Fix AArch64 "$x" symbols

[Andrew H. <ap...@re...>]

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;
+
                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.