[CSSPGO][llvm-profgen] Fix an issue in findDisjointRanges

[hoy]

Summary

We were using 0 as an indicator of invalid offset when computing disjoint ranges. In reality, 0 can be an valid code offset which stands for the first function in .text section. I'm using UINT64_MAX as an invalid code offset instead.

[wenlei]

lgtm, thanks for the fix.

llvm/tools/llvm-profgen/ProfileGenerator.cpp

if (Boundaries.find(Begin) == Boundaries.end())
       Boundaries[Begin] = BoundaryPoint();
     Boundaries[Begin].addBeginCount(Count);
 
     if (Boundaries.find(End) == Boundaries.end())
       Boundaries[End] = BoundaryPoint();
     Boundaries[End].addEndCount(Count);
   }
 
-  uint64_t BeginAddress = 0;
+  uint64_t BeginAddress = UINT64_MAX;
   int Count = 0;
   for (auto Item : Boundaries) {
     uint64_t Address = Item.first;
     BoundaryPoint &Point = Item.second;
     if (Point.BeginCount) {
-      if (BeginAddress)
+      if (BeginAddress != UINT64_MAX)
         DisjointRanges[{BeginAddress, Address - 1}] = Count;
       Count += Point.BeginCount;
       BeginAddress = Address;
     }
     if (Point.EndCount) {
-      assert(BeginAddress && "First boundary point cannot be 'end' point");
+      assert((BeginAddress != UINT64_MAX) &&
+             "First boundary point cannot be 'end' point");
       DisjointRanges[{BeginAddress, Address}] = Count;
       Count -= Point.EndCount;
       BeginAddress = Address + 1;
     }
   }
 }
 
 FunctionSamples &