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.

2 files changed, 29 insertions, 0 deletions

diff --git a/test/files/pos/t8531/MyEnum.java b/test/files/pos/t8531/MyEnum.java
new file mode 100644
index 0000000000..06cc128a79
--- /dev/null
+++ b/test/files/pos/t8531/MyEnum.java
@@ -0,0 +1,5 @@
+
+package foobar;
+public enum MyEnum {
+A1, A2, A3, A4, A5, A6, A7, A8, A9, A10, A11, A12, A13, A14, A15, A16, A17, A18, A19, A20, A21, A22, A23, A24, A25, A26, A27, A28, A29, A30, A31, A32, A33, A34, A35, A36, A37, A38, A39, A40, A41, A42, A43, A44, A45, A46, A47, A48, A49, A50, A51, A52, A53, A54, A55, A56, A57, A58, A59, A60, A61, A62, A63, A64, A65, A66, A67, A68, A69, A70, A71, A72, A73, A74, A75, A76, A77, A78, A79, A80, A81, A82, A83, A84, A85, A86, A87, A88, A89, A90, A91, A92, A93, A94, A95, A96, A97, A98, A99, A100, A101, A102, A103, A104, A105, A106, A107, A108, A109, A110, A111, A112, A113, A114, A115, A116, A117, A118, A119, A120, A121, A122, A123, A124, A125, A126, A127, A128, A129, A130, A131, A132, A133, A134, A135, A136, A137, A138, A139, A140, A141, A142, A143, A144, A145, A146, A147, A148, A149, A150, A151, A152, A153, A154, A155, A156, A157, A158, A159, A160, A161, A162, A163, A164, A165, A166, A167, A168, A169, A170, A171, A172, A173, A174, A175, A176, A177, A178, A179, A180, A181, A182, A183, A184, A185, A186, A187, A188, A189, A190, A191, A192, A193;
+}
diff --git a/test/files/pos/t8531/Test.scala b/test/files/pos/t8531/Test.scala
new file mode 100644
index 0000000000..59861435a6
--- /dev/null
+++ b/test/files/pos/t8531/Test.scala
@@ -0,0 +1,24 @@
+package test
+
+// takes > 50s and > 800M heap to compile under 2.11.0
+import foobar._
+class `SI-8531` {
+    //https://issues.scala-lang.org/browse/SI-8531
+
+  import MyEnum._
+  def foo(e1: MyEnum, e2: MyEnum) = (e1, e2) match {
+    case (A1, x) => "a1"
+    case (x, A1) => "a1"
+    case (A2, x) => "a2"
+    case (x, A2) => "a2"
+    case (A3, x) => "a3"
+    case (x, A3) => "a3"
+    case (A4, x) => "a4"
+    case (x, A4) => "a4"
+    case (A5, x) => "a5"
+    case (x, A5) => "a5"
+    case (A6, x) => "a6"
+    case (x, A6) => "a6"
+    case (a, b)  => "ab"
+  }
+}