Add support for optimizing exists-forall problems.

Modifies smt2 backend to recognize `$anyconst` etc. assigned to a wire with the `maximize` or `minimize` attribute and emit `; yosys-smt2-maximize` or `; yosys-smt2-minimize` directives as appropriate.
Modifies `backends/smt2/smtbmc.py` and `smtio.py` to recognize those directives and emit a `(maximize ...)` or `(minimize ...)` command at the end of `smt_forall_assert()`, as described in the paper "νZ - An Optimizing SMT Solver" by Nikolaj Bjørner et al.
Adds an example `examples/smtbmc/demo9.v` to show how it can be used.

3 files changed, 33 insertions, 1 deletions

diff --git a/backends/smt2/smt2.cc b/backends/smt2/smt2.cc
index 081dcda99..3e08ce37b 100644
--- a/backends/smt2/smt2.cc
+++ b/backends/smt2/smt2.cc
@@ -536,6 +536,14 @@ struct Smt2Worker
 				if (cell->attributes.count("\\reg"))
 					infostr += " " + cell->attributes.at("\\reg").decode_string();
 				decls.push_back(stringf("; yosys-smt2-%s %s#%d %d %s\n", cell->type.c_str() + 1, get_id(module), idcounter, GetSize(cell->getPort("\\Y")), infostr.c_str()));
+				if (cell->getPort("\\Y").is_wire() && cell->getPort("\\Y").as_wire()->get_bool_attribute("\\maximize")){
+					decls.push_back(stringf("; yosys-smt2-maximize %s#%d\n", get_id(module), idcounter));
+					log("Wire %s is maximized\n", cell->getPort("\\Y").as_wire()->name.str().c_str());
+				}
+				else if (cell->getPort("\\Y").is_wire() && cell->getPort("\\Y").as_wire()->get_bool_attribute("\\minimize")){
+					decls.push_back(stringf("; yosys-smt2-minimize %s#%d\n", get_id(module), idcounter));
+					log("Wire %s is minimized\n", cell->getPort("\\Y").as_wire()->name.str().c_str());
+				}
 				makebits(stringf("%s#%d", get_id(module), idcounter), GetSize(cell->getPort("\\Y")), log_signal(cell->getPort("\\Y")));
 				if (cell->type == "$anyseq")
 					ex_input_eq.push_back(stringf("  (= (|%s#%d| state) (|%s#%d| other_state))", get_id(module), idcounter, get_id(module), idcounter));
diff --git a/backends/smt2/smtbmc.py b/backends/smt2/smtbmc.py
index 3d6d3e1b3..630464419 100644
--- a/backends/smt2/smtbmc.py
+++ b/backends/smt2/smtbmc.py
@@ -1158,6 +1158,8 @@ def smt_forall_assert():
     global asserts_cache_dirty
     asserts_cache_dirty = False
 
+    assert (len(smt.modinfo[topmod].maximize) + len(smt.modinfo[topmod].minimize) <= 1)
+
     def make_assert_expr(asserts_cache):
         expr = list()
         for lst in asserts_cache:
@@ -1236,6 +1238,18 @@ def smt_forall_assert():
 
     smt.write("".join(assert_expr))
 
+    if len(smt.modinfo[topmod].maximize) > 0:
+        for s in states:
+            if s in used_states_db:
+                smt.write("(maximize (|%s| %s))\n" % (smt.modinfo[topmod].maximize.copy().pop(), s))
+                break
+
+    if len(smt.modinfo[topmod].minimize) > 0:
+        for s in states:
+            if s in used_states_db:
+                smt.write("(minimize (|%s| %s))\n" % (smt.modinfo[topmod].minimize.copy().pop(), s))
+                break
+
 def smt_push():
     global asserts_cache_dirty
     asserts_cache_dirty = True
diff --git a/backends/smt2/smtio.py b/backends/smt2/smtio.py
index 34bf7ef38..4c691716e 100644
--- a/backends/smt2/smtio.py
+++ b/backends/smt2/smtio.py
@@ -101,6 +101,8 @@ class SmtModInfo:
         self.cells = dict()
         self.asserts = dict()
         self.covers = dict()
+        self.maximize = set()
+        self.minimize = set()
         self.anyconsts = dict()
         self.anyseqs = dict()
         self.allconsts = dict()
@@ -502,6 +504,12 @@ class SmtIo:
         if fields[1] == "yosys-smt2-cover":
             self.modinfo[self.curmod].covers["%s_c %s" % (self.curmod, fields[2])] = fields[3]
 
+        if fields[1] == "yosys-smt2-maximize":
+            self.modinfo[self.curmod].maximize.add(fields[2])
+
+        if fields[1] == "yosys-smt2-minimize":
+            self.modinfo[self.curmod].minimize.add(fields[2])
+
         if fields[1] == "yosys-smt2-anyconst":
             self.modinfo[self.curmod].anyconsts[fields[2]] = (fields[4], None if len(fields) <= 5 else fields[5])
             self.modinfo[self.curmod].asize[fields[2]] = int(fields[3])
@@ -696,7 +704,9 @@ class SmtIo:
                     if msg is not None:
                         print("%s waiting for solver (%s)" % (self.timestamp(), msg), flush=True)
 
-        result = self.read()
+        result = ""
+        while result not in ["sat", "unsat"]:
+            result = self.read()
 
         if self.debug_file:
             print("(set-info :status %s)" % result, file=self.debug_file)