On Wed, Mar 30, 2011 at 11:17 AM, Devang Patel <span dir="ltr"><<a href="mailto:dpatel@apple.com">dpatel@apple.com</a>></span> wrote:<br><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;">
<div style="word-wrap:break-word"><br><div><div class="im"><div>On Mar 29, 2011, at 7:29 PM, Talin wrote:</div><br><blockquote type="cite">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:<div>
<br></div><blockquote style="margin:0 0 0 40px;border:none;padding:0px"><div><div><font face="'courier new', monospace">gdb stack crawl at point of internal error:</font></div>
</div><div><div><font face="'courier new', monospace">[ 0 ] /usr/libexec/gdb/gdb-i386-apple-darwin (align_down+0x0) [0x122300]</font></div></div><div><div><font face="'courier new', monospace">[ 1 ] /usr/libexec/gdb/gdb-i386-apple-darwin (find_partial_die_in_comp_unit+0x65) [0xc0e19]</font></div>
</div><div><div><font face="'courier new', monospace">[ 2 ] /usr/libexec/gdb/gdb-i386-apple-darwin (find_partial_die+0x2d4) [0xcf07f]</font></div></div><div><div><font face="'courier new', monospace">[ 3 ] /usr/libexec/gdb/gdb-i386-apple-darwin (fixup_partial_die+0x29) [0xcf0b3]</font></div>
</div><div><div><font face="'courier new', monospace">[ 4 ] /usr/libexec/gdb/gdb-i386-apple-darwin (scan_partial_symbols+0x26) [0xcf9e7]</font></div></div><div><div><font face="'courier new', monospace">[ 5 ] /usr/libexec/gdb/gdb-i386-apple-darwin (dwarf2_build_psymtabs+0xc54) [0xd093c]</font></div>
</div><div><div><font face="'courier new', monospace">[ 6 ] /usr/libexec/gdb/gdb-i386-apple-darwin (macho_symfile_read+0x145) [0x163b15]</font></div></div><div><div><font face="'courier new', monospace">[ 7 ] /usr/libexec/gdb/gdb-i386-apple-darwin (syms_from_objfile+0x62d) [0x52259]</font></div>
</div><div><div><font face="'courier new', monospace">[ 8 ] /usr/libexec/gdb/gdb-i386-apple-darwin (symbol_file_add_with_addrs_or_offsets_using_objfile+0x338) [0x561e7]</font></div></div>
<div>
<div><font face="'courier new', monospace">[ 9 ] /usr/libexec/gdb/gdb-i386-apple-darwin (symbol_file_add_with_addrs_or_offsets_using_objfile+0x2da) [0x56189]</font></div></div><div><div><font face="'courier new', monospace">[ 10 ] /usr/libexec/gdb/gdb-i386-apple-darwin (symbol_file_add_name_with_addrs_or_offsets+0x7a) [0x563c9]</font></div>
</div><div><div><font face="'courier new', monospace">[ 11 ] /usr/libexec/gdb/gdb-i386-apple-darwin (symbol_file_add_main_1+0xf2) [0x56e36]</font></div></div><div><div><font face="'courier new', monospace">[ 12 ] /usr/libexec/gdb/gdb-i386-apple-darwin (catch_command_errors+0x4d) [0x7ac88]</font></div>
</div><div><div><font face="'courier new', monospace">/SourceCache/gdb/gdb-966/src/gdb/dwarf2read.c:7593: internal-error: could not find partial DIE in cache</font></div></div><div><div><font face="'courier new', monospace"><br>
</font></div></div><div><div><font face="'courier new', monospace">A problem internal to GDB has been detected,</font></div></div><div><div><font face="'courier new', monospace">further debugging may prove unreliable.</font></div>
</div><div><div><font face="'courier new', monospace">Quit this debugging session? (y or n) </font></div></div></blockquote><div><div><br></div><div>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.</div>
</div></blockquote><div><br></div></div><div>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.</div>
</div></div></blockquote><div><br></div><div>OK, given that much information I was able to track it down, which is that I was passing my struct type as the context parameter to DIBuilder.createMethod. If I change it to compile unit, this problem goes away. I had thought I had read somewhere that it was legal to use the enclosing class definition as the subroutine context, but now I can't find where I read it. In any case, I guess this means that I don't know the proper way to declare member functions in DWARF - that is, how can I declare method A of class B so that I can say "B.A" in the debugger and gdb knows where to find it?</div>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;"><div style="word-wrap:break-word"><div><div class="im"><blockquote type="cite"><div>
