Fix an N^2 issue handling constant resolution due to RAUW in large arrays

this speeds up the bcreader from 6.67s to 0.12s on a testcase Daniel 
provided.  rdar://6158117



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@55090 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Bitcode/Reader/BitcodeReader.cpp

@@ -133,6 +133,15 @@ namespace {
       : ConstantExpr(Ty, Instruction::UserOp1, &Op<0>(), 1) {
       Op<0>() = UndefValue::get(Type::Int32Ty);
     }
+    
+    /// @brief Methods to support type inquiry through isa, cast, and dyn_cast.
+    static inline bool classof(const ConstantPlaceHolder *) { return true; }
+    static bool classof(const Value *V) {
+      return isa<ConstantExpr>(V) && 
+             cast<ConstantExpr>(V)->getOpcode() == Instruction::UserOp1;
+    }
+    
+    
     /// Provide fast operand accessors
     DECLARE_TRANSPARENT_OPERAND_ACCESSORS(Value);
   };
@@ -205,6 +214,85 @@ Value *BitcodeReaderValueList::getValueFwdRef(unsigned Idx, const Type *Ty) {
   return V;
 }
 
+/// ResolveConstantForwardRefs - Once all constants are read, this method bulk
+/// resolves any forward references.  The idea behind this is that we sometimes
+/// get constants (such as large arrays) which reference *many* forward ref
+/// constants.  Replacing each of these causes a lot of thrashing when
+/// building/reuniquing the constant.  Instead of doing this, we look at all the
+/// uses and rewrite all the place holders at once for any constant that uses
+/// a placeholder.
+void BitcodeReaderValueList::ResolveConstantForwardRefs() {
+  // Sort the values by-pointer so that they are efficient to look up with a 
+  // binary search.
+  std::sort(ResolveConstants.begin(), ResolveConstants.end());
+  
+  SmallVector<Constant*, 64> NewOps;
+  
+  while (!ResolveConstants.empty()) {
+    Value *RealVal = getOperand(ResolveConstants.back().second);
+    Constant *Placeholder = ResolveConstants.back().first;
+    ResolveConstants.pop_back();
+    
+    // Loop over all users of the placeholder, updating them to reference the
+    // new value.  If they reference more than one placeholder, update them all
+    // at once.
+    while (!Placeholder->use_empty()) {
+      User *U = Placeholder->use_back();
+      // If the using object isn't uniqued, just update the operands.  This
+      // handles instructions and initializers for global variables.
+      if (!isa<Constant>(U) || isa<GlobalValue>(U)) {
+        U->replaceUsesOfWith(Placeholder, RealVal);
+        continue;
+      }
+      
+      // Otherwise, we have a constant that uses the placeholder.  Replace that
+      // constant with a new constant that has *all* placeholder uses updated.
+      Constant *UserC = cast<Constant>(U);
+      for (User::op_iterator I = UserC->op_begin(), E = UserC->op_end();
+           I != E; ++I) {
+        Value *NewOp;
+        if (!isa<ConstantPlaceHolder>(*I)) {
+          // Not a placeholder reference.
+          NewOp = *I;
+        } else if (*I == Placeholder) {
+          // Common case is that it just references this one placeholder.
+          NewOp = RealVal;
+        } else {
+          // Otherwise, look up the placeholder in ResolveConstants.
+          ResolveConstantsTy::iterator It = 
+            std::lower_bound(ResolveConstants.begin(), ResolveConstants.end(), 
+                             std::pair<Constant*, unsigned>(cast<Constant>(*I),
+                                                            0));
+          assert(It != ResolveConstants.end() && It->first == *I);
+          NewOp = this->getOperand(It->second);
+        }
+
+        NewOps.push_back(cast<Constant>(NewOp));
+      }
+
+      // Make the new constant.
+      Constant *NewC;
+      if (ConstantArray *UserCA = dyn_cast<ConstantArray>(UserC)) {
+        NewC = ConstantArray::get(UserCA->getType(), &NewOps[0], NewOps.size());
+      } else if (isa<ConstantStruct>(UserC)) {
+        NewC = ConstantStruct::get(&NewOps[0], NewOps.size());
+      } else if (isa<ConstantVector>(UserC)) {
+        NewC = ConstantVector::get(&NewOps[0], NewOps.size());
+      } else {
+        // Must be a constant expression.
+        NewC = cast<ConstantExpr>(UserC)->getWithOperands(&NewOps[0],
+                                                          NewOps.size());
+      }
+      
+      UserC->replaceAllUsesWith(NewC);
+      UserC->destroyConstant();
+      NewOps.clear();
+    }
+    
+    delete Placeholder;
+  }
+}
+
 
