Engine heuristic: fix for assumptions unsupported by k-induction

The k-induction engine now correctly reports unsupported assumptions, and is
then skipped by the engine selection heuristic.

Fixes #921.

regression/ebmc/engine-heuristic/basic1.desc

@@ -0,0 +1,10 @@
+CORE
+basic1.sv
+
+^\[main\.a0\] always not s_eventually !main\.x: ASSUMED$
+^\[main\.p0\] always main\.x: PROVED up to bound 5$
+^EXIT=0$
+^SIGNAL=0$
+--
+^warning: ignoring
+--

regression/ebmc/engine-heuristic/basic1.sv

@@ -0,0 +1,8 @@
+module main(input clk, input x);
+
+  // not supported by k-induction
+  a0: assume property (not s_eventually !x);
+
+  p0: assert property (x);
+
+endmodule

regression/ebmc/engine-heuristic/basic2.desc

@@ -0,0 +1,11 @@
+CORE
+basic2.sv
+
+^\[main\.a0\] always not s_eventually !main\.y: UNSUPPORTED: unsupported by k-induction$
+^\[main\.a1\] always !main\.x: ASSUMED$
+^\[main\.p0\] always !main\.z: PROVED$
+^EXIT=0$
+^SIGNAL=0$
+--
+^warning: ignoring
+--

regression/ebmc/engine-heuristic/basic2.sv

@@ -0,0 +1,15 @@
+module main(input clk, input x, input y);
+
+  reg z = 0;
+  always_ff @(posedge clk) z <= z || x;
+
+  // unsupported assumption
+  a0: assume property (not s_eventually !y);
+
+  // supported assumption
+  a1: assume property (!x);
+
+  // inductive property
+  p0: assert property (!z);
+
+endmodule

regression/ebmc/k-induction/k-induction-unsupported2.desc

@@ -3,7 +3,7 @@ k-induction-unsupported2.sv
 --module main --k-induction
 ^EXIT=10$
 ^SIGNAL=0$
-^\[main\.p0\] always s_eventually main\.x == 10: FAILURE: property unsupported by k-induction$
+^\[main\.p0\] always s_eventually main\.x == 10: UNSUPPORTED: unsupported by k-induction$
 ^\[main\.p1\] always main\.x <= 10: PROVED$
 --
 ^warning: ignoring

regression/ebmc/k-induction/k-induction6.desc

@@ -1,9 +1,10 @@
-KNOWNBUG
+CORE
 k-induction6.sv
-
+--k-induction
+^\[main\.a0\] always not s_eventually !main\.x: UNSUPPORTED: unsupported by k-induction$
+^\[main\.p0\] always main\.x: INCONCLUSIVE$
 ^EXIT=10$
 ^SIGNAL=0$
 --
 ^warning: ignoring
 --
-The property should hold, but is reported as refuted.

regression/ebmc/k-induction/k-induction7.desc

@@ -0,0 +1,11 @@
+CORE
+k-induction7.sv
+--k-induction
+^\[main\.a0\] always not s_eventually !main\.y: UNSUPPORTED: unsupported by k-induction$
+^\[main\.a1\] always !main\.x: ASSUMED$
+^\[main\.p0\] always !main\.z: PROVED$
+^EXIT=0$
+^SIGNAL=0$
+--
+^warning: ignoring
+--

regression/ebmc/k-induction/k-induction7.sv

@@ -0,0 +1,15 @@
+module main(input clk, input x, input y);
+
+  reg z = 0;
+  always_ff @(posedge clk) z <= z || x;
+
+  // unsupported assumption
+  a0: assume property (not s_eventually !y);
+
+  // supported assumption
+  a1: assume property (!x);
+
+  // inductive property
+  p0: assert property (!z);
+
+endmodule

src/ebmc/bdd_engine.cpp

@@ -180,7 +180,7 @@ property_checker_resultt bdd_enginet::operator()()
   try
   {
     // any properties left?
-    if(!properties.has_unknown_property())
+    if(!properties.has_unfinished_property())
       return property_checker_resultt{properties};
 
     // possibly apply liveness-to-safety

src/ebmc/ebmc_properties.cpp

@@ -18,6 +18,8 @@ Author: Daniel Kroening, [email protected]
 
 std::string ebmc_propertiest::propertyt::status_as_string() const
 {
+  auto suffix = failure_reason.has_value() ? ": " + failure_reason.value() : "";
+
   switch(status)
   {
   case statust::ASSUMED:
@@ -31,14 +33,15 @@ std::string ebmc_propertiest::propertyt::status_as_string() const
   case statust::REFUTED_WITH_BOUND:
     return "REFUTED up to bound " + std::to_string(bound);
   case statust::UNKNOWN:
-    return "UNKNOWN";
+    return "UNKNOWN" + suffix;
+  case statust::UNSUPPORTED:
+    return "UNSUPPORTED" + suffix;
   case statust::INCONCLUSIVE:
-    return "INCONCLUSIVE";
+    return "INCONCLUSIVE" + suffix;
   case statust::FAILURE:
-    return failure_reason.has_value() ? "FAILURE: " + failure_reason.value()
-                                      : "FAILURE";
+    return "FAILURE" + suffix;
   case statust::DROPPED:
-    return "DROPPED";
+    return "DROPPED" + suffix;
   case statust::DISABLED:
   default:
     UNREACHABLE;
@@ -78,9 +81,9 @@ ebmc_propertiest ebmc_propertiest::from_transition_system(
         id2string(symbol.location.get_comment());
 
