[clang][CGExprConstant] handle unary negation on integrals

[nickdesaulniers]

Summary

Consider the statement:

int x = -1;

And the following AST:

`-VarDecl 0x55c4823a7670 <x.c:2:1, col:10> col:5 x 'int' cinit
  `-UnaryOperator 0x55c4823a7740 <col:9, col:10> 'int' prefix '-'
    `-IntegerLiteral 0x55c4823a7720 <col:10> 'int' 1

Return the evaluation of the subexpression negated.

[nickdesaulniers]

• fix oneline class

[efriedma]

clang/lib/CodeGen/CGExprConstant.cpp

1366

StmtVisitor supports defining a function "VisitUnaryMinus", so you don't have to switch() over the opcode.

[nickdesaulniers]

• prefer VisitUnaryMinus as per @efriedma

[efriedma]

LGTM

clang/lib/CodeGen/CGExprConstant.cpp

llvm::Constant *VisitObjCEncodeExpr(ObjCEncodeExpr *E, QualType T) {
     Str.resize(CAT->getSize().getZExtValue(), '\0');
     return llvm::ConstantDataArray::getString(VMContext, Str, false);
   }
 
   llvm::Constant *VisitUnaryExtension(const UnaryOperator *E, QualType T) {
     return Visit(E->getSubExpr(), T);
   }
 
+  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;
+  }
+
   // Utility methods
   llvm::Type *ConvertType(QualType T) {
     return CGM.getTypes().ConvertType(T);
   }
 };
 
 }  // end anonymous namespace.