Add Lfsr tests

Add LFSR tests using LFSR16 testing infrastructure. This also adds
tests that are the same as the examples shown for LFSR scaladoc.

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>

1 files changed, 118 insertions, 0 deletions

diff --git a/src/test/scala/chiselTests/util/random/LFSRSpec.scala b/src/test/scala/chiselTests/util/random/LFSRSpec.scala
new file mode 100644
index 00000000..5aedca75
--- /dev/null
+++ b/src/test/scala/chiselTests/util/random/LFSRSpec.scala
@@ -0,0 +1,118 @@
+// See LICENSE for license details.
+
+package chiselTests.util.random
+
+import chisel3._
+import chisel3.util.{Counter, Enum}
+import chisel3.util.random._
+import chisel3.testers.BasicTester
+
+import chiselTests.{ChiselFlatSpec, LFSRDistribution, LFSRMaxPeriod}
+
+import math.pow
+
+class FooLFSR(val reduction: LFSRReduce, seed: Option[BigInt]) extends PRNG(4, seed) with LFSR {
+  def delta(s: UInt): UInt = s
+}
+
+/** This tests that after reset an LFSR is not locked up. This manually sets the seed of the LFSR at run-time to the
+  * value that would cause it to lock up. It then asserts reset. The next cycle it checks that the value is NOT the
+  * locked up value.
+  * @param gen an LFSR to test
+  * @param lockUpValue the value that would lock up the LFSR
+  */
+class LFSRResetTester(gen: => LFSR, lockUpValue: BigInt) extends BasicTester {
+
+  val lfsr = Module(gen)
+  lfsr.io.seed.valid := false.B
+  lfsr.io.seed.bits := DontCare
+  lfsr.io.increment := true.B
+
+  val (count, done) = Counter(true.B, 5)
+
+  printf("%d: 0b%b\n", count, lfsr.io.out)
+
+  lfsr.io.seed.valid := count === 1.U
+  lfsr.io.seed.bits := lockUpValue.U
+  lfsr.io.increment := true.B
+
+  when (count === 2.U) {
+    assert(lfsr.io.out === lockUpValue.U, "LFSR is NOT locked up, but should be!")
+  }
+
+  lfsr.reset := count === 3.U
+
+  when (count === 4.U) {
+    assert(lfsr.io.out =/= lockUpValue.U, "LFSR is locked up, but should not be after reset!")
+  }
+
+  when (done) {
+    stop()
+  }
+
+}
+
+class LFSRSpec extends ChiselFlatSpec {
+
+  def periodCheck(gen: (Int, Set[Int], LFSRReduce) => PRNG, reduction: LFSRReduce, range: Range): Unit = {
+    it should s"have a maximal period over a range of widths (${range.head} to ${range.last}) using ${reduction.getClass}" in {
+      range
+        .foreach{ width =>
+          LFSR.tapsMaxPeriod(width).foreach{ taps =>
+            info(s"""width $width okay using taps: ${taps.mkString(", ")}""")
+            assertTesterPasses(new LFSRMaxPeriod(PRNG(gen(width, taps, reduction))))
+          }
+        }
+    }
+  }
+
+  behavior of "LFSR"
+
+  it should "throw an exception if initialized to a seed of zero for XOR configuration" in {
+    { the [IllegalArgumentException] thrownBy elaborate(new FooLFSR(XOR, Some(0))) }
+      .getMessage should include ("Seed cannot be zero")
+  }
+
+  it should "throw an exception if initialized to a seed of all ones for XNOR configuration" in {
+    { the [IllegalArgumentException] thrownBy elaborate(new FooLFSR(XNOR, Some(15))) }
+      .getMessage should include ("Seed cannot be all ones")
+  }
+
+  it should "reset correctly without a seed for XOR configuration" in {
+    assertTesterPasses(new LFSRResetTester(new FooLFSR(XOR, None), 0))
+  }
+
+  it should "reset correctly without a seed for XNOR configuration" in {
+    assertTesterPasses(new LFSRResetTester(new FooLFSR(XNOR, None), 15))
+  }
+
+  behavior of "MaximalPeriodGaloisLFSR"
+
+  it should "throw an exception if no LFSR taps are known" in {
+    { the [IllegalArgumentException] thrownBy elaborate(new MaxPeriodGaloisLFSR(787)) }
+      .getMessage should include ("No max period LFSR taps stored for requested width")
+  }
+
+  periodCheck((w: Int, t: Set[Int], r: LFSRReduce) => new GaloisLFSR(w, t, reduction=r), XOR, 2 to 16)
+  periodCheck((w: Int, t: Set[Int], r: LFSRReduce) => new GaloisLFSR(w, t, reduction=r), XNOR, 2 to 16)
+
+  ignore should "have a sane distribution for larger widths" in {
+    ((17 to 32) ++ Seq(64, 128, 256, 512, 1024, 2048, 4096))
+      .foreach{ width =>
+        info(s"width $width okay!")
+        assertTesterPasses(new LFSRDistribution(LFSR(width), math.pow(2, 22).toInt))
+      }
+  }
+
+  behavior of "MaximalPeriodFibonacciLFSR"
+
+  periodCheck((w: Int, t: Set[Int], r: LFSRReduce) => new FibonacciLFSR(w, t, reduction=r), XOR, 2 to 16)
+  periodCheck((w: Int, t: Set[Int], r: LFSRReduce) => new FibonacciLFSR(w, t, reduction=r), XNOR, 2 to 16)
+
+  behavior of "LFSR maximal period taps"
+
+  it should "contain all the expected widths" in {
+    ((2 to 786) ++ Seq(1024, 2048, 4096)).foreach(LFSR.tapsMaxPeriod.keys should contain (_))
+  }
+
+}