Compute the loop nest depth of each CfgNode and weight Variables by it.

src/IceOperand.h

Index: src/IceOperand.h
diff --git a/src/IceOperand.h b/src/IceOperand.h
index 946f47dc7a630a43ec3a20c52f52fe34eff96aeb..9638066ec89f157fde02511aa98edc716fc595e6 100644
--- a/src/IceOperand.h
+++ b/src/IceOperand.h
@@ -321,13 +321,15 @@ public:
   explicit RegWeight(uint32_t Weight) : Weight(Weight) {}
   RegWeight(const RegWeight &) = default;
   RegWeight &operator=(const RegWeight &) = default;
-  const static uint32_t Inf = ~0; /// Force regalloc to give a register
-  const static uint32_t Zero = 0; /// Force regalloc NOT to give a register
+  const static uint32_t Inf = ~0;      /// Force regalloc to give a register
+  const static uint32_t Zero = 0;      /// Force regalloc NOT to give a register
+  const static uint32_t Max = Inf - 1; /// Max natural weight.
   void addWeight(uint32_t Delta) {
     if (Delta == Inf)
       Weight = Inf;
     else if (Weight != Inf)
-      Weight += Delta;
+      if (Utils::add_overflow(Weight, Delta, &Weight) || Weight == Inf)

[Jim Stichnoth, 2015/09/03 23:23:30]

This seems to have the interesting property that if Weight is Max, and Delta is
1, then the new Weight is actually Inf, whereas we probably want it to stay Max,
right?

[ascull, 2015/09/04 00:23:51]

In that case Max + 1 = Inf which does not overflow but does == Inf so it will be
set to Max as it matches the second part of the logical or. maybe the confusion
is that add_overflow is actually doing the addition before comparing to Inf. I
believe this is defied behavior with the shortcut or?

[Jim Stichnoth, 2015/09/04 00:47:26]

OK, thanks, I'm convinced now.

+        Weight = Max;
   }
   void addWeight(const RegWeight &Other) { addWeight(Other.Weight); }
   void setWeight(uint32_t Val) { Weight = Val; }
@@ -579,7 +581,7 @@ public:
   const Inst *getSingleDefinition() const;
   const InstDefList &getLatterDefinitions() const { return Definitions; }
   CfgNode *getNode() const { return SingleUseNode; }
-  uint32_t getUseWeight() const { return UseWeight; }
+  RegWeight getUseWeight() const { return UseWeight; }
   void markUse(MetadataKind TrackingKind, const Inst *Instr, CfgNode *Node,
                bool IsImplicit);
   void markDef(MetadataKind TrackingKind, const Inst *Instr, CfgNode *Node);
@@ -594,7 +596,7 @@ private:
   InstDefList Definitions; /// Only used if Kind==VMK_All
   const Inst *FirstOrSingleDefinition =
       nullptr; /// Is a copy of Definitions[0] if Kind==VMK_All
-  uint32_t UseWeight = 0;
+  RegWeight UseWeight;
 };
 
 /// VariablesMetadata analyzes and summarizes the metadata for the complete set
@@ -649,7 +651,7 @@ public:
 
   /// Returns the total use weight computed as the sum of uses multiplied by a
   /// loop nest depth factor for each use.
-  uint32_t getUseWeight(const Variable *Var) const;
+  RegWeight getUseWeight(const Variable *Var) const;
 
 private:
   const Cfg *Func;