Updated backpatching logic during object deserialization to perform

eager backpatching instead of waithing until all objects have been
deserialized.  This allows us to reduce the memory footprint needed
for backpatching.

llvm-svn: 43422

2 files changed, 91 insertions, 43 deletions

diff --git a/llvm/include/llvm/Bitcode/Deserialize.h b/llvm/include/llvm/Bitcode/Deserialize.h
index 4a9b06381ad..508b4deee82 100644
--- a/llvm/include/llvm/Bitcode/Deserialize.h
+++ b/llvm/include/llvm/Bitcode/Deserialize.h
@@ -26,11 +26,11 @@
 namespace llvm {
   
 class Deserializer {  
-  BitstreamReader& Stream;
-  SmallVector<uint64_t,10> Record;
-  unsigned RecIdx;
-  BumpPtrAllocator Allocator;
-  
+
+  //===----------------------------------------------------------===//
+  // Internal type definitions.
+  //===----------------------------------------------------------===//
+
   struct PtrIdInfo {
     static inline unsigned getEmptyKey() { return ~((unsigned) 0x0); }
     static inline unsigned getTombstoneKey() { return getEmptyKey()-1; }
@@ -39,26 +39,58 @@ class Deserializer {
     static inline bool isPod() { return true; }
   };
   
-  struct BPatchNode {
-    BPatchNode* const Next;
+  struct BPNode {
+    BPNode* Next;
     uintptr_t& PtrRef;
-    BPatchNode(BPatchNode* n, void*& pref) 
-      : Next(n), PtrRef(reinterpret_cast<uintptr_t&>(pref)) {
+    BPNode(BPNode* n, uintptr_t& pref) 
+      : Next(n), PtrRef(pref) {
         PtrRef = 0;
       }
   };
   
-  struct BPatchEntry {
-    BPatchNode* Head;
-    void* Ptr;    
-    BPatchEntry() : Head(NULL), Ptr(NULL) {}
+  class BPatchEntry {
+    uintptr_t Ptr;
+  public:
+    
+    BPatchEntry() : Ptr(0x1) {}
+      
+    BPatchEntry(void* P) : Ptr(reinterpret_cast<uintptr_t>(P)) {}
+
+    bool hasFinalPtr() const { return Ptr & 0x1 ? true : false; }
+    void setFinalPtr(BPNode*& FreeList, void* P);
+
+    BPNode* getBPNode() const {
+      assert (!hasFinalPtr());
+      return reinterpret_cast<BPNode*>(Ptr & ~0x1);
+    }
+    
+    void setBPNode(BPNode* N) {
+      assert (!hasFinalPtr());
+      Ptr = reinterpret_cast<uintptr_t>(N) | 0x1;
+    }
+    
+    uintptr_t getRawPtr() const { return Ptr; }
+
     static inline bool isPod() { return true; }
-  };  
-  
+  };
+
   typedef llvm::DenseMap<unsigned,BPatchEntry,PtrIdInfo,BPatchEntry> MapTy;
 
+  //===----------------------------------------------------------===//
+  // Internal data members.
+  //===----------------------------------------------------------===//
+  
+  BitstreamReader& Stream;
+  SmallVector<uint64_t,10> Record;
+  unsigned RecIdx;
+  BumpPtrAllocator Allocator;
+  BPNode* FreeList;
   MapTy BPatchMap;  
   
+  //===----------------------------------------------------------===//
+  // Public Interface.
+  //===----------------------------------------------------------===//
+  
 public:
   Deserializer(BitstreamReader& stream);
   ~Deserializer();
@@ -99,13 +131,15 @@ public:
     return x;
   }
   
-  void ReadPtr(void*& PtrRef);
-  void ReadPtr(uintptr_t& PtrRef) { ReadPtr(reinterpret_cast<void*&>(PtrRef)); }
+  template <typename T>
+  void ReadPtr(T*& PtrRef) { ReadUIntPtr(reinterpret_cast<uintptr_t&>(PtrRef));}
+
+  void ReadPtr(uintptr_t& PtrRef) { ReadUIntPtr(PtrRef); }
+  
+  void ReadUIntPtr(uintptr_t& PtrRef);
   
   void RegisterPtr(unsigned PtrId, void* Ptr);
 
