add check for INT_MIN % -1

INT_MIN % -1 is, in principle, defined to be zero in
ANSI C, C99, C++98, and C++11. Most compilers, however,
fail to produce zero, and in some cases generate an exception.
C11 explicitly makes this case undefined.

This commit adds a check for this particular case, classified
as signed overflow.

Author: kroening

regression/cbmc/overflow/mod_overflow.c

@@ -0,0 +1,13 @@
+#include <limits.h>
+
+int main()
+{
+  signed int x, y;
+
+  // this is undefined in C11
+  y = -1;
+  x = INT_MIN % y;
+
+  // always ok
+  x = y % 2;
+}

regression/cbmc/overflow/mod_overflow.desc

@@ -0,0 +1,10 @@
+CORE
+mod_overflow.c
+--signed-overflow-check
+^EXIT=10$
+^SIGNAL=0$
+^\[.*\] line 9 result of signed mod is not representable in .*: FAILURE$
+^VERIFICATION FAILED$
+--
+^\[.*\] line 12 result of signed mod is not representable in .*: FAILURE$
+^warning: ignoring

src/analyses/goto_check.cpp

@@ -104,6 +104,7 @@ class goto_checkt
   void bounds_check(const index_exprt &, const guardt &);
   void div_by_zero_check(const div_exprt &, const guardt &);
   void mod_by_zero_check(const mod_exprt &, const guardt &);
+  void mod_overflow_check(const mod_exprt &, const guardt &);
   void undefined_shift_check(const shift_exprt &, const guardt &);
   void pointer_rel_check(const exprt &, const guardt &);
   void pointer_overflow_check(const exprt &, const guardt &);
@@ -348,6 +349,39 @@ void goto_checkt::mod_by_zero_check(
     guard);
 }
 
+/// check a mod expression for the case INT_MIN % -1
+void goto_checkt::mod_overflow_check(const mod_exprt &expr, const guardt &guard)
+{
+  if(!enable_signed_overflow_check)
+    return;
+
+  const auto &type = expr.type();
+
+  if(type.id() == ID_signedbv)
+  {
+    // INT_MIN % -1 is, in principle, defined to be zero in
+    // ANSI C, C99, C++98, and C++11. Most compilers, however,
+    // fail to produce 0, and in some cases generate an exception.
+    // C11 explicitly makes this case undefined.
+
+    notequal_exprt int_min_neq(
+      expr.op0(), to_signedbv_type(type).smallest_expr());
+
+    notequal_exprt minus_one_neq(
+      expr.op1(), from_integer(-1, expr.op1().type()));
+
+    add_guarded_claim(
+      or_exprt(int_min_neq, minus_one_neq),
+      "result of signed mod is not representable",
+      "overflow",
+      expr.find_source_location(),
+      expr,
+      guard);
+  }
+
+  return;
+}
+
 void goto_checkt::conversion_check(
   const exprt &expr,
   const guardt &guard)
@@ -563,11 +597,6 @@ void goto_checkt::integer_overflow_check(
 
     return;
   }
-  else if(expr.id()==ID_mod)
-  {
-    // these can't overflow
-    return;
-  }
   else if(expr.id()==ID_unary_minus)
   {
     if(type.id()==ID_signedbv)
@@ -1543,6 +1572,7 @@ void goto_checkt::check_rec(const exprt &expr, guardt &guard, bool address)
   else if(expr.id()==ID_mod)
   {
     mod_by_zero_check(to_mod_expr(expr), guard);
+    mod_overflow_check(to_mod_expr(expr), guard);
   }
   else if(expr.id()==ID_plus || expr.id()==ID_minus ||
           expr.id()==ID_mult ||