concretizer: unify handling of single- and multi-valued variants

Multi-valued variants previously maximized default values to handle
cases where the default contained two values, e.g.:

    variant("foo", default="bar,baz")

This is because previously we were minimizing non-default values, and
`foo=bar`, `foo=baz`, and `foo=bar,baz` all had the same score, as
none of them had any "non-default" values.

This commit changes the approach and considers a non-default value
to be either a value set to something not default *or* the absence
of a default value from the set value.  This allows multi- and
single-valued variants to be handled the same way, with the same
minimization criterion.  It alse means that the "best" value for every
optimization criterion is now zero, which allows us to make useful
assumptions about the optimization criteria.

1 files changed, 5 insertions, 18 deletions

diff --git a/lib/spack/spack/solver/concretize.lp b/lib/spack/spack/solver/concretize.lp
index 2cb1512a33..cab54b9bd0 100644
--- a/lib/spack/spack/solver/concretize.lp
+++ b/lib/spack/spack/solver/concretize.lp
@@ -429,9 +429,11 @@ variant_value(Package, Variant, Value)
 % whenever possible. If a variant is set in a spec, or if it is
 % specified in an external, we score it as if it was a default value.
 variant_not_default(Package, Variant, Value, 1)
- :- variant_value(Package, Variant, Value),
-    not variant_default_value(Package, Variant, Value),
-    not variant_set(Package, Variant, Value),
+ :- variant_value(Package, Variant, Actual),
+    not variant_value(Package, Variant, Value),
+    variant_default_value(Package, Variant, Value),
+    Actual != Value,
+    not variant_set(Package, Variant, Actual),
     not external_with_variant_set(Package, Variant, Value),
     node(Package).
 
@@ -937,21 +939,6 @@ opt_criterion(5, "version badness").
       build_priority(Package, Priority)
 }.
 
-% If the value is a multivalued variant there could be multiple
-% values set as default. Since a default value has a weight of 0 we
-% need to maximize their number below to ensure they're all set
-opt_criterion(104, "(build) number of values in multi valued variants (non-root)").
-opt_criterion(4, "number of values in multi valued variants (non-root)").
-#minimize{ 0@104 : #true }.
-#minimize{ 0@4 : #true }.
-#maximize {
-    1@4+Priority,Package,Variant,Value
-    : variant_not_default(Package, Variant, Value, _),
-      not variant_single_value(Package, Variant),
-      not root(Package),
-      build_priority(Package, Priority)
-}.
-
 % Try to use preferred compilers
 opt_criterion(103, "(build) non-preferred compilers").
 opt_criterion(3, "non-preferred compilers").