Do not fold on overflow for CABS Unop

Author: michael-schwarz

src/cil.mli

@@ -2584,6 +2584,8 @@ val mkCilint : ikind -> int64 -> cilint
 
 val mkCilintIk : ikind -> cilint -> cilint
 
+val const_if_not_overflow : exp -> ikind -> cilint -> exp
+
 (** The size of a type, in bytes. Returns a constant expression or a
  * "sizeof" expression if it cannot compute the size. This function
  * is architecture dependent, so you should only call this after you

src/frontc/cabs2cil.ml

@@ -3805,10 +3805,11 @@ and doExp (asconst: bool)   (* This expression is used as a constant *)
         let (se, e', t) = doExp asconst e (AExp None) in
         if isIntegralType t then
           let tres = integralPromotion t in
+          let fallback = UnOp(Neg, makeCastT ~e:e' ~oldt:t ~newt:tres, tres) in
           let e'' =
             match e', tres with
-            | Const(CInt(i, _, _)), TInt(ik, _) -> kintegerCilint ik (neg_cilint i)
-            | _ -> UnOp(Neg, makeCastT ~e:e' ~oldt:t ~newt:tres, tres)
+            | Const(CInt(i, _, _)), TInt(ik, _) -> const_if_not_overflow fallback ik (neg_cilint i)
+            | _ -> fallback
           in
           finishExp se e'' tres
         else