SI-8531 Better space efficiency for patmat analysis

By adding logging to `clause`, I found that the majority of
calls provide 0 or 1 elements. In SI-7020 / 69557da55, we changed this
method to use a `LinkedHashSet` to have deterministic results
for clauses with more elements. But I suspect that this
contributes to higher memory usage from the pattern matcher.

The enclosed test case, carefully whittled down by @oxbowlakes,
used to consume an inordinate amount of memory and time.
After this patch, it is back to 2.10.4 performance.

I have run `neg/t7020.scala` in a loop and it still is deterministic.

1 files changed, 6 insertions, 2 deletions

diff --git a/src/compiler/scala/tools/nsc/transform/patmat/Solving.scala b/src/compiler/scala/tools/nsc/transform/patmat/Solving.scala
index 1902606d86..31b1ffa912 100644
--- a/src/compiler/scala/tools/nsc/transform/patmat/Solving.scala
+++ b/src/compiler/scala/tools/nsc/transform/patmat/Solving.scala
@@ -29,8 +29,12 @@ trait Solving extends Logic {
     type Clause  = collection.Set[Lit]
     // a clause is a disjunction of distinct literals
     def clause(l: Lit*): Clause = (
-      // neg/t7020.scala changes output 1% of the time, the non-determinism is quelled with this linked set
-      mutable.LinkedHashSet(l: _*)
+      if (l.lengthCompare(1) <= 0) {
+        l.toSet // SI-8531 Avoid LinkedHashSet's bulk for 0 and 1 element clauses
+      } else {
+        // neg/t7020.scala changes output 1% of the time, the non-determinism is quelled with this linked set
+        mutable.LinkedHashSet(l: _*)
+      }
     )
 
     type Lit