Re: Fix AArch64 "$x" symbols

On 07/30/2014 11:37 AM, Andrew Haley wrote:
> There's a bug in Oprofile for AArch64 where "opreport -l" prints out lots
> of symbols as "x".  Like this:
> 
> 21690    604433         1.0010  27.8939    libjvm.so                PhaseChaitin::build_ifg_physical(ResourceArea*)
> 20856    625289         0.9625  28.8564    libjvm.so                ConnectionGraph::add_fields_to_worklist(FieldNode*, PointsToNode*)
> 20707    645996         0.9556  29.8120    libjvm.so                $x
> 19824    665820         0.9149  30.7269    libjvm.so                PhaseIdealLoop::is_dominator(Node*, Node*)
> 19483    685303         0.8991  31.6260    libjvm.so                $x
> 17863    703166         0.8244  32.4504    libjvm.so                SymbolTable::lookup_only(char const*, int, unsigned int&)
> 17530    720696         0.8090  33.2593    libjvm.so                PhaseChaitin::interfere_with_live(unsigned int, IndexSet*)
> 16381    737077         0.7560  34.0153    libjvm.so                PhaseIdealLoop::Dominators()
> 16124    753201         0.7441  34.7594    libjvm.so                MachNode::rematerialize() const
> 15886    769087         0.7331  35.4925    libjvm.so                SpinPause
> 14797    783884         0.6829  36.1754    libjvm.so                $x
> 
> 
> It turns out that these "$x" symbols aren't symbols at all, but
> "mapping symbols".  The AArch64 ELF specification requires that
> special symbols be inserted into object files to mark certain
> features:
> 
> $x
>     At the start of a region of code containing AArch64 instructions.
> 
> $d
>     At the start of a region of data.
> 
> These symbols should be ignored by tools.  The fix in Oprofile is
> trivial:
> 
> diff --git a/libpp/profile_container.cpp b/libpp/profile_container.cpp
> index e543f51..ec8438c 100644
> --- a/libpp/profile_container.cpp
> +++ b/libpp/profile_container.cpp
> @@ -81,6 +81,9 @@ void profile_container::add(profile_t const & profile,
> 
>         for (symbol_index_t i = 0; i < abfd.syms.size(); ++i) {
> 
> +               if (abfd.syms[i].name() == "$x")
> +                       continue;
Andrew,
This seems to be just applying a band aid to the problem.  We end up losing samples that were incorrectly attributed to this special symbol, right? Those samples are probably for a real symbol I would expect.  Is there any way to figure that out?

-Maynard
> +
>                 unsigned long long start = 0, end = 0;
>                 symbol_entry symb_entry;
> 
> And now the output looks like this:
> 
> 21690    604433         1.1579  32.2665    libjvm.so                PhaseChaitin::build_ifg_physical(ResourceArea*)
> 20856    625289         1.1134  33.3798    libjvm.so                ConnectionGraph::add_fields_to_worklist(FieldNode*, PointsToNode*)
> 19824    645113         1.0583  34.4381    libjvm.so                PhaseIdealLoop::is_dominator(Node*, Node*)
> 17863    662976         0.9536  35.3917    libjvm.so                SymbolTable::lookup_only(char const*, int, unsigned int&)
> 17530    680506         0.9358  36.3275    libjvm.so                PhaseChaitin::interfere_with_live(unsigned int, IndexSet*)
> 16381    696887         0.8745  37.2019    libjvm.so                PhaseIdealLoop::Dominators()
> 16124    713011         0.8607  38.0627    libjvm.so                MachNode::rematerialize() const
> 15886    728897         0.8480  38.9107    libjvm.so                SpinPause
> 14687    743584         0.7840  39.6948    libjvm.so                PhaseLive::add_liveout(Block*, IndexSet*, VectorSet&)
> 
> Andrew.
> 
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