<div>In the past, the way that I have dealt with DWARF-related problems is to try a number of strategies:</div><div><br></div><div>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...)</div>
<div><br></div><div>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.</div>
<div><br></div><div>2) Use dwarfdump to try and verify the validity of the debug symbols.</div><div><br></div><div>Unfortunately, the information from dwarfdump is not too useful in this case. Here's what I get:</div>
<div><ul><li>On OS X, with the "small" test cases I created, I get no errors at all.</li><li>On OS X, with my normal unit tests (with the standard library) I get hundreds of error messages of the following form:</li>
</ul></div></div><blockquote style="margin:0 0 0 40px;border:none;padding:0px"><div><div><div><div> 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.</div>
</div></div></div></blockquote></blockquote><div><br></div></div><div>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.</div>
<div><br></div><div>In DwarfDebug.cpp, you'll see code like</div><div><br></div><div><div> addDIEEntry(VariableSpecDIE, dwarf::DW_AT_specification, dwarf::DW_FORM_ref4, VariableDIE);</div><div><br></div><div>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(). </div>
<div><br></div></div></div></div></blockquote><div>OK I'm still trying to track this one down, it is apparently unrelated to the earlier problem. After fixing the problem with the subroutine context mentioned above, I now see the following in gdb:</div>
<div><br></div><div><div>Die: DW_TAG_formal_parameter (abbrev = 27, offset = 14760)</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>has children: FALSE</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>attributes:</div>
<div><span class="Apple-tab-span" style="white-space:pre"> </span>DW_AT_name (DW_FORM_strp) string: "testType"</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>DW_AT_decl_file (DW_FORM_data1) constant: 74</div>
<div><span class="Apple-tab-span" style="white-space:pre"> </span>DW_AT_decl_line (DW_FORM_data1) constant: 47</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>DW_AT_type (DW_FORM_ref4) constant ref: 43711 (adjusted)</div>
<div><span class="Apple-tab-span" style="white-space:pre"> </span>DW_AT_location (DW_FORM_block1) block: size 2</div><div>Dwarf Error: Cannot find type of die [in module /Users/talin/Projects/tart/build-eclipse/test/stdlib/BitTricksTest.dSYM/Contents/Resources/DWARF/BitTricksTest]</div>
</div><div><br></div><div>This is good because I know exactly where that parameter is - now the question is to figure out what is wrong with it.</div><div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;">
<div style="word-wrap:break-word"><div><div><div></div><div>-</div><div>Devang</div></div><br><blockquote type="cite"><div class="im"><blockquote style="margin:0 0 0 40px;border:none;padding:0px"><div><div><div>
</div></div></div><div><div><div><div> 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.</div></div></div></div><div><div>
<div><div> 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.</div></div></div></div><div><div><div><div> 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.</div>
</div></div></div><div><div><div><div> 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.</div></div></div></div><div><div>
<div><div> 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.</div></div></div></div><div><div><div><div> 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.</div>
</div></div></div><div><div><div><div> 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.</div></div></div></div><div><div>
<div><div> 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.</div></div></div></div><div><div><div><div> 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.</div>
</div></div></div></blockquote><div><div><ul><li>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.</li>
</ul></div><div>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.</div>
<div><br></div>-- <br>-- Talin<br>
</div></div>
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<a href="http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev" target="_blank">http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev</a><br></blockquote></div><br></div></blockquote></div><br><br clear="all"><br>-- <br>-- Talin<br>