[Lldb-commits] [PATCH] D62634: Improve DWARF parsing and accessing by 1% to 2%

Pavel Labath via Phabricator via lldb-commits lldb-commits at lists.llvm.org
Tue Jun 4 04:15:28 PDT 2019

labath added a comment.

In D62634#1527575 <https://reviews.llvm.org/D62634#1527575>, @dblaikie wrote:

> > 1. We need to figure out whether there's a bug in clang and fix it. + @dblaikie for help with that
> This is intentional behavior in clang (controllable under the -f[no-]split-dwarf-inlining flag, and modified by the -f[no-]debug-info-for-profiling flag).
> This extra debug info is used for online symbolication (in the absence of .dwo files) - such as for the sanitizers (accurate symbolication is necessary for correctness in msan, due to msan's necessary use of blacklisting to avoid certain false positives) and some forms of sample based profiling collection.
> In the default mode, clang includes, as you noted, just the subprograms that have inlined subroutines in them in this split-dwarf-inlining data.
>  In -fdebug-info-for-profiling all subprograms are described in the skeleton CU with some minimal attributes (they don't need parameters, local variables/scopes, etc) necessary to do certain profile things I don't know lots about.
> Chances are it's probably best for a debugger to ignore these entries.

Thanks for explaining David. It sounds like we can safely ignore these entries if we have successfully loaded the dwo file, as it will contain a superset of information. If we cannot find the dwo file for any reason, then this information might be useful as a rough approximation of debug info. However, in this case, there's no dwo file around, so there isn't a possibility for confusion. I'll prepare a patch doing something like that.

Over email, @clayborg wrote:

> > Actually, there was a subtle change of behavior here. Before this change, if we tried to parse a DIE using an abbreviation table from a different file, we would most likely fail immediately because of the fail-safe in GetAbbreviationDeclarationPtr. Now, we just do a blind read and return bogus data.
> This seems just like a straight up bug we need to fix in LLDB. It shouldn't affect or require any changes from any compilers. Seems like a lldbassert that maybe verifies that the abbreviation we use is valid for the current CU in debug builds would be nice so we can trigger this bug when we run the test suite or locally against an example program would be good. Depending on that fact that we have a mismatch in the abbrev index seems fragile. Most .debug_abbrev tables start out with a DW_TAG_compile_unit. So if the abbrev index magically matches, that still doesn't mean the we will be decoding the right data even if the index matches.

Yes, this part is a bug, but in order to figure out how to fix it, I had to know whether the compiler behavior is intentional, and whether we need to access that data. If it turned out we did, the fix would be more involved, because we'd also need to change the UID encoding so that we can reference these DIEs correctly, etc. Given that it seems we can just ignore these debug info entries, the fix will be relatively simple.



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