[LLVMdev] "Cannot fine DIE"

Talin viridia at gmail.com
Tue Aug 31 23:47:06 PDT 2010

On Sat, Aug 28, 2010 at 10:58 PM, Talin <viridia at gmail.com> wrote:

> On Sat, Aug 28, 2010 at 4:05 PM, Talin <viridia at gmail.com> wrote:
>> On Mon, Aug 23, 2010 at 5:58 PM, Devang Patel <devang.patel at gmail.com>wrote:
>>> On Sun, Aug 22, 2010 at 12:50 PM, Talin <viridia at gmail.com> wrote:
>>>> I recently started getting this error when I try to debug my
>>>> LLVM-compiled program in GDB:
>>>>     Dwarf Error: Cannot find DIE at 0x16769 referenced from DIE at
>>>> 0x1713c [in module
>>>> /home/talin/Projects/tart/build-eclipse/test/stdlib/ReflectionTest]
>>>> I'm not sure if it's something I did or not. Is there any way to track
>>>> down the cause of this error? The hex addresses in the error message are
>>>> meaningless to me.
>>> There is not much info here. The error says one debug info entry (DIE) is
>>> referring to another debug info entry that does not exist. This usually
>>> indicates that dwarf generator in llvm codegen knew about the other DIE but
>>> some how gdb does not find the DIE. May be the code generator did not emit
>>> the DIE ? If yes then why ?  Otherwise, the DIE was dropped somewhere after
>>>  compiler generated code.
>>> I've discovered a little bit more, but not enough to solve the problem.
>> Here's what my code looks like for setting the debug location (some
>> details removed for clarity):
>> void CodeGenerator::setDebugLocation(const SourceLocation & loc) {
>>   if (loc != dbgLocation_ && dbgContext_.isScope()) {
>>     dbgLocation_ = loc;
>>     if (loc.file == NULL) {
>>        builder_.SetCurrentDebugLocation(llvm::DebugLoc());
>>     } else if (loc.file == module_->moduleSource()) {
>>       TokenPosition pos = tokenPosition(loc);
>>       // ** Comment out this line and the DIE errors disappear **
>>       builder_.SetCurrentDebugLocation(
>>           DebugLoc::get(pos.beginLine, pos.beginCol, dbgContext_));
>>     }
>>   }
>> }
>> As noted in the comment above, if I comment out the line that calls
>> builder_.SetCurrentDebugLocation(), then gdb no longer reports not being
>> able to find the DIE. Of course, I don't get any source-line debugging
>> information either.
>> Note that even in this case, I'm still generating tons of DWARF info for
>> data types, files, functions, and so on. This all apparently works (or at
>> least, it doesn't seem to generate this particular problem). It's only the
>> generating of source line information that causes the error. This is
>> fortunate, since as you can see the code is quite trivial, whereas I was
>> afraid that if the problem was in the data type code, it would take forever
>> to locate the problem as that code is pretty complicated.
>> As far as why this code is causing the error, I can only guess that I am
>> calling it wrong somehow. But I can't think what I could do differently from
>> what I am doing already.
> OK some progress on this: Changing the 'scope' argument passed to
> DebugLoc::get() - from a DICompileUnit to a DIFile (I'm not sure which is
> right, the docs and comments aren't real clear on this) - gets rid of the
> "Can't find DIE" error. Instead I get:
>     "Line number -1 out of range;"
> ...when I try to examine a stack frame. Which is strange, because  *know*
> my line numbers are not -1. There's even an assert for that. And the
> comments in my generated assembly language look perfectly valid to me:
> Ltmp26:
>         subl    $8, %esp                ## Array.tart:103:11[
> Array.tart:103:11 ]
> Ltmp27:                                 ## Array.tart:103:11[
> Array.tart:103:11 ]
>         movl    $0, 4(%esp)             ## Array.tart:103:11[
> Array.tart:103:11 ]
>         movl    $16, (%esp)             ## Array.tart:103:11[
> Array.tart:103:11 ]
>         call    _malloc                 ## Array.tart:103:11[
> Array.tart:103:11 ]
> --
> -- Talin

Well, I figured out why the line numbers were getting set to -1: My
"add_custom_command" directive in my CMake file which was supposed to run
dsymutil was silently failing:

     OUTPUT "${EXE_FILE}.dSYM/Contents/Resources/DWARF/${EXE_FILE}"
        COMMENT "Generating debug symbols for ${EXE_FILE}")

I don't know why, but this directive does nothing and prints nothing. In any
case, that's not an LLVM problem so I'll stop talking about it.

However, this leads to a new problem: Now when I manually run dsymutil and
attempt to debug my executable, I get this:

DW_FORM_strp pointing outside of .debug_str section [in module

Once again, I have no idea what this means or how to go about debugging it.
This is my biggest frustration with DIFactory - there's absolutely no way to
verify that the DWARF debugging information that I've emitted into my module
is correct or even sensible. The only way to test it is to try and debug it
with gdb, but all that will tell you is that *something* failed - it won't
tell you where or what. It's not so much like looking for a needle in a
haystack - more like looking for a particular needle in a needlestack.

-- Talin
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