performance: Prevent needless DWARFCompileUnit::Clear() on freshly ctor-ed object

[jankratochvil]

Summary

DWARFDebugInfo::ParseCompileUnitHeadersIfNeeded() was clearing each DWARFCompileUnit twice (ctor+Clear()).

Sure I can drop it if it is not worth the change.

Is such patch obvious enough to check it in without an approval?

[clayborg]

See inline comments.

source/Plugins/SymbolFile/DWARF/DWARFCompileUnit.cpp

74

Make this static as mentioned in above comment. Assertions are ok to detect things in debug build, but please use lldbassert and make sure it returns an empty shared pointer if things fail (code must function properly when assert is not compiled in the program.

source/Plugins/SymbolFile/DWARF/DWARFCompileUnit.h

34–36

If you want to enforce this, then make this function static and have it return a shared pointer to a DWARFCompileUnit and make the constructor private. Commenting isn't enough on its own

source/Plugins/SymbolFile/DWARF/DWARFDebugInfo.cpp

294–297

I don't see this code in top of tree? My DWARFDebugInfo.cpp ends at line 259

[jankratochvil]

Thanks for the review but then it would become a performance regression, not the performance improvement I was trying to make.
Withdrawing this patch.

source/Plugins/SymbolFile/DWARF/DWARFCompileUnit.cpp

74

In such case each Extract() call will always make a new allocation of DWARFCompileUnit instance which is much more performance expensive that the double in-place reinitialization this patch was trying to avoid.

source/Plugins/SymbolFile/DWARF/DWARFDebugInfo.cpp

294–297

I see it in:
https://github.com/llvm-mirror/lldb/blob/master/source/Plugins/SymbolFile/DWARF/DWARFDebugInfo.cpp#L295
and also in:
https://llvm.org/svn/llvm-project/lldb/trunk/source/Plugins/SymbolFile/DWARF/DWARFDebugInfo.cpp

source/Plugins/SymbolFile/DWARF/DWARFCompileUnit.h

   DWARFCompileUnit(SymbolFileDWARF *dwarf2Data);
   ~DWARFCompileUnit();
 
+  // Object must be clear - either constructed now or after calling Clear().
   bool Extract(const lldb_private::DWARFDataExtractor &debug_info,
                lldb::offset_t *offset_ptr);
   size_t ExtractDIEsIfNeeded(bool cu_die_only);

source/Plugins/SymbolFile/DWARF/DWARFCompileUnit.cpp

 
 bool DWARFCompileUnit::Extract(const DWARFDataExtractor &debug_info,
                                lldb::offset_t *offset_ptr) {
-  Clear();
-
+  assert(m_offset == DW_INVALID_OFFSET);
   m_offset = *offset_ptr;
 
   if (debug_info.ValidOffset(*offset_ptr)) {

source/Plugins/SymbolFile/DWARF/DWARFDebugInfo.cpp

       // and create a new one for our own use!
       if (!cu.unique())
         cu.reset(new DWARFCompileUnit(dwarf2Data));
+      else
+        cu->Clear();
 
       // Make sure we start on a proper
       offset = next_cu_offset;