[clang][ConstExprEmitter] handle Unary Not on integers

[nickdesaulniers]

Summary

Consider the following statement:

int x = ~0;

With the following AST:

`-VarDecl 0x5647bdb12d60 <x.c:1:1, col:10> col:5 x 'int' cinit
  `-UnaryOperator 0x5647bdb12e30 <col:9, col:10> 'int' prefix '~' cannot overflow
    `-IntegerLiteral 0x5647bdb12e10 <col:10> 'int' 0

Return the evaluation of the subexpression bitwise inverted.

Link: https://github.com/ClangBuiltLinux/linux/issues/1906

[nickdesaulniers]

clang/lib/CodeGen/CGExprConstant.cpp

1402

A lot of these visitor methods for UnaryOperator are going to look almost the same; I think I might prefer to have a VisitUnaryOperator method instead, and simply switch on the kind of operator.

But perhaps, I should flush out more of these methods, then do such refactoring? WDYT @efriedma

clang/lib/CodeGen/CGExprConstant.cpp

public:
 
   llvm::Constant *VisitUnaryMinus(UnaryOperator *U, QualType T) {
     if (llvm::Constant *C = Visit(U->getSubExpr(), T))
       if (auto *CI = dyn_cast<llvm::ConstantInt>(C))
         return llvm::ConstantInt::get(CGM.getLLVMContext(), -CI->getValue());
     return nullptr;
   }
 
+  llvm::Constant *VisitUnaryNot(UnaryOperator *U, QualType T) {
+    if (llvm::Constant *C = Visit(U->getSubExpr(), T))
+      if (auto *CI = dyn_cast<llvm::ConstantInt>(C))
+        return llvm::ConstantInt::get(CGM.getLLVMContext(), ~CI->getValue());
+    return nullptr;
+  }
+
   // Utility methods
   llvm::Type *ConvertType(QualType T) {
     return CGM.getTypes().ConvertType(T);
   }
 };
 
 }  // end anonymous namespace.