From 6f00cacc6d33a5385374e1c98a0445ee2aa83ff0 Mon Sep 17 00:00:00 2001
From: Karol Stasiak <karol.m.stasiak@gmail.com>
Date: Mon, 18 Mar 2019 20:01:40 +0100
Subject: [PATCH] 6502: Optimize 16-bit multiplication by a constant

---
 .../scala/millfork/compiler/mos/BuiltIns.scala     |  4 ++++
 .../compiler/mos/PseudoregisterBuiltIns.scala      | 14 ++++++++++++++
 src/main/scala/millfork/env/Constant.scala         |  3 +++
 3 files changed, 21 insertions(+)

diff --git a/src/main/scala/millfork/compiler/mos/BuiltIns.scala b/src/main/scala/millfork/compiler/mos/BuiltIns.scala
index 180ce211..68047505 100644
--- a/src/main/scala/millfork/compiler/mos/BuiltIns.scala
+++ b/src/main/scala/millfork/compiler/mos/BuiltIns.scala
@@ -862,6 +862,8 @@ object BuiltIns {
     }
   }
 
+  private def isPowerOfTwoUpTo15(n: Long): Boolean = if (n <= 0 || n >= 0x8000) false else 0 == ((n-1) & n)
+
   def compileInPlaceWordMultiplication(ctx: CompilationContext, v: LhsExpression, addend: Expression): List[AssemblyLine] = {
     val b = ctx.env.get[Type]("byte")
     val w = ctx.env.get[Type]("word")
@@ -870,6 +872,8 @@ object BuiltIns {
         MosExpressionCompiler.compile(ctx, v, None, NoBranching) ++ MosExpressionCompiler.compileAssignment(ctx, LiteralExpression(0, 2), v)
       case Some(NumericConstant(1, _)) =>
         MosExpressionCompiler.compile(ctx, v, None, NoBranching)
+      case Some(NumericConstant(n, _)) if isPowerOfTwoUpTo15(n) =>
+        BuiltIns.compileInPlaceWordOrLongShiftOps(ctx, v, LiteralExpression(java.lang.Long.bitCount(n - 1), 1), aslRatherThanLsr = true)
       case _ =>
         // TODO: optimize?
         PseudoregisterBuiltIns.compileWordMultiplication(ctx, Some(v), addend, storeInRegLo = true) ++
diff --git a/src/main/scala/millfork/compiler/mos/PseudoregisterBuiltIns.scala b/src/main/scala/millfork/compiler/mos/PseudoregisterBuiltIns.scala
index e707308b..ab7c8d5d 100644
--- a/src/main/scala/millfork/compiler/mos/PseudoregisterBuiltIns.scala
+++ b/src/main/scala/millfork/compiler/mos/PseudoregisterBuiltIns.scala
@@ -380,6 +380,8 @@ object PseudoregisterBuiltIns {
     load ++ calculate
   }
 
+  private def isPowerOfTwoUpTo15(n: Long): Boolean = if (n <= 0 || n >= 0x8000) false else 0 == ((n-1) & n)
+
   def compileWordMultiplication(ctx: CompilationContext, param1OrRegister: Option[Expression], param2: Expression, storeInRegLo: Boolean): List[AssemblyLine] = {
     if (ctx.options.zpRegisterSize < 3) {
       ctx.log.error("Variable word multiplication requires the zeropage pseudoregister of size at least 3", param1OrRegister.flatMap(_.position))
@@ -391,6 +393,18 @@ object PseudoregisterBuiltIns {
       case (2 | 1, 1) => // ok
       case _ => ctx.log.fatal("Invalid code path", param2.position)
     }
+    if (!storeInRegLo && param1OrRegister.isDefined) {
+      (ctx.env.eval(param1OrRegister.get), ctx.env.eval(param2)) match {
+        case (Some(l), Some(r)) =>
+          val product = CompoundConstant(MathOperator.Times, l, r).quickSimplify
+          return List(AssemblyLine.immediate(LDA, product.loByte), AssemblyLine.immediate(LDX, product.hiByte))
+        case (Some(NumericConstant(c, _)), _) if isPowerOfTwoUpTo15(c)=>
+          return compileWordShiftOps(left = true, ctx, param2, LiteralExpression(java.lang.Long.bitCount(c - 1), 1))
+        case (_, Some(NumericConstant(c, _))) if isPowerOfTwoUpTo15(c)=>
+          return compileWordShiftOps(left = true, ctx, param1OrRegister.get, LiteralExpression(java.lang.Long.bitCount(c - 1), 1))
+        case _ =>
+      }
+    }
     val b = ctx.env.get[Type]("byte")
     val w = ctx.env.get[Type]("word")
     val reg = ctx.env.get[VariableInMemory]("__reg")
diff --git a/src/main/scala/millfork/env/Constant.scala b/src/main/scala/millfork/env/Constant.scala
index 515b9675..11ff753d 100644
--- a/src/main/scala/millfork/env/Constant.scala
+++ b/src/main/scala/millfork/env/Constant.scala
@@ -98,6 +98,9 @@ sealed trait Constant {
   def refersTo(name: String): Boolean
 
   def fitsInto(typ: Type): Boolean = true // TODO
+
+  final def succ: Constant = (this + 1).quickSimplify
+
 }
 
 case class AssertByte(c: Constant) extends Constant {