[svsim] Fix initial ordering issue (#4614)

Fix a bug in svsim where the entire simulation is run inside an `initial`
block from a generated testbench.  This creates undefined simulation
ordering with `initial` blocks in the simulated modules.  Fix this by
switching to an explicit state machine inside the testbench.

Fixes #3962.

Signed-off-by: Schuyler Eldridge <[email protected]>

Author: seldridge

svsim/src/main/scala/Workspace.scala

@@ -141,8 +141,14 @@ final class Workspace(
       }
       l("  // Simulation")
       l("  import \"DPI-C\" context task simulation_body();")
-      l("  initial begin")
-      l("    simulation_body();")
+      l("  enum {INIT, RUN, DONE} simulationState = INIT;")
+      l("  initial")
+      l("    simulationState = RUN;")
+      l("  always @(simulationState) begin")
+      l("    if (simulationState == RUN) begin")
+      l("      simulation_body();")
+      l("      simulationState = DONE;")
+      l("    end")
       l("  end")
       l("  `ifdef ", Backend.HarnessCompilationFlags.supportsDelayInPublicFunctions)
       l("  export \"DPI-C\" task run_simulation;")

svsim/src/test/resources/Initial.sv

@@ -0,0 +1,12 @@
+// SPDX-License-Identifier: Apache-2.0
+
+module Initial(output b);
+
+  reg a = 1'b0;
+
+  assign b = a;
+
+  initial
+    a = 1'b1;
+
+endmodule

svsim/src/test/scala/BackendSpec.scala

@@ -6,6 +6,7 @@ import org.scalatest.funspec.AnyFunSpec
 import org.scalatest.matchers.must.Matchers
 import svsim._
 import java.io.{BufferedReader, FileReader}
+import svsimTests.Resources.TestWorkspace
 
 class VCSSpec extends BackendSpec {
   import vcs.Backend.CompilationSettings._
@@ -272,6 +273,29 @@ trait BackendSpec extends AnyFunSpec with Matchers {
           }
         }
       }
+
+      it("handles initial statements correctly (#3962)") {
+        workspace.reset()
+        workspace.elaborateInitialTest()
+        workspace.generateAdditionalSources()
+        simulation = workspace.compile(
+          backend
+        )(
+          workingDirectoryTag = name,
+          commonSettings = CommonCompilationSettings(),
+          backendSpecificSettings = compilationSettings,
+          customSimulationWorkingDirectory = None,
+          verbose = false
+        )
+        simulation.run(
+          verbose = false,
+          executionScriptLimit = None
+        ) { controller =>
+          controller.port("b").check(isSigned = false) { value =>
+            assert(value.asBigInt === 1)
+          }
+        }
+      }
     }
   }
 }

svsim/src/test/scala/Resources.scala

@@ -116,5 +116,20 @@ object Resources {
         )
       )
     }
+    def elaborateInitialTest(): Unit = {
+      workspace.addPrimarySourceFromResource(getClass, "/Initial.sv")
+      workspace.elaborate(
+        ModuleInfo(
+          name = "Initial",
+          ports = Seq(
+            new ModuleInfo.Port(
+              name = "b",
+              isSettable = false,
+              isGettable = true
+            )
+          )
+        )
+      )
+    }
   }
 }