optimize assigning word array value to byte variable

codeGenCpu6502/src/prog8/codegen/cpu6502/BuiltinFunctionsAsmGen.kt

@@ -1115,7 +1115,25 @@ internal class BuiltinFunctionsAsmGen(private val program: PtProgram,
                 else -> throw AssemblyError("invalid reg")
             }
         } else {
-            when(resultRegister) {
+            if(arg is PtArrayIndexer && resultRegister in setOf(null, RegisterOrPair.A, RegisterOrPair.Y, RegisterOrPair.X)) {
+                // just read the lsb byte out of the word array
+                val arrayVar = if(arg.splitWords) asmgen.asmVariableName(arg.variable)+"_lsb" else asmgen.asmVariableName(arg.variable)
+                when(resultRegister) {
+                    null, RegisterOrPair.A -> {
+                        asmgen.loadScaledArrayIndexIntoRegister(arg, arg.type, CpuRegister.Y)
+                        asmgen.out("  lda  $arrayVar,y")
+                    }
+                    RegisterOrPair.Y -> {
+                        asmgen.loadScaledArrayIndexIntoRegister(arg, arg.type, CpuRegister.X)
+                        asmgen.out("  lda  $arrayVar,x")
+                    }
+                    RegisterOrPair.X -> {
+                        asmgen.loadScaledArrayIndexIntoRegister(arg, arg.type, CpuRegister.Y)
+                        asmgen.out("  ldx  $arrayVar,y")
+                    }
+                    else -> throw AssemblyError("invalid reg")
+                }
+            } else  when(resultRegister) {
                 null, RegisterOrPair.A -> {
                     asmgen.assignExpressionToRegister(arg, RegisterOrPair.AY)
                     // NOTE: we rely on the fact that the above assignment to AY, assigns the Lsb to A as the last instruction.

codeGenCpu6502/src/prog8/codegen/cpu6502/assignment/AssignmentAsmGen.kt

@@ -1787,6 +1787,12 @@ internal class AssignmentAsmGen(private val program: PtProgram,
                 }
             }
             is PtNumber -> throw AssemblyError("a cast of a literal value should have been const-folded away")
+            is PtArrayIndexer -> {
+                if(targetDt in ByteDatatypes && valueDt in WordDatatypes) {
+                    // just assign the lsb from the array value
+                    return assignCastViaLsbFunc(value, target)
+                }
+            }
             else -> {}
         }
 

examples/test.p8

@@ -1,10 +1,30 @@
 %import syslib
-%zpreserved $a0,$ff
+%import textio
-%zpallowed $70,$b0
+%zeropage basicsafe
 
 main {
     sub start() {
-        repeat {
+        uword[] wordarray = [0,1,2,3,4,5,6,7,8,9]
-        }
+        ubyte n = 5
+        n = lsb(wordarray[n])
+        n = wordarray[n] as ubyte
+        test(wordarray[n] as ubyte,wordarray[n] as ubyte,0)
+        test(lsb(wordarray[n]), lsb(wordarray[n]),0)
+    }
+
+    asmsub test(ubyte a1 @A, ubyte a2 @X, ubyte a3 @Y) {
+        %asm {{
+            phy
+            phx
+            jsr  txt.print_ub
+            jsr  txt.spc
+            pla
+            jsr  txt.print_ub
+            jsr  txt.spc
+            pla
+            jsr  txt.print_ub
+            jsr  txt.nl
+            rts
+        }}
     }
 }