[LLVMdev] More DWARF problems

Talin viridia at gmail.com
Sat Apr 2 23:03:45 PDT 2011

On Wed, Mar 30, 2011 at 11:17 AM, Devang Patel <dpatel at apple.com> wrote:

> On Mar 29, 2011, at 7:29 PM, Talin wrote:
> I've been trying to track down the problem with the DWARF info that is
> being emitted by my front end, which has been broken for about a month now.
> Here's what happens when I attempt to use gdb to debug one of my programs on
> OS X:
> gdb stack crawl at point of internal error:
> [ 0 ] /usr/libexec/gdb/gdb-i386-apple-darwin (align_down+0x0) [0x122300]
> [ 1 ] /usr/libexec/gdb/gdb-i386-apple-darwin
> (find_partial_die_in_comp_unit+0x65) [0xc0e19]
> [ 2 ] /usr/libexec/gdb/gdb-i386-apple-darwin (find_partial_die+0x2d4)
> [0xcf07f]
> [ 3 ] /usr/libexec/gdb/gdb-i386-apple-darwin (fixup_partial_die+0x29)
> [0xcf0b3]
> [ 4 ] /usr/libexec/gdb/gdb-i386-apple-darwin (scan_partial_symbols+0x26)
> [0xcf9e7]
> [ 5 ] /usr/libexec/gdb/gdb-i386-apple-darwin (dwarf2_build_psymtabs+0xc54)
> [0xd093c]
> [ 6 ] /usr/libexec/gdb/gdb-i386-apple-darwin (macho_symfile_read+0x145)
> [0x163b15]
> [ 7 ] /usr/libexec/gdb/gdb-i386-apple-darwin (syms_from_objfile+0x62d)
> [0x52259]
> [ 8 ] /usr/libexec/gdb/gdb-i386-apple-darwin
> (symbol_file_add_with_addrs_or_offsets_using_objfile+0x338) [0x561e7]
>  [ 9 ] /usr/libexec/gdb/gdb-i386-apple-darwin
> (symbol_file_add_with_addrs_or_offsets_using_objfile+0x2da) [0x56189]
> [ 10 ] /usr/libexec/gdb/gdb-i386-apple-darwin
> (symbol_file_add_name_with_addrs_or_offsets+0x7a) [0x563c9]
> [ 11 ] /usr/libexec/gdb/gdb-i386-apple-darwin (symbol_file_add_main_1+0xf2)
> [0x56e36]
> [ 12 ] /usr/libexec/gdb/gdb-i386-apple-darwin (catch_command_errors+0x4d)
> [0x7ac88]
> /SourceCache/gdb/gdb-966/src/gdb/dwarf2read.c:7593: internal-error: could
> not find partial DIE in cache
> A problem internal to GDB has been detected,
> further debugging may prove unreliable.
> Quit this debugging session? (y or n)
> Now, all of this was working earlier, and I don't know whether it was
> something I did or a change in LLVM, but that's not important. The real
> question is how to track down the problem.
> I have seen gdb crash with this back trace when it has seen a subprogram
> specification DIE at top level, but the actual subprogram definition is not
> found. The definition DIE may not be found because either it is hiding deep
> in nested subclass or it may be missing  all together in compiler output.
>  One easy way to rule out this is to check all specification DIE's
> indentation level in dwarfdump output and check corresponding level of
> definition die referred by it.
> In the past, the way that I have dealt with DWARF-related problems is to
> try a number of strategies:
> 1) Reduce the problem to the smallest reproducible case. In the past I have
> had some success with this, but not in this case. You see, one of the
> problems with object-oriented languages is that even simple operations -
> such as appending an element to an array - can end up pulling in a very
> large number of classes (For example, the array class might throw an
> exception if your index is invalid, which pulls in the exception hierarchy
> and so on...)
> I have a special script which attempts to compile a "minimal" test case,
> without the standard library and with garbage collection disabled.
> Unfortunately, none of the "small" test cases that I have been able to come
> up with exhibit the problem, and any time I use certain language features I
> am forced to link in the standard library which makes the test program huge.
> I have plenty of example cases which exhibit the problem, but they are all
> bitcode files on the order of 100K or more in size. And I'm not going to
> have much luck tracking down a needle in such a large haystack.
> 2) Use dwarfdump to try and verify the validity of the debug symbols.
> Unfortunately, the information from dwarfdump is not too useful in this
> case. Here's what I get:
>    - On OS X, with the "small" test cases I created, I get no errors at
>    all.
>    - On OS X, with my normal unit tests (with the standard library) I get
>    hundreds of error messages of the following form:
>     0x00000882: DIE attribute 0x00000883:  AT_type/FORM_ref4 has a value
> 0x00000592 that is not in the current compile unit in the .debug_info
> section.
> This indicates that while DwarfDebug.cpp was preparing dwarf info, it
> created a DIE  0x00000592 that was referred by another DIE 0x00000883 but
> somehow DIE 0x00000592 was not emitted. This could be a bug in
> DwarfDebug.cpp or how debug info is  generated by FE.
> In DwarfDebug.cpp, you'll see code like
>       addDIEEntry(VariableSpecDIE,
> dwarf::DW_AT_specification,  dwarf::DW_FORM_ref4, VariableDIE);
> Here VariableSpecDIE is referring VariableDIE, but VariableDIE is missing
> from the output. There are other uses of DW_FORM_ref4 also. So check in our
> dwarfdump output what is  0x00000883 and set appropriate breakpoint in
> debugger and see why it is not reaching to DwarfDebug::emitDIE().

