Implement compressed Namespace (backport #2856) (#2860)

* Implement compressed Namespace (#2856)

The namespace disambiguates requests for the same name with _<idx>.
Rather than storing every disambiguated name in the underlying HashMap,
it now only stores the base along with the "next available" index. This
makes the logic for checking if a name is already contained in the
namespace slightly more sophisticated because users can name things in a
way that will collide with disambiguated names from a common substring.

For example, in naming the sequence "foo", "foo", "foo_1", the 2nd "foo"
takes the name "foo_1" so the following "foo_1" gets disambiguated to
"foo_1_1". But since we compressed that original "foo_1" into the same
HashMap entry as just "foo", we have to do a form of "prefix checking"
whenever naming something that ends in "_<idx>".

In practice, the saved memory allocations more than make up for the more
complicated logic to disambiguate names because the common case is still
fast.

(cherry picked from commit 1654d87a02ca799bf12805a611a91e7524d49843)

# Conflicts:
#	core/src/main/scala/chisel3/internal/Builder.scala

* Resolve backport conflicts

Co-authored-by: Jack Koenig <koenig@sifive.com>

1 files changed, 81 insertions, 0 deletions

diff --git a/src/test/scala/chisel3/internal/NamespaceSpec.scala b/src/test/scala/chisel3/internal/NamespaceSpec.scala
new file mode 100644
index 00000000..fd808ff0
--- /dev/null
+++ b/src/test/scala/chisel3/internal/NamespaceSpec.scala
@@ -0,0 +1,81 @@
+// SPDX-License-Identifier: Apache-2.0
+
+package chisel3.internal
+
+import org.scalatest.flatspec.AnyFlatSpec
+import org.scalatest.matchers.should.Matchers._
+
+class NamespaceSpec extends AnyFlatSpec {
+  behavior.of("Namespace")
+
+  they should "support basic disambiguation" in {
+    val namespace = Namespace.empty
+    val name = namespace.name(_, false)
+    name("x") should be("x")
+    name("x") should be("x_1")
+    name("x") should be("x_2")
+  }
+
+  they should "support explicit <prefix>_# names before <prefix> names" in {
+    val namespace = Namespace.empty
+    val name = namespace.name(_, false)
+    name("x_1") should be("x_1")
+    name("x_2") should be("x_2")
+    name("x") should be("x")
+    name("x") should be("x_3")
+  }
+
+  they should "support explicit <prefix>_# names in the middle of <prefix> names" in {
+    val namespace = Namespace.empty
+    val name = namespace.name(_, false)
+    name("x") should be("x")
+    name("x") should be("x_1")
+    name("x_1") should be("x_1_1")
+    name("x_2") should be("x_2")
+    name("x") should be("x_3")
+  }
+
+  // For some reason, multi-character names tickled a different failure mode than single character
+  they should "support explicit <prefix>_# names in the middle of longer <prefix> names" in {
+    val namespace = Namespace.empty
+    val name = namespace.name(_, false)
+    name("foo") should be("foo")
+    name("foo") should be("foo_1")
+    name("foo_1") should be("foo_1_1")
+    name("foo_2") should be("foo_2")
+    name("foo") should be("foo_3")
+  }
+
+  they should "support collisions in recursively growing names" in {
+    val namespace = Namespace.empty
+    val name = namespace.name(_, false)
+    name("x") should be("x")
+    name("x") should be("x_1")
+    name("x_1") should be("x_1_1")
+    name("x_1") should be("x_1_2")
+    name("x_1_1") should be("x_1_1_1")
+    name("x_1_1") should be("x_1_1_2")
+  }
+
+  they should "support collisions in recursively shrinking names" in {
+    val namespace = Namespace.empty
+    val name = namespace.name(_, false)
+    name("x_1_1") should be("x_1_1")
+    name("x_1_1") should be("x_1_1_1")
+    name("x_1") should be("x_1")
+    name("x_1") should be("x_1_2")
+    name("x") should be("x")
+    name("x") should be("x_2")
+  }
+
+  // The namespace never generates names with _0 so it's actually a false collision case
+  they should "properly handle false collisions with signals ending in _0" in {
+    val namespace = Namespace.empty
+    val name = namespace.name(_, false)
+    name("x") should be("x")
+    name("x") should be("x_1")
+    name("x_0") should be("x_0")
+    name("x") should be("x_2")
+    name("x_0") should be("x_0_1")
+  }
+}