targets: print a warning message before downgrading (#13513)

* Make package preferences a soft failure for targets, instead of a hard failure.
* Added unit tests for preferences expressed via packages.yaml

lib/spack/spack/concretize.py

@@ -281,6 +281,24 @@ def concretize_architecture(self, spec):
             else:
                 # To get default platform, consider package prefs
                 if PackagePrefs.has_preferred_targets(spec.name):
+                    new_target = self.target_from_package_preferences(spec)
+                else:
+                    new_target = new_plat.target('default_target')
+
+        # Construct new architecture, compute whether spec changed
+        arch_spec = (str(new_plat), str(new_os), str(new_target))
+        new_arch = spack.spec.ArchSpec(arch_spec)
+        spec_changed = new_arch != spec.architecture
+        spec.architecture = new_arch
+        return spec_changed
+
+    def target_from_package_preferences(self, spec):
+        """Returns the preferred target from the package preferences if
+        there's any.
+
+        Args:
+            spec: abstract spec to be concretized
+        """
         target_prefs = PackagePrefs(spec.name, 'target')
         target_specs = [spack.spec.Spec('target=%s' % tname)
                         for tname in cpu.targets]
@@ -298,17 +316,8 @@ def tspec_filter(s):
         # Sort filtered targets by package prefs
         target_specs = list(filter(tspec_filter, target_specs))
         target_specs.sort(key=target_prefs)
-
         new_target = target_specs[0].architecture.target
-                else:
+        return new_target
-                    new_target = new_plat.target('default_target')
-
-        # Construct new architecture, compute whether spec changed
-        arch_spec = (str(new_plat), str(new_os), str(new_target))
-        new_arch = spack.spec.ArchSpec(arch_spec)
-        spec_changed = new_arch != spec.architecture
-        spec.architecture = new_arch
-        return spec_changed
 
     def concretize_variants(self, spec):
         """If the spec already has variants filled in, return.  Otherwise, add
@@ -526,7 +535,12 @@ def _adjust_target(self, spec):
         current_platform = spack.architecture.get_platform(
             spec.architecture.platform
         )
-        if current_target != current_platform.target('default_target') or \
+
+        default_target = current_platform.target('default_target')
+        if PackagePrefs.has_preferred_targets(spec.name):
+            default_target = self.target_from_package_preferences(spec)
+
+        if current_target != default_target or \
             (self.abstract_spec.architecture is not None and
              self.abstract_spec.architecture.target is not None):
             return False
@@ -544,6 +558,11 @@ def _adjust_target(self, spec):
                     continue
 
                 if candidate is not None:
+                    msg = ('{0.name}@{0.version} cannot build optimized '
+                           'binaries for "{1}". Using best target possible: '
+                           '"{2}"')
+                    msg = msg.format(spec.compiler, current_target, candidate)
+                    tty.warn(msg)
                     spec.architecture.target = candidate
                     return True
             else:

lib/spack/spack/test/concretize.py

@@ -11,6 +11,7 @@
 import spack.repo
 
 from spack.concretize import find_spec, NoValidVersionError
+from spack.package_prefs import PackagePrefs
 from spack.spec import Spec, CompilerSpec
 from spack.spec import ConflictsInSpecError, SpecError
 from spack.version import ver
@@ -83,13 +84,25 @@ def spec(request):
 
 
 @pytest.fixture(params=[
-    'haswell', 'broadwell', 'skylake', 'icelake'
+    # Mocking the host detection
+    'haswell', 'broadwell', 'skylake', 'icelake',
+    # Using preferred targets from packages.yaml
+    'icelake-preference', 'cannonlake-preference'
 ])
 def current_host(request, monkeypatch):
-    target = llnl.util.cpu.targets[request.param]
+    # is_preference is not empty if we want to supply the
+    # preferred target via packages.yaml
+    cpu, _, is_preference = request.param.partition('-')
+    target = llnl.util.cpu.targets[cpu]
+    if not is_preference:
         monkeypatch.setattr(llnl.util.cpu, 'host', lambda: target)
-    monkeypatch.setattr(spack.platforms.test.Test, 'default', request.param)
+        monkeypatch.setattr(spack.platforms.test.Test, 'default', cpu)
-    return target
+        yield target
+    else:
+        # There's a cache that needs to be cleared for unit tests
+        PackagePrefs._packages_config_cache = None
+        with spack.config.override('packages:all', {'target': [cpu]}):
+            yield target
 
 
 @pytest.mark.usefixtures('config', 'mock_packages')