 const Type *BitcodeReader::getTypeByID(unsigned ID, bool isTypeTable) {
   // If the TypeID is in range, return it.
@@ -602,14 +690,8 @@ bool BitcodeReader::ParseConstants() {
   unsigned NextCstNo = ValueList.size();
   while (1) {
     unsigned Code = Stream.ReadCode();
-    if (Code == bitc::END_BLOCK) {
-      if (NextCstNo != ValueList.size())
-        return Error("Invalid constant reference!");
-      
-      if (Stream.ReadBlockEnd())
-        return Error("Error at end of constants block");
-      return false;
-    }
+    if (Code == bitc::END_BLOCK)
+      break;
     
     if (Code == bitc::ENTER_SUBBLOCK) {
       // No known subblocks, always skip them.
@@ -852,6 +934,17 @@ bool BitcodeReader::ParseConstants() {
     ValueList.AssignValue(V, NextCstNo);
     ++NextCstNo;
   }
+  
+  if (NextCstNo != ValueList.size())
+    return Error("Invalid constant reference!");
+  
+  if (Stream.ReadBlockEnd())
+    return Error("Error at end of constants block");
+  
+  // Once all the constants have been read, go through and resolve forward
+  // references.
+  ValueList.ResolveConstantForwardRefs();
+  return false;
 }
 
 /// RememberAndSkipFunctionBody - When we see the block for a function body,

lib/Bitcode/Reader/BitcodeReader.h

@@ -32,9 +32,22 @@ namespace llvm {
 
 class BitcodeReaderValueList : public User {
   unsigned Capacity;
+  
+  /// ResolveConstants - As we resolve forward-referenced constants, we add
+  /// information about them to this vector.  This allows us to resolve them in
+  /// bulk instead of resolving each reference at a time.  See the code in
+  /// ResolveConstantForwardRefs for more information about this.
+  ///
+  /// The key of this vector is the placeholder constant, the value is the slot
+  /// number that holds the resolved value.
+  typedef std::vector<std::pair<Constant*, unsigned> > ResolveConstantsTy;
+  ResolveConstantsTy ResolveConstants;
 public:
   BitcodeReaderValueList() : User(Type::VoidTy, Value::ArgumentVal, 0, 0)
                            , Capacity(0) {}
+  ~BitcodeReaderValueList() {
+    assert(ResolveConstants.empty() && "Constants not resolved?");
+  }
 
   /// Provide fast operand accessors
   DECLARE_TRANSPARENT_OPERAND_ACCESSORS(Value);
@@ -50,6 +63,7 @@ public:
   }
   
   void clear() {
+    assert(ResolveConstants.empty() && "Constants not resolved?");
     if (OperandList) dropHungoffUses(OperandList);
     Capacity = 0;
   }
@@ -73,15 +87,26 @@ public:
     if (Idx == size()) {
       push_back(V);
     } else if (Value *OldV = getOperand(Idx)) {
-      // If there was a forward reference to this value, replace it.
-      setOperand(Idx, V);
-      OldV->replaceAllUsesWith(V);
-      delete OldV;
+      // Handle constants and non-constants (e.g. instrs) differently for
+      // efficiency.
+      if (Constant *PHC = dyn_cast<Constant>(OldV)) {
+        ResolveConstants.push_back(std::make_pair(PHC, Idx));
+        setOperand(Idx, V);
+      } else {
+        // If there was a forward reference to this value, replace it.
+        setOperand(Idx, V);
+        OldV->replaceAllUsesWith(V);
+        delete OldV;
+      }
     } else {
       initVal(Idx, V);
     }
   }
   
+  /// ResolveConstantForwardRefs - Once all constants are read, this method bulk
+  /// resolves any forward references.
+  void ResolveConstantForwardRefs();
+  
 private:
   void initVal(unsigned Idx, Value *V) {
     if (Idx >= size()) {
@@ -94,7 +119,8 @@ private:
 };
 
 template <>
-struct OperandTraits<BitcodeReaderValueList> : HungoffOperandTraits</*16 FIXME*/> {
+struct OperandTraits<BitcodeReaderValueList>
+  : HungoffOperandTraits</*16 FIXME*/> {
 };
 
 DEFINE_TRANSPARENT_OPERAND_ACCESSORS(BitcodeReaderValueList, Value)