r189320 - PR14569: Omit debug info for thunks

[David Blaikie]

On Tue, Aug 27, 2013 at 1:45 AM, Dmitry Vyukov <dvyukov at google.com> wrote:
> On Tue, Aug 27, 2013 at 12:19 PM, Alexey Samsonov <samsonov at google.com> wrote:
>>
>> On Tue, Aug 27, 2013 at 9:21 AM, David Blaikie <dblaikie at gmail.com> wrote:
>>>
>>> Author: dblaikie
>>> Date: Tue Aug 27 00:21:11 2013
>>> New Revision: 189320
>>>
>>> URL: http://llvm.org/viewvc/llvm-project?rev=189320&view=rev
>>> Log:
>>> PR14569: Omit debug info for thunks
>>>
>>> This was added in r166676 based on PR13942 on the basis that tools may
>>> need debug information for any executable code/function for some fairly
>>> broad/non-specific purposes. It seems to me (as noted in PR14569) that
>>> the major/only purpose is in backtraces, which should generally not
>>> apply to thunks as they won't appear in the stack themselves.
>>
>>
>> Are you sure that thunks may not appear in backtraces? I think I saw them in
>> stack traces,
>> more specifically - in stack traces produced by ThreadSanitizer.
>> It's possible that I'm wrong, though...
>
> Debug info for thunks is necessary for sampling-based profilers and
> gdb (if you step or stop in thunk).

In theory I would think this could just be handled using the symbol
table - the only advantage of emitting debug info is to allow inlining
to correctly attribute the inlined instructions to the original
function, as I understand it (for tasks such as this anyway - where
there's no user code that maps into the function, etc).

Please let me know what your thoughts on that are.

In the mean time, given that there was no observable debug info size
savings (which I still don't really understand/believe, but can't
refute), I have reverted & reworked this patch to keep thunk debug
info but to encode the name only in the linkage name in the debug
info, not the plain name. This is sufficient to fix the relevant GDB
test case.

- David

>
>
>>> By
>>> removing them we fix PR14569 and reduce the size of Clang's debug info.
>>>
>>> Strangely enough this doesn't seem to have a substantial impact on
>>> Clang's self-hosted debug info (at least looking at DWO file size) size
>>> at all. Not sure if I failed to test this correctly but I only observed
>>> a 0.004% change in DWO file size over Clang+LLVM.
>>>
>>> With thanks to Dinesh Dwivedi for work on this PR.
>>>
>>> Modified:
>>>     cfe/trunk/lib/CodeGen/CGVTables.cpp
>>>     cfe/trunk/test/CodeGenCXX/debug-info-thunk.cpp
>>>
>>> Modified: cfe/trunk/lib/CodeGen/CGVTables.cpp
>>> URL:
>>> http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/CodeGen/CGVTables.cpp?rev=189320&r1=189319&r2=189320&view=diff
>>>
>>> ==============================================================================
>>> --- cfe/trunk/lib/CodeGen/CGVTables.cpp (original)
>>> +++ cfe/trunk/lib/CodeGen/CGVTables.cpp Tue Aug 27 00:21:11 2013
>>> @@ -292,6 +292,7 @@ void CodeGenFunction::GenerateVarArgsThu
>>>  void CodeGenFunction::GenerateThunk(llvm::Function *Fn,
>>>                                      const CGFunctionInfo &FnInfo,
>>>                                      GlobalDecl GD, const ThunkInfo
>>> &Thunk) {
>>> +  DebugInfo = NULL; // debug info for thunks is not required or desired
>>>    const CXXMethodDecl *MD = cast<CXXMethodDecl>(GD.getDecl());
>>>    const FunctionProtoType *FPT =
>>> MD->getType()->getAs<FunctionProtoType>();
>>>    QualType ThisType = MD->getThisType(getContext());
>>>
>>> Modified: cfe/trunk/test/CodeGenCXX/debug-info-thunk.cpp
>>> URL:
>>> http://llvm.org/viewvc/llvm-project/cfe/trunk/test/CodeGenCXX/debug-info-thunk.cpp?rev=189320&r1=189319&r2=189320&view=diff
>>>
>>> ==============================================================================
>>> --- cfe/trunk/test/CodeGenCXX/debug-info-thunk.cpp (original)
>>> +++ cfe/trunk/test/CodeGenCXX/debug-info-thunk.cpp Tue Aug 27 00:21:11
>>> 2013
>>> @@ -14,4 +14,4 @@ struct C : A, B {
>>>
>>>  void C::f() { }
>>>
>>> -// CHECK: [ DW_TAG_subprogram ] [line 15] [def] [_ZThn{{4|8}}_N1C1fEv]
>>> +// CHECK-NOT: [ DW_TAG_subprogram ] [line 15] [def]
>>> [_ZThn{{4|8}}_N1C1fEv]
>>>
>>>
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>>
>>
>>
>>
>> --
>> Alexey Samsonov, MSK