OK I've been checking this out some more, and the DIEs don't look valid to
me. Take a look at this output from dwarfdump -v:

0x000000c7:     TAG_subprogram [3]
0x000000c8:      AT_name( .debug_str[0x000001bd] = "construct" )
0x000000cc:      AT_MIPS_linkage_name( .debug_str[0x000001c7] =
"tart.reflect.Parameter.construct(tart.core.String)" )
0x000000d0:      AT_decl_file( 0x3d (
"/Users/talin/Projects/tart/trunk/lib/std/tart/reflect/Parameter.tart" ) )
0x000000d1:      AT_decl_line( 0x0d ( 13 ) )
0x000000d2:      AT_type( cu + 0x00000066 => {0x00000103} (  ) )
0x000000d6:      AT_external( 0x01 )
0x000000d7:      AT_low_pc( 0x0000f780 )
0x000000db:      AT_high_pc( 0x0000f7b1 )
0x000000df:      AT_frame_base( <0x1> 55  ( reg5 ) )

0x000000e1:     NULL

0x000000e2: Compile Unit: length = 0x00000071  version = 0x0002  abbr_offset
= 0x00000000  addr_size = 0x04  (next CU at 0x00000157)

0x000000ed: TAG_compile_unit [1] *
0x000000ee:  AT_producer( .debug_str[0x00000001] = "0.1 tartc" )
0x000000f2:  AT_language( 0x0002 ( DW_LANG_C ) )
0x000000f4:  AT_name( .debug_str[0x000001fa] = "range.tart" )
0x000000f8:  AT_entry_pc( 0x00004360 )
0x000000fc:  AT_stmt_list( 0x00000000 ( 0x00000000 ) )
0x00000100:  AT_comp_dir( .debug_str[0x00000205] =
"/Users/talin/Projects/tart/trunk/lib/std/tart/core" )
0x00000104:  AT_APPLE_major_runtime_vers( 0x01 )

In particular note that the DIE starting at 0x0c7, which is a
TAG_subprogram, has a return type (AT_type) which points to 0x103. However
if you look further down, you'll see that there is no DIE at offset 0x103.
Instead it looks like it's pointing into the middle of another DIE.

At least, this is true if I'm interpreting this right.

> -
> Devang
>     0x000009a9: DIE attribute 0x000009ae:  AT_type/FORM_ref4 has a value
> 0x000001c2 that is not in the current compile unit in the .debug_info
> section.
>     0x00000b85: DIE attribute 0x00000b8a:  AT_type/FORM_ref4 has a value
> 0x0000055c that is not in the current compile unit in the .debug_info
> section.
>     0x00000c88: DIE attribute 0x00000c89:  AT_type/FORM_ref4 has a value
> 0x0000055c that is not in the current compile unit in the .debug_info
> section.
>     0x00000d2f: DIE attribute 0x00000d34:  AT_type/FORM_ref4 has a value
> 0x0000055c that is not in the current compile unit in the .debug_info
> section.
>     0x00000d9a: DIE attribute 0x00000d9f:  AT_type/FORM_ref4 has a value
> 0x00000584 that is not in the current compile unit in the .debug_info
> section.
>     0x00000e43: DIE attribute 0x00000e48:  AT_type/FORM_ref4 has a value
> 0x000011ac that is not in the current compile unit in the .debug_info
> section.
>     0x00000ea3: DIE attribute 0x00000ea8:  AT_type/FORM_ref4 has a value
> 0x00001225 that is not in the current compile unit in the .debug_info
> section.
>     0x00000ebe: DIE attribute 0x00000ebf:  AT_type/FORM_ref4 has a value
> 0x00001248 that is not in the current compile unit in the .debug_info
> section.
>     0x00000ee3: DIE attribute 0x00000ee4:  AT_type/FORM_ref4 has a value
> 0x00001285 that is not in the current compile unit in the .debug_info
> section.
>    - On Linux - well the problem here is that even when my DWARF info was
>    working, dwarfdump would spit out a ton of error messages about bad file
>    DIEs and other spam - in other words, I've never been able to use LLVM to
>    produce a binary on Linux that was dwarfdump-error free. So any "new" errors
>    are mixed in with all of the "old" errors I was seeing before.
> 3) Use llbrowse to manually inspect the DIEs and see if they make sense.
> (Which is part of the reason why I wrote llbrowse.) Again, the problem is
> that I don't know where to look, and the files are simply too large to
> inspect manually.
> --
> -- Talin
>  _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

-- Talin
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