ChiselSim `simulate` hangs when testing >2802 inputs

[gdnmhr]

Type of issue: Bug Report

Please provide the steps to reproduce the problem:

1. Put this in build.sbt

scalaVersion := "2.13.18"
scalacOptions ++= Seq(
  "-encoding", "UTF-8",
)

val chiselVersion = "7.5.0"
addCompilerPlugin("org.chipsalliance" % "chisel-plugin" % chiselVersion cross CrossVersion.full)

libraryDependencies += "org.chipsalliance" % "chisel_2.13" % chiselVersion
libraryDependencies +=  "org.chipsalliance" % "chisel-plugin_2.13.18" % chiselVersion

2. And this in src/main/scala/Fail.scala:

import chisel3._
import chisel3.util._
import chisel3.simulator.EphemeralSimulator._

import scala.language.reflectiveCalls

object Fail extends App {

// Print test duration on pass when using simulate()
def simulate[T <: chisel3.RawModule](dut: => T)(body: (T) => Unit): Unit  = {
  val start = System.currentTimeMillis()
  chisel3.simulator.EphemeralSimulator.simulate(dut)(body)
  val end = System.currentTimeMillis()
  val duration = end - start
  println(s"PASS ($duration ms)")
}

class Incrementer(bitWidth: Int) extends Module {
  val io = IO(new Bundle {
    val a = Input(UInt(bitWidth.W))
    val c = Input(Bool())
    val out = Output(UInt((bitWidth+1).W))
  })

  io.out := Mux(io.c, io.a.zext.asUInt + 1.U, io.a.zext.asUInt)
}

def randomIncrementerTest(
        bitWidth: Int,
        numberOfTests: Int
    ): Unit = {
    simulate(new Incrementer(bitWidth)) { c =>
        // val rand = new scala.util.Random(987654321L)
        for (i <- 1 to numberOfTests) {
            val a = BigInt(1) //BigInt(bitWidth, rand)
            c.io.a.poke(a.U)
            c.io.c.poke(1.B)
            c.io.out.expect((a+1).U((bitWidth+1).W))
            c.io.c.poke(0.B)
            c.io.out.expect(a.U((bitWidth+1).W))
            if (i % 100 == 0) println(s"Progress $i of $numberOfTests")
        }
        println("Loop done.")
    }
    println("Success!")
}
randomIncrementerTest(8, 2802)
randomIncrementerTest(8, 2803)
}

3. Run sbt run
4. Observe that the first call terminates successfully while the second call hangs at Loop done..

What is the current behavior?

The simulation with 2802 inputs finishes in ~3s on my machine, the simulation with 2803 inputs hangs.

What is the expected behavior?

The simulation would take slightly longer, but would still complete within seconds.

Please tell us about your environment:

• Chisel: 7.5.0
• sbt: 1.11.4
• Java: 21.0.9
• Scala: 2.13.18
• OS: Linux 6.8.0-90-generic # 91-Ubuntu SMP PREEMPT_DYNAMIC Tue Nov 18 14:14:30 UTC 2025 x86_64 x86_64 x86_64 GNU/Linux

Other Information

What is the use case for changing the behavior?

[seldridge]

Weird... Here's a slightly simpler example that doesn't use EphemeralSimulator. That really should be deprecated as it provides almost zero ability to debug what is going on as everything is written to temporary areas:

//> using repository https://central.sonatype.com/repository/maven-snapshots
//> using scala 2.13.18
//> using dep org.chipsalliance::chisel:7.5.0
//> using plugin org.chipsalliance:::chisel-plugin:7.5.0
//> using options -unchecked -deprecation -language:reflectiveCalls -feature -Xcheckinit
//> using options -Xfatal-warnings -Ywarn-dead-code -Ywarn-unused -Ymacro-annotations

import chisel3._
import chisel3.simulator.ChiselSim

class Incrementer(bitWidth: Int) extends Module {
  val io = IO(new Bundle {
    val a = Input(UInt(bitWidth.W))
    val c = Input(Bool())
    val out = Output(UInt((bitWidth + 1).W))
  })

  io.out := Mux(io.c, io.a.zext.asUInt + 1.U, io.a.zext.asUInt)
}

object Fail extends App with ChiselSim {

  // Print test duration on pass when using simulate()
  def simulateWithDuration[T <: chisel3.RawModule](
      dut: => T
  )(body: (T) => Unit): Unit = {
    val start = System.currentTimeMillis()
    simulateRaw(dut)(body)
    val end = System.currentTimeMillis()
    val duration = end - start
    println(s"PASS ($duration ms)")
  }

  def randomIncrementerTest(
      bitWidth: Int,
      numberOfTests: Int
  ): Unit = {
    simulateWithDuration(new Incrementer(bitWidth)) { c =>
      // val rand = new scala.util.Random(987654321L)
      for (i <- 1 to numberOfTests) {
        val a = BigInt(1) // BigInt(bitWidth, rand)
        c.io.a.poke(a.U)
        c.io.c.poke(1.B)
        c.io.out.expect((a + 1).U((bitWidth + 1).W))
        c.io.c.poke(0.B)
        c.io.out.expect(a.U((bitWidth + 1).W))
        if (i % 100 == 0) println(s"Progress $i of $numberOfTests")
      }
      println("Loop done.")
    }
    println("Success!")
  }
  randomIncrementerTest(8, 2802)
  randomIncrementerTest(8, 2803)
}

What I'm seeing:

1. If I run this locally on macOS w/ Java 22, there is a hang.
2. If I run this on a Linux box w/ Java 17, there is a hang.
3. If I remove the timing information and just use simulateRaw directly, I don't see a hang on Linux / Java 17.

If I go into the build area for that test and manually rerun things with make replay, I see e Could not read next command when rerunning the 2803 test. This may indicate that the problem is that the simulation isn't returning the final command and this is hanging. I don't have a read on why, though, as this seems unrelated.

LLM-assistance indicates that this may be due to the stdout buffer not being drained while the process is waiting for it to finish. This sounds plausible...

[seldridge]

I golfed this down to a smaller failing test in: #5129

The problem that I'm seeing is that the actual body never returns here: https://github.com/chipsalliance/chisel/blob/main/svsim/src/main/scala/Simulation.scala#L70

This isn't what the LLM was suggesting, so this needs more investigation. I don't have time to look at this more right now, but possibly over the weekend. If you want to dig into, please go ahead!

[Emin017]

I opened #5132. This patch should fix the issue.