From 2d56c6738c8712621999e3c7fd75a8deb566aa91 Mon Sep 17 00:00:00 2001
From: Richard Petri <git@rpls.de>
Date: Sun, 20 Oct 2019 22:07:43 +0200
Subject: [PATCH] Multiplication Plugin using 16-bit DSPs

---
 .../scala/vexriscv/plugin/Mul16Plugin.scala   | 119 ++++++++++++++++++
 1 file changed, 119 insertions(+)
 create mode 100644 src/main/scala/vexriscv/plugin/Mul16Plugin.scala

diff --git a/src/main/scala/vexriscv/plugin/Mul16Plugin.scala b/src/main/scala/vexriscv/plugin/Mul16Plugin.scala
new file mode 100644
index 0000000..f2a63c3
--- /dev/null
+++ b/src/main/scala/vexriscv/plugin/Mul16Plugin.scala
@@ -0,0 +1,119 @@
+package vexriscv.plugin
+
+import vexriscv._
+import vexriscv.plugin._
+import spinal.core._
+
+/**
+  * A multiplication plugin using only 16-bit multiplications
+  */
+class Mul16Plugin extends Plugin[VexRiscv]{
+
+  object MUL_LL extends Stageable(UInt(32 bits))
+  object MUL_LH extends Stageable(UInt(32 bits))
+  object MUL_HL extends Stageable(UInt(32 bits))
+  object MUL_HH extends Stageable(UInt(32 bits))
+
+  object MUL     extends Stageable(Bits(64 bits))
+
+  object IS_MUL  extends Stageable(Bool)
+
+  override def setup(pipeline: VexRiscv): Unit = {
+    import Riscv._
+    import pipeline.config._
+
+
+    val actions = List[(Stageable[_ <: BaseType],Any)](
+      SRC1_CTRL                -> Src1CtrlEnum.RS,
+      SRC2_CTRL                -> Src2CtrlEnum.RS,
+      REGFILE_WRITE_VALID      -> True,
+      BYPASSABLE_EXECUTE_STAGE -> False,
+      BYPASSABLE_MEMORY_STAGE  -> False,
+      RS1_USE                  -> True,
+      RS2_USE                  -> True,
+      IS_MUL                   -> True
+    )
+
+    val decoderService = pipeline.service(classOf[DecoderService])
+    decoderService.addDefault(IS_MUL, False)
+    decoderService.add(List(
+      MULX  -> actions
+    ))
+
+  }
+
+  override def build(pipeline: VexRiscv): Unit = {
+    import pipeline._
+    import pipeline.config._
+
+    // Prepare signed inputs for the multiplier in the next stage.
+    // This will map them best to an FPGA DSP.
+    execute plug new Area {
+      import execute._
+      val a,b = Bits(32 bit)
+
+      a := input(SRC1)
+      b := input(SRC2)
+
+      val aLow = a(15 downto 0).asUInt
+      val bLow = b(15 downto 0).asUInt
+      val aHigh = a(31 downto 16).asUInt
+      val bHigh = b(31 downto 16).asUInt
+
+      insert(MUL_LL) := aLow * bLow
+      insert(MUL_LH) := aLow * bHigh
+      insert(MUL_HL) := aHigh * bLow
+      insert(MUL_HH) := aHigh * bHigh
+    }
+
+    memory plug new Area {
+      import memory._
+
+      val ll = UInt(32 bits)
+      val lh = UInt(33 bits)
+      val hl = UInt(32 bits)
+      val hh = UInt(32 bits)
+
+      ll := input(MUL_LL)
+      lh := input(MUL_LH).resized
+      hl := input(MUL_HL)
+      hh := input(MUL_HH)
+
+      val hllh = lh + hl
+      insert(MUL) := ((hh ## ll(31 downto 16)).asUInt + hllh) ## ll(15 downto 0)
+    }
+
+    writeBack plug new Area {
+      import writeBack._
+      val aSigned,bSigned = Bool
+      switch(input(INSTRUCTION)(13 downto 12)) {
+        is(B"01") {
+          aSigned := True
+          bSigned := True
+        }
+        is(B"10") {
+          aSigned := True
+          bSigned := False
+        }
+        default {
+          aSigned := False
+          bSigned := False
+        }
+      }
+
+      val a = (aSigned && input(SRC1).msb) ? input(SRC2).asUInt | U(0)
+      val b = (bSigned && input(SRC2).msb) ? input(SRC1).asUInt | U(0)
+
+      when(arbitration.isValid && input(IS_MUL)){
+        switch(input(INSTRUCTION)(13 downto 12)){
+          is(B"00"){
+            output(REGFILE_WRITE_DATA) := input(MUL)(31 downto 0)
+          }
+          is(B"01",B"10",B"11"){
+            output(REGFILE_WRITE_DATA) := (((input(MUL)(63 downto 32)).asUInt + ~a) + (~b + 2)).asBits
+          }
+        }
+      }
+    }
+  }
+}