[ValueLattice] Use union to shave off ptr size bytes from elements.

By using a union for Constant* and ConstantRange we can shave off ptr
size bytes off lattice elements. On 64 bit systems, it brings down the
size to 40 bytes from 48 bytes.

Initialization of Range happens on-demand using placement new, if the
state changes to constantrange from non-constantrange. Similarly, the
Range object is destroyed if the state changes from constantrange to
non-constantrange.

Reviewers: reames, anna, davide

Reviewed By: reames, davide

Differential Revision: https://reviews.llvm.org/D41903


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@323049 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Analysis/ValueLattice.h

@@ -49,14 +49,73 @@ class ValueLatticeElement {
     overdefined
   };
 
-  /// Val: This stores the current lattice value along with the Constant* for
-  /// the constant if this is a 'constant' or 'notconstant' value.
   ValueLatticeElementTy Tag;
-  Constant *Val;
-  ConstantRange Range;
+
+  /// The union either stores a pointer to a constant or a constant range,
+  /// associated to the lattice element. We have to ensure that Range is
+  /// initialized or destroyed when changing state to or from constantrange.
+  union {
+    Constant *ConstVal;
+    ConstantRange Range;
+  };
 
 public:
-  ValueLatticeElement() : Tag(undefined), Val(nullptr), Range(1, true) {}
+  // Const and Range are initialized on-demand.
+  ValueLatticeElement() : Tag(undefined) {}
+
+  /// Custom destructor to ensure Range is properly destroyed, when the object
+  /// is deallocated.
+  ~ValueLatticeElement() {
+    switch (Tag) {
+    case overdefined:
+    case undefined:
+    case constant:
+    case notconstant:
+      break;
+    case constantrange:
+      Range.~ConstantRange();
+      break;
+    };
+  }
+
+  /// Custom copy constructor, to ensure Range gets initialized when
+  /// copying a constant range lattice element.
+  ValueLatticeElement(const ValueLatticeElement &Other) : Tag(undefined) {
+    *this = Other;
+  }
+
+  /// Custom assignment operator, to ensure Range gets initialized when
+  /// assigning a constant range lattice element.
+  ValueLatticeElement &operator=(const ValueLatticeElement &Other) {
+    // If we change the state of this from constant range to non constant range,
+    // destroy Range.
+    if (isConstantRange() && !Other.isConstantRange())
+      Range.~ConstantRange();
+
+    // If we change the state of this from a valid ConstVal to another a state
+    // without a valid ConstVal, zero the pointer.
+    if ((isConstant() || isNotConstant()) && !Other.isConstant() &&
+        !Other.isNotConstant())
+      ConstVal = nullptr;
+
+    switch (Other.Tag) {
+    case constantrange:
+      if (!isConstantRange())
+        new (&Range) ConstantRange(Other.Range);
+      else
+        Range = Other.Range;
+      break;
+    case constant:
+    case notconstant:
+      ConstVal = Other.ConstVal;
+      break;
+    case overdefined:
+    case undefined:
+      break;
+    }
+    Tag = Other.Tag;
+    return *this;
+  }
 
   static ValueLatticeElement get(Constant *C) {
     ValueLatticeElement Res;
@@ -89,12 +148,12 @@ class ValueLatticeElement {
 
   Constant *getConstant() const {
     assert(isConstant() && "Cannot get the constant of a non-constant!");
-    return Val;
+    return ConstVal;
   }
 
   Constant *getNotConstant() const {
     assert(isNotConstant() && "Cannot get the constant of a non-notconstant!");
-    return Val;
+    return ConstVal;
   }
 
   const ConstantRange &getConstantRange() const {
@@ -104,10 +163,10 @@ class ValueLatticeElement {
   }
 
   Optional<APInt> asConstantInteger() const {
-    if (isConstant() && isa<ConstantInt>(Val)) {
-      return cast<ConstantInt>(Val)->getValue();
-    } else if (isConstantRange() && Range.isSingleElement()) {
-      return *Range.getSingleElement();
+    if (isConstant() && isa<ConstantInt>(getConstant())) {
+      return cast<ConstantInt>(getConstant())->getValue();
+    } else if (isConstantRange() && getConstantRange().isSingleElement()) {
+      return *getConstantRange().getSingleElement();
     }
     return None;
   }
@@ -116,6 +175,10 @@ class ValueLatticeElement {
   void markOverdefined() {
     if (isOverdefined())
       return;
+    if (isConstant() || isNotConstant())
+      ConstVal = nullptr;
+    if (isConstantRange())
+      Range.~ConstantRange();
     Tag = overdefined;
   }
 
@@ -132,7 +195,7 @@ class ValueLatticeElement {
            "Marking constant with different value");
     assert(isUndefined());
     Tag = constant;
-    Val = V;
+    ConstVal = V;
   }
 
   void markNotConstant(Constant *V) {
@@ -150,7 +213,7 @@ class ValueLatticeElement {
            "Marking !constant with different value");
     assert(isUndefined() || isConstant());
     Tag = notconstant;
-    Val = V;
+    ConstVal = V;
   }
 
   void markConstantRange(ConstantRange NewR) {
@@ -168,7 +231,7 @@ class ValueLatticeElement {
       markOverdefined();
     else {
       Tag = constantrange;
-      Range = std::move(NewR);
+      new (&Range) ConstantRange(std::move(NewR));
     }
   }