unit DW_CHILDREN_yes
>> +; CHECK: DW_AT_GNU_dwo_name  DW_FORM_GNU_str_index
>>  ; CHECK: DW_AT_producer  DW_FORM_GNU_str_index
>>  ; CHECK: DW_AT_language  DW_FORM_data2
>>  ; CHECK: DW_AT_name      DW_FORM_GNU_str_index
>> @@ -64,38 +65,41 @@
>>  ; Check that the rest of the compile units have information.
>>  ; CHECK: .debug_info.dwo contents:
>>  ; CHECK: DW_TAG_compile_unit
>> -; CHECK: DW_AT_producer [DW_FORM_GNU_str_index] ( indexed (00000000)
>> string = "clang version 3.3 (trunk 169021) (llvm/trunk 169020)")
>> +; CHECK: DW_AT_GNU_dwo_name [DW_FORM_GNU_str_index] ( indexed (00000000)
>> string = "baz.dwo")
>> +; CHECK: DW_AT_producer [DW_FORM_GNU_str_index] ( indexed (00000001)
>> string = "clang version 3.3 (trunk 169021) (llvm/trunk 169020)")
>>  ; CHECK: DW_AT_language [DW_FORM_data2]        (DW_LANG_C99)
>> -; CHECK: DW_AT_name [DW_FORM_GNU_str_index]    ( indexed (00000001)
>> string = "baz.c")
>> +; CHECK: DW_AT_name [DW_FORM_GNU_str_index]    ( indexed (00000002)
>> string = "baz.c")
>>  ; CHECK-NOT: DW_AT_low_pc
>>  ; CHECK-NOT: DW_AT_stmt_list
>>  ; CHECK-NOT: DW_AT_comp_dir
>>  ; CHECK: DW_AT_GNU_dwo_id [DW_FORM_data8]  (0x1f1f859683d49324)
>>  ; CHECK: DW_TAG_variable
>> -; CHECK: DW_AT_name [DW_FORM_GNU_str_index]     ( indexed (00000002)
>> string = "a")
>> +; CHECK: DW_AT_name [DW_FORM_GNU_str_index]     ( indexed (00000003)
>> string = "a")
>>  ; CHECK: DW_AT_type [DW_FORM_ref4]       (cu + 0x{{[0-9a-f]*}} =>
>> {[[TYPE:0x[0-9a-f]*]]})
>>  ; CHECK: DW_AT_external [DW_FORM_flag_present]   (true)
>>  ; CHECK: DW_AT_decl_file [DW_FORM_data1] (0x01)
>>  ; CHECK: DW_AT_decl_line [DW_FORM_data1] (1)
>>  ; CHECK: DW_AT_location [DW_FORM_exprloc] (<0x2> fb 00 )
>>  ; CHECK: [[TYPE]]: DW_TAG_base_type
>> -; CHECK: DW_AT_name [DW_FORM_GNU_str_index]     ( indexed (00000003)
>> string = "int")
>> +; CHECK: DW_AT_name [DW_FORM_GNU_str_index]     ( indexed (00000004)
>> string = "int")
>>
>>  ; CHECK: .debug_str contents:
>>  ; CHECK: 0x00000000: "baz.dwo"
>>  ; CHECK: 0x00000008: "/usr/local/google/home/echristo/tmp"
>>
>>  ; CHECK: .debug_str.dwo contents:
>> -; CHECK: 0x00000000: "clang version 3.3 (trunk 169021) (llvm/trunk
>> 169020)"
>> -; CHECK: 0x00000035: "baz.c"
>> -; CHECK: 0x0000003b: "a"
>> -; CHECK: 0x0000003d: "int"
>> +; CHECK: 0x00000000: "baz.dwo"
>> +; CHECK: 0x00000008: "clang version 3.3 (trunk 169021) (llvm/trunk
>> 169020)"
>> +; CHECK: 0x0000003d: "baz.c"
>> +; CHECK: 0x00000043: "a"
>> +; CHECK: 0x00000045: "int"
>>
>>  ; CHECK: .debug_str_offsets.dwo contents:
>>  ; CHECK: 0x00000000: 00000000
>> -; CHECK: 0x00000004: 00000035
>> -; CHECK: 0x00000008: 0000003b
>> -; CHECK: 0x0000000c: 0000003d
>> +; CHECK: 0x00000004: 00000008
>> +; CHECK: 0x00000008: 0000003d
>> +; CHECK: 0x0000000c: 00000043
>> +; CHECK: 0x00000010: 00000045
>>
>>  ; Object file checks
>>  ; For x86-64-linux we should have this set of relocations for the debug
>> info section
>>
>>
>> _______________________________________________
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>> llvm-commits at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-commits
>>
>
>
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