r189320 - PR14569: Omit debug info for thunks

[Alexey Samsonov]

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...



> 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
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