Defer DiaSession's symbol init (it is unused by WinPix (or anyone?)) (#…

…6187)

The init of the dia symbol manager presents an unneccessary performance
hit for WinPix. It's quite simple to defer the init, so here we are.
(This dia implementation was originally added specifically for WinPix,
but was then superseded by a simpler API. It's entirely possible that
there are no other clients of the dia interface at all, but it has been
published, so we're probably bound to maintain it.)
(Note that there are other deferment opportunities here, but the larger
pending work is to disentangle the dia implementation from that of the
new "IDxcPix*" API that WinPix uses, which would obviate deferment
altogether, but that's way out of scope for me for now.)

lib/DxilDia/DxilDiaSession.cpp

@@ -96,13 +96,18 @@ void dxil_dia::Session::Init(std::shared_ptr<llvm::LLVMContext> context,
     DXASSERT(m_rvaMap[It->second] == It->first,
              "instruction mapped to wrong rva");
   }
+}
 
-  // Initialize symbols
-  try {
-    m_symsMgr.Init(this);
-  } catch (const hlsl::Exception &) {
-    m_symsMgr = dxil_dia::SymbolManager();
+const dxil_dia::SymbolManager &dxil_dia::Session::SymMgr() {
+  if (!m_symsMgr) {
+    try {
+      m_symsMgr.reset(new dxil_dia::SymbolManager());
+      m_symsMgr->Init(this);
+    } catch (const hlsl::Exception &) {
+      m_symsMgr.reset(new dxil_dia::SymbolManager());
+    }
   }
+  return *m_symsMgr;
 }
 
 HRESULT dxil_dia::Session::getSourceFileIdByName(llvm::StringRef fileName,
@@ -138,7 +143,7 @@ STDMETHODIMP dxil_dia::Session::get_globalScope(
   *pRetVal = nullptr;
 
   Symbol *ret;
-  IFR(m_symsMgr.GetGlobalScope(&ret));
+  IFR(SymMgr().GetGlobalScope(&ret));
   *pRetVal = ret;
   return S_OK;
 }
@@ -378,7 +383,7 @@ STDMETHODIMP dxil_dia::Session::findInlineFramesByAddr(
 
   HRESULT hr;
   SymbolChildrenEnumerator *ChildrenEnum;
-  IFR(hr = m_symsMgr.DbgScopeOf(It->second, &ChildrenEnum));
+  IFR(hr = SymMgr().DbgScopeOf(It->second, &ChildrenEnum));
 
   *ppResult = ChildrenEnum;
   return hr;

lib/DxilDia/DxilDiaSession.h

@@ -61,7 +61,7 @@ class Session : public IDiaSession, public IDxcPixDxilDebugInfoFactory {
   hlsl::DxilModule &DxilModuleRef() { return *m_dxilModule.get(); }
   llvm::Module &ModuleRef() { return *m_module.get(); }
   llvm::DebugInfoFinder &InfoRef() { return *m_finder.get(); }
-  const SymbolManager &SymMgr() const { return m_symsMgr; }
+  const SymbolManager &SymMgr();
   const RVAMap &InstructionsRef() const { return m_instructions; }
   const std::vector<const llvm::Instruction *> &InstructionLinesRef() const {
     return m_instructionLines;
@@ -499,7 +499,7 @@ class Session : public IDiaSession, public IDxcPixDxilDebugInfoFactory {
   std::unordered_map<const llvm::Instruction *, RVA>
       m_rvaMap; // Map instruction to its RVA.
   LineToInfoMap m_lineToInfoMap;
-  SymbolManager m_symsMgr;
+  std::unique_ptr<SymbolManager> m_symsMgr;
 
 private:
   CComPtr<IDiaEnumTables> m_pEnumTables;