Use Treadle (on pull requests only) to speed up CI (backport #2341) (#…

…2748)

* Support using Treadle for 'sbt test'

Treadle will be used as the "defaultBackend" when the environment
variable CHISEL3_CI_USE_TREADLE is set. The intent is to set this
variable during CI for pre-merge CI (aka on pull requests).

(cherry picked from commit 7d39b7b)

* Use Treadle for CI on pull requests

(cherry picked from commit 8266067)

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

.github/workflows/test.yml

@@ -50,6 +50,9 @@ jobs:
       - name: Documentation (Scala 2.12 only)
         if: startsWith(matrix.scala, '2.12')
         run: sbt ++${{ matrix.scala }} docs/mdoc unidoc
+      - name: Use Treadle for Pull Requests
+        if: github.event_name == 'pull_request'
+        run: echo "CHISEL3_CI_USE_TREADLE=1" >> $GITHUB_ENV
       - name: Test
         run: sbt ++${{ matrix.scala }} test
       - name: Binary compatibility

src/test/scala/chiselTests/ChiselSpec.scala

@@ -38,7 +38,10 @@ trait ChiselRunners extends Assertions with BackendCompilationUtilities {
     annotations:          AnnotationSeq = Seq()
   ): Boolean = {
     // Change this to enable Treadle as a backend
-    val defaultBackend = chisel3.testers.TesterDriver.defaultBackend
+    val defaultBackend = {
+      val useTreadle = sys.env.get("CHISEL3_CI_USE_TREADLE").isDefined
+      if (useTreadle) chisel3.testers.TreadleBackend else chisel3.testers.TesterDriver.defaultBackend
+    }
     val hasBackend = TestUtils.containsBackend(annotations)
     val annos: Seq[Annotation] = if (hasBackend) annotations else defaultBackend +: annotations
     TesterDriver.execute(() => t, additionalVResources, annos)

src/test/scala/chiselTests/Vec.scala

@@ -6,7 +6,7 @@ import org.scalacheck._
 
 import chisel3._
 import chisel3.stage.ChiselStage
-import chisel3.testers.BasicTester
+import chisel3.testers.{BasicTester, TesterDriver}
 import chisel3.util._
 import org.scalacheck.Shrink
 import scala.annotation.tailrec
@@ -456,7 +456,7 @@ class VecSpec extends ChiselPropSpec with Utils {
   }
 
   property("Infering widths on huge Vecs should not cause a stack overflow") {
-    assertTesterPasses { new HugeVecTester(10000) }
+    assertTesterPasses(new HugeVecTester(10000), annotations = TesterDriver.verilatorOnly)
   }
 
   property("A Reg of a Vec of a single 1 bit element should compile and work") {