Add missing '::' when hitting cached scope names

Fixes C++ symbol names in some situations as shown by the changes
to the expected test data. Because the code to build the scope
names is complex, I added a longer comment in the hope that this
better explains the behavior and logic of this code.

The test e.g. now has this changed behavior:
```
-       201650 201700   /usr/include/c++/12.1.0/bits/shared_ptr_base.h:611:7    201650 dc       std_Sp_counted_ptr_inplace<std::__future_base::_Async_state_impl<std::thread::_Invoker<std::tuple<main(int, char**)::<lambda()> > >, double>, std::allocator<void>, (__gnu_cxx::_Lock_policy)2>_M_dispose parallel_static_gcc /home/milian/projects/kdab/rnd/hotspot/3rdparty/perfparser/tests/manual/clients/parallel_static_gcc
+       201650 201700   /usr/include/c++/12.1.0/bits/shared_ptr_base.h:611:7    201650 dc       std::_Sp_counted_ptr_inplace<std::__future_base::_Async_state_impl<std::thread::_Invoker<std::tuple<main(int, char**)::<lambda()> > >, double>, std::allocator<void>, (__gnu_cxx::_Lock_policy)2>::_M_dispose parallel_static_gcc /home/milian/projects/kdab/rnd/hotspot/3rdparty/perfparser/tests/manual/clients/parallel_static_gcc
```

Change-Id: Iaa82add2c878796890decb4365b3ca783b46f355
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>

3 files changed, 14 insertions, 0 deletions

diff --git a/app/perfdwarfdiecache.cpp b/app/perfdwarfdiecache.cpp
index 123ebac..fb87b25 100644
--- a/app/perfdwarfdiecache.cpp
+++ b/app/perfdwarfdiecache.cpp
@@ -139,6 +139,14 @@ void prependScopeNames(QByteArray &name, Dwarf_Die *die, QHash<Dwarf_Off, QByteA
     Dwarf_Die *scopes = nullptr;
     auto nscopes = dwarf_getscopes_die(die, &scopes);
 
+    // We essentially visit and build the scope name in reverse order.
+    // The cache ops encode offsets we can lookup directly that then map to fully
+    // qualified identifiers, which must obviously _not_ end on a double-colon separator.
+    // Note that while filling cacheOps below, we thus always have to prepend the double-colon
+    // first to the name, then store the cacheOps with the size of `name`. While that may sound
+    // confusing, that gives us the desired results: `ScopesToCache::trailing` will then be set
+    // to the size _following_ the current entry, which may get more identifiers appended to
+    // it when we continue to visit the other DIEs next.
     struct ScopesToCache
     {
         Dwarf_Off offset;
@@ -154,6 +162,10 @@ void prependScopeNames(QByteArray &name, Dwarf_Die *die, QHash<Dwarf_Off, QByteA
 
         auto it = cache.find(scopeOffset);
         if (it != cache.end()) {
+            // prepend the fully qualified cached identifier
+            // that won't end on `::`, so we have to add that manually here
+            if (!name.isEmpty())
+                name.prepend("::");
             name.prepend(*it);
             // we can stop, cached names are always fully qualified
             break;
diff --git a/tests/auto/perfdata/parallel_static_gcc/perf.data.zstd.expected.txt.zlib b/tests/auto/perfdata/parallel_static_gcc/perf.data.zstd.expected.txt.zlib
index eaaa310..39f0703 100644
--- a/tests/auto/perfdata/parallel_static_gcc/perf.data.zstd.expected.txt.zlib
+++ b/tests/auto/perfdata/parallel_static_gcc/perf.data.zstd.expected.txt.zlib
diff --git a/tests/auto/perfdata/tst_perfdata.cpp b/tests/auto/perfdata/tst_perfdata.cpp
index b4a72dd..d0706c9 100644
--- a/tests/auto/perfdata/tst_perfdata.cpp
+++ b/tests/auto/perfdata/tst_perfdata.cpp
@@ -348,6 +348,8 @@ void TestPerfData::testFiles()
     }
 
     if (actualText != expectedText) {
+        compressFile(actualOutputFile);
+
         const auto diff = QStandardPaths::findExecutable("diff");
         if (!diff.isEmpty()) {
             {