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.
In the past, the way that I have dealt with DWARF-related problems is to try a number of strategies:
- 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.
- 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.
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.
- 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.