       // Don't try to prove assumption properties.
-      if(symbol.value.id() == ID_sva_assume)
+      if(properties.properties.back().is_assumption())
       {
-        properties.properties.back().status = propertyt::statust::ASSUMED;
+        properties.properties.back().assumed();
         properties.properties.back().normalized_expr =
           normalize_property(to_sva_assume_expr(symbol.value).op());
       }

src/ebmc/ebmc_properties.h

@@ -40,6 +40,7 @@ class ebmc_propertiest
       UNKNOWN,            // no work done yet
       DISABLED,           // turned off by user
       ASSUMED,            // property is assumed to be true, unbounded
+      UNSUPPORTED,        // property is unsupported
       PROVED,             // property is true, unbounded
       PROVED_WITH_BOUND,  // property is true, with bound
       REFUTED,            // property is false, possibly counterexample
@@ -98,24 +99,38 @@ class ebmc_propertiest
       return status == statust::FAILURE;
     }
 
+    bool is_unsupported() const
+    {
+      return status == statust::UNSUPPORTED;
+    }
+
     bool is_inconclusive() const
     {
       return status == statust::INCONCLUSIVE;
     }
 
+    void assumed()
+    {
+      status = statust::ASSUMED;
+      failure_reason = {};
+    }
+
     void unknown()
     {
       status = statust::UNKNOWN;
+      failure_reason = {};
     }
 
     void disable()
     {
       status = statust::DISABLED;
+      failure_reason = {};
     }
 
     void proved()
     {
       status = statust::PROVED;
+      failure_reason = {};
     }
 
     void proved_with_bound(std::size_t _bound)
@@ -127,17 +142,20 @@ class ebmc_propertiest
     void refuted()
     {
       status = statust::REFUTED;
+      failure_reason = {};
     }
 
     void refuted_with_bound(std::size_t _bound)
     {
       status = statust::REFUTED_WITH_BOUND;
       bound = _bound;
+      failure_reason = {};
     }
 
     void drop()
     {
       status = statust::DROPPED;
+      failure_reason = {};
     }
 
     void failure(const std::optional<std::string> &reason = {})
@@ -146,9 +164,16 @@ class ebmc_propertiest
       failure_reason = reason;
     }
 
+    void unsupported(const std::optional<std::string> &reason = {})
+    {
+      status = statust::UNSUPPORTED;
+      failure_reason = reason;
+    }
+
     void inconclusive()
     {
       status = statust::INCONCLUSIVE;
+      failure_reason = {};
     }
 
     std::string status_as_string() const;
@@ -164,16 +189,30 @@ class ebmc_propertiest
     {
       return ::is_exists_path(original_expr);
     }
+
+    bool is_assumption() const
+    {
+      return original_expr.id() == ID_sva_assume;
+    }
   };
 
   typedef std::list<propertyt> propertiest;
   propertiest properties;
 
-  bool has_unknown_property() const
+  bool has_unfinished_property() const
   {
     for(const auto &p : properties)
-      if(p.is_unknown())
+    {
+      if(p.is_assumption())
+      {
+      }
+      else if(
+        p.is_unknown() || p.is_unsupported() || p.is_failure() ||
+        p.is_inconclusive())
+      {
         return true;
+      }
+    }
 
     return false;
   }
@@ -209,12 +248,40 @@ class ebmc_propertiest
     return result;
   }
 
-  void reset_failure_to_unknown()
+  /// Resets properties/assumptions in FAILURE state to
+  /// ASSUMED/UNKNOWN respectively.
+  void reset_failure()
   {
     for(auto &p : properties)
       if(p.is_failure())
+      {
+        if(p.is_assumption())
+          p.assumed();
+        else
+          p.unknown();
+      }
+  }
+
+  /// Resets properties in INCONCLUSIVE state to UNKNOWN.
+  void reset_inconclusive()
+  {
+    for(auto &p : properties)
+      if(p.is_inconclusive())
         p.unknown();
   }
+
+  /// Resets properties in UNSUPPORTED state to UNKNOWN/ASSUMED.
+  void reset_unsupported()
+  {
+    for(auto &p : properties)
+      if(p.is_unsupported())
+      {
+        if(p.is_assumption())
+          p.assumed();
+        else
+          p.unknown();
+      }
+  }
 };
 
 #endif // CPROVER_EBMC_PROPERTIES_H

src/ebmc/k_induction.cpp

@@ -170,16 +170,42 @@ Function: k_inductiont::operator()
 
 void k_inductiont::operator()()
 {
+  // Unsupported assumption? Mark as such.
+  bool assumption_unsupported = false;
+  for(auto &property : properties.properties)
+  {
+    if(!supported(property) && property.is_assumed())
+    {
+      assumption_unsupported = true;
+      property.unsupported("unsupported by k-induction");
+    }
+  }
+
   // Fail unsupported properties
   for(auto &property : properties.properties)
   {
-    if(!supported(property))
-      property.failure("property unsupported by k-induction");
+    if(
+      !supported(property) && !property.is_assumed() && !property.is_disabled())
+    {
+      property.unsupported("unsupported by k-induction");
+    }
   }
 
   // do induction base
   induction_base();
 
+  // Any refuted properties are really inconclusive if there are
+  // unsupported assumptions, as the assumption might have
+  // proven the property.
+  if(assumption_unsupported)
+  {
+    for(auto &property : properties.properties)
+    {
+      if(property.is_refuted())
+        property.unknown();
+    }
+  }
+
   // do induction step
   induction_step();
 }
@@ -231,8 +257,12 @@ void k_inductiont::induction_step()
 
   for(auto &p_it : properties.properties)
   {
-    if(p_it.is_disabled() || p_it.is_failure() || p_it.is_assumed())
+    if(
+      p_it.is_disabled() || p_it.is_failure() || p_it.is_assumed() ||
+      p_it.is_unsupported())
+    {
       continue;
+    }
 
     // If it's not failed, then it's supported.
     DATA_INVARIANT(supported(p_it), "property must be supported");