[clang][Interp] Check Neg ops for errors

[tbaeder]

Summary

I'm not sure what to do with the float case here, maybe it's not relevant at all.

[aaron.ballman]

clang/lib/AST/Interp/Interp.h

438–441

I'm confused -- why would negation be UB for a floating-point type? For integer types, it's a matter of not being able to represent the value when the source is INT_MIN.

[tbaeder]

clang/lib/AST/Interp/Interp.h

438–441

No idea, this was just something that came to mind when I wrote the code. If it's not an issue, that's even better can I can just remove the comment.

[aaron.ballman]

clang/lib/AST/Interp/Interp.h

438–441

I think we should probably assert that the failure only occurs for integer types and not other arithmetic types.

[aaron.ballman]

LGTM with a request to add a message to the assertion.

clang/lib/AST/Interp/Interp.h

441

clang/lib/AST/Interp/Integral.h

public:
   }
 
   static bool bitXor(Integral A, Integral B, unsigned OpBits, Integral *R) {
     *R = Integral(A.V ^ B.V);
     return false;
   }
 
   static bool neg(Integral A, Integral *R) {
+    if (Signed && A.isMin())
+      return true;
+
     *R = -A;
     return false;
   }
 
   static bool comp(Integral A, Integral *R) {
     *R = Integral(~A.V);
     return false;
   }

clang/lib/AST/Interp/Interp.h

 }
 
 //===----------------------------------------------------------------------===//
 // Neg
 //===----------------------------------------------------------------------===//
 
 template <PrimType Name, class T = typename PrimConv<Name>::T>
 bool Neg(InterpState &S, CodePtr OpPC) {
-  const T &Val = S.Stk.pop<T>();
+  const T &Value = S.Stk.pop<T>();
   T Result;
-  T::neg(Val, &Result);
 
+  if (!T::neg(Value, &Result)) {
     S.Stk.push<T>(Result);
     return true;
   }
 
+  S.Stk.push<T>(Result);
+
+  // FIXME: This code Just Works[tm] for floats, but it's probably not doing
+  //   the right thing. At least the diagnostic could be better without
+  //   the conversion to an APInt.
+
+  unsigned Bits = Value.bitWidth();
+  APSInt NegatedValue = -Value.toAPSInt(Bits + 1);
+  const Expr *E = S.Current->getExpr(OpPC);
+  QualType Type = E->getType();
+
+  if (S.checkingForUndefinedBehavior()) {
+    SmallString<32> Trunc;
+    NegatedValue.trunc(Result.bitWidth()).toString(Trunc, 10);
+    auto Loc = E->getExprLoc();
+    S.report(Loc, diag::warn_integer_constant_overflow) << Trunc << Type;
+    return true;
+  }
+
+  S.CCEDiag(E, diag::note_constexpr_overflow) << NegatedValue << Type;
+  return S.noteUndefinedBehavior();
+}
+
 enum class PushVal : bool {
   No,
   Yes,
 };
 enum class IncDecOp {
   Inc,
   Dec,
 };

clang/test/AST/Interp/literals.cpp

 
 static_assert(~0 == -1, "");
 static_assert(~1 == -2, "");
 static_assert(~-1 == 0, "");
 static_assert(~255 == -256, "");
 static_assert(~INT_MIN == INT_MAX, "");
 static_assert(~INT_MAX == INT_MIN, "");
 
+static_assert(-(1 << 31), ""); // expected-error {{not an integral constant expression}} \
+                               // expected-note {{outside the range of representable values}} \
+                               // ref-error {{not an integral constant expression}} \
+                               // ref-note {{outside the range of representable values}} \
+
+
 enum E {};
 constexpr E e = static_cast<E>(0);
 static_assert(~e == -1, "");
 
 
 constexpr int m = 10;
 constexpr const int *p = &m;
 static_assert(p != nullptr, "");