namespace llvm {
+/*==============================================================================
+
+
+ -----------------------------------------------------------------
+ --- Interaction and relationship between User and Use objects ---
+ -----------------------------------------------------------------
+
+
+A subclass of User can choose between incorporating its Use objects
+or refer to them out-of-line by means of a pointer. A mixed variant
+(some Uses inline others hung off) is impractical and breaks the invariant
+that the Use objects belonging to the same User form a contiguous array.
+
+We have 2 different layouts in the User (sub)classes:
+
+Layout a)
+The Use object(s) are inside (resp. at fixed offset) of the User
+object and there are a fixed number of them.
+
+Layout b)
+The Use object(s) are referenced by a pointer to an
+array from the User object and there may be a variable
+number of them.
+
+Initially each layout will possess a direct pointer to the
+start of the array of Uses. Though not mandatory for layout a),
+we stick to this redundancy for the sake of simplicity.
+The User object will also store the number of Use objects it
+has. (Theoretically this information can also be calculated
+given the scheme presented below.)
+
+Special forms of allocation operators (operator new)
+will enforce the following memory layouts:
+
+
+# Layout a) will be modelled by prepending the User object
+# by the Use[] array.
+#
+# ...---.---.---.---.-------...
+# | P | P | P | P | User
+# '''---'---'---'---'-------'''
+
+
+# Layout b) will be modelled by pointing at the Use[] array.
+#
+# .-------...
+# | User
+# '-------'''
+# |
+# v
+# .---.---.---.---...
+# | P | P | P | P |
+# '---'---'---'---'''
+
+ (In the above figures 'P' stands for the Use** that
+ is stored in each Use object in the member Use::Prev)
+
+
+Since the Use objects will be deprived of the direct pointer to
+their User objects, there must be a fast and exact method to
+recover it. This is accomplished by the following scheme:
+
+A bit-encoding in the 2 LSBits of the Use::Prev will allow to find the
+start of the User object:
+
+00 --> binary digit 0
+01 --> binary digit 1
+10 --> stop and calc (s)
+11 --> full stop (S)
+
+Given a Use*, all we have to do is to walk till we get
+a stop and we either have a User immediately behind or
+we have to walk to the next stop picking up digits
+and calculating the offset:
+
+.---.---.---.---.---.---.---.---.---.---.---.---.---.---.---.---.----------------
+| 1 | s | 1 | 0 | 1 | 0 | s | 1 | 1 | 0 | s | 1 | 1 | s | 1 | S | User (or User*)
+'---'---'---'---'---'---'---'---'---'---'---'---'---'---'---'---'----------------
+ |+15 |+10 |+6 |+3 |+1
+ | | | | |__>
+ | | | |__________>
+ | | |______________________>
+ | |______________________________________>
+ |__________________________________________________________>
+
+
+Only the significant number of bits need to be stored between the
+stops, so that the worst case is 20 memory accesses when there are
+1000 Use objects.
+
+The following literate Haskell fragment demonstrates the concept:
+
+> import Test.QuickCheck
+>
+> digits :: Int -> [Char] -> [Char]
+> digits 0 acc = '0' : acc
+> digits 1 acc = '1' : acc
+> digits n acc = digits (n `div` 2) $ digits (n `mod` 2) acc
+>
+> dist :: Int -> [Char] -> [Char]
+> dist 0 [] = ['S']
+> dist 0 acc = acc
+> dist 1 acc = let r = dist 0 acc in 's' : digits (length r) r
+> dist n acc = dist (n - 1) $ dist 1 acc
+>
+> takeLast n ss = reverse $ take n $ reverse ss
+>
+> test = takeLast 40 $ dist 20 []
+>
+
+Printing <test> gives: "1s100000s11010s10100s1111s1010s110s11s1S"
+
+The reverse algorithm computes the
+length of the string just by examining
+a certain prefix:
+
+> pref :: [Char] -> Int
+> pref "S" = 1
+> pref ('s':'1':rest) = decode 2 1 rest
+> pref (_:rest) = 1 + pref rest
+>
+> decode walk acc ('0':rest) = decode (walk + 1) (acc * 2) rest
+> decode walk acc ('1':rest) = decode (walk + 1) (acc * 2 + 1) rest
+> decode walk acc _ = walk + acc
+>
+
+Now, as expected, printing <pref test> gives 40.
