Better handling of memory alignment

src/main/scala/millfork/env/Constant.scala

@@ -201,7 +201,11 @@ case class MemoryAddressConstant(var thing: ThingInMemory) extends Constant {
   }
 
   override def isProvablyDivisibleBy256: Boolean = thing match {
-    case t:PreallocableThing => t.alignment match {
+    case t: PreallocableThing => t.alignment match {
+      case DivisibleAlignment(divisor) => divisor.&(0xff) == 0
+      case _ => false
+    }
+    case t: UninitializedMemory => t.alignment match {
       case DivisibleAlignment(divisor) => divisor.&(0xff) == 0
       case _ => false
     }
@@ -228,7 +232,9 @@ case class SubbyteConstant(base: Constant, index: Int) extends Constant {
       } else {
         NumericConstant((x >> (index * 8)) & 0xff, 1)
       }
-      case _ => SubbyteConstant(simplified, index)
+      case _ =>
+        if (index == 0 && simplified.isProvablyDivisibleBy256) Constant.Zero
+        else SubbyteConstant(simplified, index)
     }
   }
 

src/main/scala/millfork/env/Environment.scala

@@ -1002,6 +1002,9 @@ class Environment(val parent: Option[Environment], val prefix: String, val cpuFa
   }
 
   def registerArray(stmt: ArrayDeclarationStatement, options: CompilationOptions): Unit = {
+    if (options.flag(CompilationFlag.LUnixRelocatableCode) && stmt.alignment.exists(_.isMultiplePages)) {
+      log.error("Invalid alignment for LUnix code", stmt.position)
+    }
     val b = get[VariableType]("byte")
     val w = get[VariableType]("word")
     val p = get[Type]("pointer")
@@ -1036,7 +1039,7 @@ class Environment(val parent: Option[Environment], val prefix: String, val cpuFa
                               declaredBank = stmt.bank, indexType, b)
                 }
                 addThing(array, stmt.position)
-                registerAddressConstant(UninitializedMemoryVariable(stmt.name, p, VariableAllocationMethod.None, stmt.bank, NoAlignment, isVolatile = false), stmt.position, options)
+                registerAddressConstant(UninitializedMemoryVariable(stmt.name, p, VariableAllocationMethod.None, stmt.bank, alignment, isVolatile = false), stmt.position, options)
                 val a = address match {
                   case None => array.toAddress
                   case Some(aa) => aa

src/main/scala/millfork/output/MemoryAlignment.scala

@@ -3,8 +3,18 @@ package millfork.output
 /**
   * @author Karol Stasiak
   */
-sealed trait MemoryAlignment
+sealed trait MemoryAlignment {
+  def isMultiplePages: Boolean
+}
 
-case object NoAlignment extends MemoryAlignment
-case object WithinPageAlignment extends MemoryAlignment
-case class DivisibleAlignment(divisor: Int) extends MemoryAlignment
+case object NoAlignment extends MemoryAlignment {
+  override def isMultiplePages: Boolean = false
+}
+
+case object WithinPageAlignment extends MemoryAlignment {
+  override def isMultiplePages: Boolean = false
+}
+
+case class DivisibleAlignment(divisor: Int) extends MemoryAlignment {
+  override def isMultiplePages: Boolean = divisor > 256
+}