diff --git a/slothy/core/core.py b/slothy/core/core.py
index cddf4fd..113319e 100644
--- a/slothy/core/core.py
+++ b/slothy/core/core.py
@@ -1231,7 +1231,6 @@ def __init__(self, config):
         self._kernel_input_output = None
         self._pre_core_post_dict = None
         self._codesize_with_bubbles = None
-        self._register_used = None
         self._optimization_wall_time = None
         self._optimization_user_time = None
 
@@ -2201,7 +2200,7 @@ def _allow_renaming(_):
         if self.config.constraints.minimize_register_usage is not None:
             ty = self.config.constraints.minimize_register_usage
             regs = self.arch.RegisterType.list_registers(ty)
-            self._register_used = { reg : self._NewBoolVar(f"reg_used[{reg}]") for reg in regs }
+            self._model._register_used = { reg : self._NewBoolVar(f"reg_used[{reg}]") for reg in regs }
 
         # Create variables for register renaming
 
@@ -2506,9 +2505,9 @@ def _add_constraints_register_renaming(self):
             for reg in self.arch.RegisterType.list_registers(ty):
                 arr = self._model.register_usage_vars.get(reg,[])
                 if len(arr) > 0:
-                    self._model.AddMaxEquality(self._register_used[reg], arr)
+                    self._AddMaxEquality(self._model._register_used[reg], arr)
                 else:
-                    self._Add(self._register_used[reg] is False)
+                    self._Add(self._model._register_used[reg] == False)
 
         # Ensure that outputs are unambiguous
         for t in self._get_nodes(allnodes=True):
@@ -3064,8 +3063,6 @@ def _add_objective(self, force_objective=False):
         name = None
         printer = None
 
-        # We only support objectives of the form: Maximize/Minimize the sum of a set of variables.
-
         # If the number of stalls is variable, its minimization is our objective
         if force_objective is False and self.config.variable_size:
             name = "minimize cycles"
@@ -3090,8 +3087,8 @@ def _add_objective(self, force_objective=False):
                 maxlist = [ t.program_start_var for t in self._get_nodes() ]
                 name = "move stalls to top"
             elif self.config.constraints.minimize_register_usage is not None:
-                # Minimize the number of registers used
-                minlist = list(self._register_used.values())
+                minlist = list(self._model._register_used.values())
+                name = "minimize number of registers used"
             elif self.config.constraints.minimize_use_of_extra_registers is not None:
                 ty = self.config.constraints.minimize_use_of_extra_registers
                 minlist = []
@@ -3169,6 +3166,8 @@ def _AddHint(self,var,val): # pylint:disable=invalid-name
         return self._model.cp_model.AddHint(var,val)
     def _AddNoOverlap(self,interval_list): # pylint:disable=invalid-name
         return self._model.cp_model.AddNoOverlap(interval_list)
+    def _AddMaxEquality(self,varlist,var): # pylint:disable=invalid-name
+        return self._model.cp_model.AddMaxEquality(varlist, var)
 
     def _export_model(self):
         if self.config.log_model is None:
diff --git a/slothy/core/heuristics.py b/slothy/core/heuristics.py
index 79f2ad4..63acdf7 100644
--- a/slothy/core/heuristics.py
+++ b/slothy/core/heuristics.py
@@ -128,10 +128,14 @@ def optimize_binsearch(source, logger, conf, **kwargs):
             The Result object for the succceeding optimization with the smallest
             number of stalls.
         """
+        flexible = not conf.constraints.functional_only
+
         if conf.variable_size:
-            return Heuristics.optimize_binsearch_internal(source, logger, conf, **kwargs)
+            return Heuristics.optimize_binsearch_internal(source, logger, conf,
+                                                          flexible=flexible, **kwargs)
 
-        return Heuristics.optimize_binsearch_external(source, logger, conf, **kwargs)
+        return Heuristics.optimize_binsearch_external(source, logger, conf,
+                                                      flexible=flexible, **kwargs)
 
     @staticmethod
     def _log_reoptimization_failure(log):
@@ -201,7 +205,7 @@ def optimize_binsearch_external(source, logger, conf, flexible=True, **kwargs):
         return core.result
 
     @staticmethod
-    def optimize_binsearch_internal(source, logger, conf, **kwargs):
+    def optimize_binsearch_internal(source, logger, conf, flexible=True, **kwargs):
         """Internally optimize for minimum number of stalls, and potentially a secondary objective.
 
         This finds the minimum number of stalls for which a one-shot SLOTHY optimization succeeds.
@@ -213,11 +217,21 @@ def optimize_binsearch_internal(source, logger, conf, **kwargs):
             logger: The logger to be used.
             conf: The configuration to apply. This is fixed for all one-shot SLOTHY
                 runs invoked by this call, except for variation of stall count.
+            flexible: Indicates whether the number of stalls should be minimized
+                through a binary search, or whether a single one-shot SLOTHY optimization
+                for a fixed number of stalls (encoded in the configuration) should be
+                conducted.
 
         Returns:
             A Result object representing the final optimization result.
         """
 
+        if not flexible:
+            core = SlothyBase(conf.arch, conf.target, logger=logger,config=conf)
+            if not core.optimize(source):
+                raise SlothyException("Optimization failed")
+            return core.result
+
         logger.info("Perform internal binary search for minimal number of stalls...")
 
         start_attempt = conf.constraints.stalls_first_attempt