+
+We can quickCheck this with following property:
+
+> testcase = dist 2000 []
+> testcaseLength = length testcase
+>
+> identityProp n = n > 0 && n <= testcaseLength ==> length arr == pref arr
+> where arr = takeLast n testcase
+
+As expected <quickCheck identityProp> gives:
+
+*Main> quickCheck identityProp
+OK, passed 100 tests.
+
+Let's be a bit more exhaustive:
+
+>
+> deepCheck p = check (defaultConfig { configMaxTest = 500 }) p
+>
+
+And here is the result of <deepCheck identityProp>:
+
+*Main> deepCheck identityProp
+OK, passed 500 tests.
+
+
+To maintain the invariant that the 2 LSBits of each Use** in Use
+never change after being set up, setters of Use::Prev must re-tag the
+new Use** on every modification. Accordingly getters must strip the
+tag bits.
+
+For layout b) instead of the User we will find a pointer (User* with LSBit set).
+Following this pointer brings us to the User. A portable trick will ensure
+that the first bytes of User (if interpreted as a pointer) will never have
+the LSBit set.
+
+==============================================================================*/
+
+/// OperandTraits - Compile-time customization of
+/// operand-related allocators and accessors
+/// for use of the User class
+template <class>
+struct OperandTraits;
+
+class User;
+
+/// OperandTraits<User> - specialization to User
+template <>
+struct OperandTraits<User> {
+ static inline Use *op_begin(User*);
+ static inline Use *op_end(User*);
+ static inline unsigned operands(const User*);
+ template <class U>
+ struct Layout {
+ typedef U overlay;
+ };
+ static inline void *allocate(unsigned);
+};
+
class User : public Value {
User(const User &); // Do not implement
void *operator new(size_t); // Do not implement
+ template <unsigned>
+ friend struct HungoffOperandTraits;
protected:
/// OperandList - This is a pointer to the array of Users for this operand.
/// For nodes of fixed arity (e.g. a binary operator) this array will live
- /// embedded into the derived class. For nodes of variable arity
- /// (e.g. ConstantArrays, CallInst, PHINodes, ReturnInst etc), this memory
- /// will be dynamically allocated and should be destroyed by the classes
+ /// prefixed to the derived class. For nodes of resizable variable arity
+ /// (e.g. PHINodes, SwitchInst etc.), this memory will be dynamically
+ /// allocated and should be destroyed by the classes'
/// virtual dtor.
Use *OperandList;
///
unsigned NumOperands;
- void *operator new(size_t s, size_t) {
- return ::operator new(s);
+ void *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;
}
User(const Type *Ty, unsigned vty, Use *OpList, unsigned NumOps)
: Value(Ty, vty), OperandList(OpList), NumOperands(NumOps) {}
-
+ Use *allocHungoffUses(unsigned) const;
+ void dropHungoffUses(Use *U) {
+ if (OperandList == U) {
+ OperandList = 0;
+ NumOperands = 0;
+ }
+ Use::zap(U, U->getImpliedUser(), true);
+ }
public:
+ ~User() {
+ Use::zap(OperandList, OperandList + NumOperands);
+ }
+ void operator delete(void *Usr) {
+ User *Start = static_cast<User*>(Usr);
+ Use *Storage = static_cast<Use*>(Usr) - Start->NumOperands;
+ ::operator delete(Storage == Start->OperandList
+ ? Storage
+ : Usr);
+ }
+ template <unsigned Idx> Use &Op() {
+ return OperandTraits<User>::op_begin(this)[Idx];
+ }
+ template <unsigned Idx> const Use &Op() const {
+ return OperandTraits<User>::op_begin(const_cast<User*>(this))[Idx];
+ }
Value *getOperand(unsigned i) const {
assert(i < NumOperands && "getOperand() out of range!");
return OperandList[i];
}
};
+inline Use *OperandTraits<User>::op_begin(User *U) {
+ return U->op_begin();
+}
+
+inline Use *OperandTraits<User>::op_end(User *U) {
+ return U->op_end();
+}
+
+inline unsigned OperandTraits<User>::operands(const User *U) {
+ return U->getNumOperands();
+}
+
template<> struct simplify_type<User::op_iterator> {
typedef Value* SimpleType;
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