Rearrange operands of the BranchInst, to be able to

access each with a fixed negative index from op_end().

This has two important implications:
- getUser() will work faster, because there are less iterations
  for the waymarking algorithm to perform. This is important
  when running various analyses that want to determine callers
  of basic blocks.
- getSuccessor() now runs faster, because the indirection via OperandList
  is not necessary: Uses corresponding to the successors are at fixed
  offset to "this".

The price we pay is the slightly more complicated logic in the operator
User::delete, as it has to pick up the information whether it has to free
the memory of an original unconditional BranchInst or a BranchInst that
was originally conditional, but has been shortened to unconditional.
I was not able to come up with a nicer solution to this problem. (And
rest assured, I tried *a lot*).

Similar reorderings will follow for InvokeInst and CallInst. After that
some optimizations to pred_iterator and CallSite will fall out naturally.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@66815 91177308-0d34-0410-b5e6-96231b3b80d8

lib/VMCore/Use.cpp

@@ -163,4 +163,71 @@ Use *User::allocHungoffUses(unsigned N) const {
   return Use::initTags(Begin, End);
 }
 
+//===----------------------------------------------------------------------===//
+//                         User operator new Implementations
+//===----------------------------------------------------------------------===//
+
+void *User::operator new(size_t s, unsigned Us) {
+  void *Storage = ::operator new(s + sizeof(Use) * Us);
+  Use *Start = static_cast<Use*>(Storage);
+  Use *End = Start + Us;
+  User *Obj = reinterpret_cast<User*>(End);
+  Obj->OperandList = Start;
+  Obj->NumOperands = Us;
+  Use::initTags(Start, End);
+  return Obj;
+}
+
+/// Prefixed allocation - just before the first Use, allocate a NULL pointer.
+/// The destructor can detect its presence and readjust the OperandList
+/// for deletition.
+///
+void *User::operator new(size_t s, unsigned Us, bool Prefix) {
+  // currently prefixed allocation only admissible for
+  // unconditional branch instructions
+  if (!Prefix)
+    return operator new(s, Us);
+
+  assert(Us == 1 && "Other than one Use allocated?");
+  typedef PointerIntPair<void*, 2, Use::PrevPtrTag> TaggedPrefix;
+  void *Raw = ::operator new(s + sizeof(TaggedPrefix) + sizeof(Use) * Us);
+  TaggedPrefix *Pre = static_cast<TaggedPrefix*>(Raw);
+  Pre->setFromOpaqueValue(0);
+  void *Storage = Pre + 1; // skip over prefix
+  Use *Start = static_cast<Use*>(Storage);
+  Use *End = Start + Us;
+  User *Obj = reinterpret_cast<User*>(End);
+  Obj->OperandList = Start;
+  Obj->NumOperands = Us;
+  Use::initTags(Start, End);
+  return Obj;
+}
+
+//===----------------------------------------------------------------------===//
+//                         User operator delete Implementation
+//===----------------------------------------------------------------------===//
+
+void User::operator delete(void *Usr) {
+  User *Start = static_cast<User*>(Usr);
+  Use *Storage = static_cast<Use*>(Usr) - Start->NumOperands;
+  //
+  // look for a variadic User
+  if (Storage == Start->OperandList) {
+    ::operator delete(Storage);
+    return;
+  }
+  //
+  // check for the flag whether the destructor has detected a prefixed
+  // allocation, in which case we remove the flag and delete starting
+  // at OperandList
+  if (reinterpret_cast<intptr_t>(Start->OperandList) & 1) {
+    ::operator delete(reinterpret_cast<char*>(Start->OperandList) - 1);
+    return;
+  }
+  //
+  // in all other cases just delete the nullary User (covers hung-off
+  // uses also
+  ::operator delete(Usr);
+}
+
 } // End llvm namespace