asp: memoize the list of all target_specs to speed-up setup phase (#20473)

* asp: memoize the list of all target_specs to speed-up setup phase

* asp: memoize using a cache per solver object

lib/spack/spack/solver/asp.py

@@ -497,6 +497,9 @@ def __init__(self):
         # id for dummy variables
         self.card = 0
 
+        # Caches to optimize the setup phase of the solver
+        self.target_specs_cache = None
+
     def pkg_version_rules(self, pkg):
         """Output declared versions of a package.
 
@@ -910,10 +913,14 @@ def preferred_variants(self, pkg_name):
 
     def preferred_targets(self, pkg_name):
         key_fn = spack.package_prefs.PackagePrefs(pkg_name, 'target')
-        target_specs = [
+
+        if not self.target_specs_cache:
+            self.target_specs_cache = [
                 spack.spec.Spec('target={0}'.format(target_name))
                 for target_name in archspec.cpu.TARGETS
             ]
+
+        target_specs = self.target_specs_cache
         preferred_targets = [x for x in target_specs if key_fn(x) < 0]
         if not preferred_targets:
             return