[analyzer] Introduce minor refactoring of SVal::getSubKind function

[ASDenysPetrov]

Summary

BaseMask occupies the lowest bits. Effect of applying the mask is neutralized by right shift operation, thus making it useless.
Also change mask init value from hex to bin since it's more natural for mask.

Fix: Remove a redundant bitwise operation.

P.S. It hurts my eyes everytime I see it. I gave up.

[steakhal]

I like the change.
I also proved the equivalence, just for fun really :D

from z3 import *

def proof_equality(F, G):
  s = Solver()
  s.add(Not(F == G))
  r = s.check()
  if r == unsat:
    print("proved")
  else:
    print("counterexample")
    print(s.model())


Kind = BitVec('Kind', 32)
BaseMask = BitVecVal(0b11, 32)
BaseBits = BitVecVal(2, 32)

Before = (Kind & ~BaseMask) >> BaseBits
After = Kind >> BaseBits

proof_equality(Before, After)
# prints: proved 

[vsavchenko]

Hey, nice catch!
However, I'm going to complain about commit messages again 😅 I would prefer having imperative mood in the message, something like "Refactor ..." or "Introduce minor refactoring..."

[NoQ]

That's a fair point.

[ASDenysPetrov]

@vsavchenko:

However, I'm going to complain about commit messages again 😅 I would prefer having imperative mood in the message, something like "Refactor ..." or "Introduce minor refactoring..."

Changed title.

Thank you, guys!

clang/include/clang/StaticAnalyzer/Core/PathSensitive/SVals.h

 class SVal {
 public:
   enum BaseKind {
     // The enumerators must be representable using 2 bits.
 #define BASIC_SVAL(Id, Parent) Id ## Kind,
 #define ABSTRACT_SVAL_WITH_KIND(Id, Parent) Id ## Kind,
 #include "clang/StaticAnalyzer/Core/PathSensitive/SVals.def"
   };
-  enum { BaseBits = 2, BaseMask = 0x3 };
+  enum { BaseBits = 2, BaseMask = 0b11 };
 
 protected:
   const void *Data = nullptr;
 
   /// The lowest 2 bits are a BaseKind (0 -- 3).
   ///  The higher bits are an unsigned "kind" value.
   unsigned Kind = 0;
 
public:
   Optional<T> getAs() const {
     if (!T::isKind(*this))
       return None;
     return *static_cast<const T *>(this);
   }
 
   unsigned getRawKind() const { return Kind; }
   BaseKind getBaseKind() const { return (BaseKind) (Kind & BaseMask); }
-  unsigned getSubKind() const { return (Kind & ~BaseMask) >> BaseBits; }
+  unsigned getSubKind() const { return Kind >> BaseBits; }
 
   // This method is required for using SVal in a FoldingSetNode.  It
   // extracts a unique signature for this SVal object.
   void Profile(llvm::FoldingSetNodeID &ID) const {
     ID.AddInteger((unsigned) getRawKind());
     ID.AddPointer(Data);
   }