-
-  void BackpatchPointers();
 private:
   void ReadRecord();  
   bool inRecord();
diff --git a/llvm/lib/Bitcode/Reader/Deserialize.cpp b/llvm/lib/Bitcode/Reader/Deserialize.cpp
index 66df46ea23e..9caef6dae1e 100644
--- a/llvm/lib/Bitcode/Reader/Deserialize.cpp
+++ b/llvm/lib/Bitcode/Reader/Deserialize.cpp
@@ -16,14 +16,14 @@
 using namespace llvm;
 
 Deserializer::Deserializer(BitstreamReader& stream)
-  : Stream(stream), RecIdx(0) {
+  : Stream(stream), RecIdx(0), FreeList(NULL) {
 }
 
 Deserializer::~Deserializer() {
   assert (RecIdx >= Record.size() && 
           "Still scanning bitcode record when deserialization completed.");
   
-  BackpatchPointers();
+  assert (FreeList == NULL && "Some pointers were not backpatched.");
 }
 
 
@@ -96,11 +96,11 @@ void Deserializer::ReadCStr(std::vector<char>& buff, bool isNullTerm) {
 
 void Deserializer::RegisterPtr(unsigned PtrId,void* Ptr) {
   BPatchEntry& E = BPatchMap[PtrId];
-  assert (E.Ptr == NULL && "Pointer already registered.");
-  E.Ptr = Ptr;
+  assert (E.hasFinalPtr() && "Pointer already registered.");
+  E.setFinalPtr(FreeList,Ptr);
 }
 
-void Deserializer::ReadPtr(void*& PtrRef) {
+void Deserializer::ReadUIntPtr(uintptr_t& PtrRef) {
   unsigned PtrId = ReadInt();
   
   if (PtrId == 0) {
@@ -110,32 +110,46 @@ void Deserializer::ReadPtr(void*& PtrRef) {
   
   BPatchEntry& E = BPatchMap[PtrId];
   
-  if (E.Ptr == NULL) {
-    // Register backpatch.
-    void* P = Allocator.Allocate<BPatchNode>();    
-    E.Head = new (P) BPatchNode(E.Head,PtrRef);
+  if (E.hasFinalPtr())
+    PtrRef = E.getRawPtr();
+  else {
+    // Register backpatch.  Check the freelist for a BPNode.
+    BPNode* N;
+
+    if (FreeList) {
+      N = FreeList;
+      FreeList = FreeList->Next;
+    }
+    else // No available BPNode.  Allocate one.
+      N = (BPNode*) Allocator.Allocate<BPNode>();
+    
+    new (N) BPNode(E.getBPNode(),PtrRef);
+    E.setBPNode(N);
   }
-  else
-    PtrRef = E.Ptr;
 }
 
-void Deserializer::BackpatchPointers() {
-  for (MapTy::iterator I=BPatchMap.begin(),E=BPatchMap.end(); I!=E; ++I) {
-    
-    BPatchEntry& Entry = I->second;
-    assert (Entry.Ptr && "No pointer found for backpatch.");
-    
-    for (BPatchNode* N = Entry.Head; N != NULL; N = N->Next)
-      // Bitwise-OR in the pointer to support "smart" pointers that use
-      // unused bits to store extra data.
-      N->PtrRef |= reinterpret_cast<uintptr_t>(Entry.Ptr);
-    
-    Entry.Head = NULL;
+
+void Deserializer::BPatchEntry::setFinalPtr(BPNode*& FreeList, void* P) {
+  assert (!hasFinalPtr());
+  
+  // Perform backpatching.
+  
+  BPNode* Last = NULL;
+  
+  for (BPNode* N = getBPNode() ; N != NULL; N = N->Next) {
+    Last = N;
+    N->PtrRef |= reinterpret_cast<uintptr_t>(P);
   }
   
-  Allocator.Reset();
+  if (Last) {
+    Last->Next = FreeList;
+    FreeList = getBPNode();    
+  }
+  
+  Ptr = reinterpret_cast<uintptr_t>(P);
 }
 
+
 #define INT_READ(TYPE)\
 void SerializeTrait<TYPE>::Read(Deserializer& D, TYPE& X) {\
   X = (TYPE) D.ReadInt(); }\