concretizer: handle target preferences from packages.yaml

The weight of the target used in concretization is, in order:
1. A specific per package weight, if set in packages.yaml
2. Inherited from the parent, if possible
3. The default target weight (always set)

lib/spack/spack/solver/asp.py

@@ -997,14 +997,13 @@ def external_packages(self):
             self.gen.out.write(external_rule)
             self.gen.control.add("base", [], external_rule)
 
-    def concretization_preferences(self, pkg_name):
+    def preferred_variants(self, pkg_name):
         """Facts on concretization preferences, as read from packages.yaml"""
         preferences = spack.package_prefs.PackagePrefs
         preferred_variants = preferences.preferred_variants(pkg_name)
         if not preferred_variants:
             return
 
-        self.gen.h2('Concretization preferences {0}'.format(pkg_name))
         for variant_name in sorted(preferred_variants):
             variant = preferred_variants[variant_name]
             values = variant.value
@@ -1017,6 +1016,21 @@ def concretization_preferences(self, pkg_name):
                     pkg_name, variant.name, value
                 ))
 
+    def preferred_targets(self, pkg_name):
+        key_fn = spack.package_prefs.PackagePrefs(pkg_name, 'target')
+        target_specs = [
+            spack.spec.Spec('target={0}'.format(target_name))
+            for target_name in llnl.util.cpu.targets
+        ]
+        preferred_targets = [x for x in target_specs if key_fn(x) < 0]
+        if not preferred_targets:
+            return
+
+        preferred = preferred_targets[0]
+        self.gen.fact(fn.package_target_weight(
+            str(preferred.architecture.target), pkg_name, -10
+        ))
+
     def flag_defaults(self):
         self.gen.h2("Compiler flag defaults")
 
@@ -1193,8 +1207,6 @@ def target_defaults(self, specs):
         for compiler in sorted(compilers):
             supported = self._supported_targets(compiler, compatible_targets)
 
-#            print("   ", compiler, "supports", [t.name for t in supported])
-
             if not supported:
                 continue
 
@@ -1227,10 +1239,10 @@ def target_defaults(self, specs):
             # prefer best possible targets; weight others poorly so
             # they're not used unless set explicitly
             if target.name in best_targets:
-                self.gen.fact(fn.target_weight(target.name, i))
+                self.gen.fact(fn.default_target_weight(target.name, i))
                 i += 1
             else:
-                self.gen.fact(fn.target_weight(target.name, 100))
+                self.gen.fact(fn.default_target_weight(target.name, 100))
 
             self.gen.newline()
 
@@ -1394,7 +1406,8 @@ def setup(self, driver, specs):
         for pkg in sorted(pkgs):
             self.gen.h2('Package: %s' % pkg)
             self.pkg_rules(pkg)
-            self.concretization_preferences(pkg)
+            self.preferred_variants(pkg)
+            self.preferred_targets(pkg)
 
         self.gen.h1('Spec Constraints')
         for spec in sorted(specs):

lib/spack/spack/solver/concretize.lp

@@ -235,6 +235,22 @@ node_os(Package, OS)
 % one target per node -- optimization will pick the "best" one
 1 { node_target(Package, Target) : target(Target) } 1 :- node(Package).
 
+% The target weight is either the default target weight
+% or a more specific per-package weight if set
+target_weight(Target, Package, Weight)
+  :- default_target_weight(Target, Weight),
+     node(Package),
+     not derive_target_from_parent(_, Package),
+     not package_target_weight(Target, Package, _).
+
+target_weight(Target, Dependency, Weight)
+  :- depends_on(Package, Dependency),
+     derive_target_from_parent(Package, Dependency),
+     target_weight(Target, Package, Weight).
+
+target_weight(Target, Package, Weight)
+  :- package_target_weight(Target, Package, Weight).
+
 % can't use targets on node if the compiler for the node doesn't support them
 :- node_target(Package, Target),
    not compiler_supports_target(Compiler, Version, Target),
@@ -247,7 +263,9 @@ node_target(Package, Target)
 
 % each node has the weight of its assigned target
 node_target_weight(Package, Weight)
- :- node(Package), node_target(Package, Target), target_weight(Target, Weight).
+ :- node(Package),
+    node_target(Package, Target),
+    target_weight(Target, Package, Weight).
 
 % compatibility rules for targets among nodes
 node_target_match_pref(Package, Target) :- node_target_set(Package, Target).
@@ -257,7 +275,12 @@ node_target_match_pref(Dependency, Target)
 node_target_match(Package, 1)
   :- node_target(Package, Target), node_target_match_pref(Package, Target).
 
+derive_target_from_parent(Parent, Package)
+  :- depends_on(Parent, Package), not package_target_weight(_, Package, _).
+
+
 #defined node_target_set/2.
+#defined package_target_weight/3.
 
 %-----------------------------------------------------------------------------
 % Compiler semantics
@@ -422,7 +445,7 @@ root(Dependency, 1) :- not root(Dependency), node(Dependency).
 % fastest target for node
 
 % TODO: if these are slightly different by compiler (e.g., skylake is
-% best, gcc supports skylake and broadweell, clang's best is haswell)
+% best, gcc supports skylake and broadwell, clang's best is haswell)
 % things seem to get really slow.
 #maximize{ Weight@5,Package : node_target_match(Package, Weight) }.
 #minimize{ Weight@4,Package : node_target_weight(Package, Weight) }.