fix invalid handling of X register functioncall result value

compiler/src/prog8/ast/processing/AstChecker.kt

@@ -1029,8 +1029,6 @@ internal class AstChecker(private val program: Program,
                 }
             }
         } else if(target is Subroutine) {
-            if(target.regXasResult())
-                errors.warn("subroutine call return value in X register is discarded and replaced by 0", position)
             if(target.isAsmSubroutine) {
                 for (arg in args.zip(target.parameters)) {
                     val argIDt = arg.first.inferType(program)

compiler/src/prog8/compiler/target/c64/codegen/AsmGen.kt

@@ -1,5 +1,6 @@
 package prog8.compiler.target.c64.codegen
 
+import prog8.ast.IFunctionCall
 import prog8.ast.INameScope
 import prog8.ast.Node
 import prog8.ast.Program
@@ -792,6 +793,12 @@ internal class AsmGen(private val program: Program,
     internal fun translateFunctionCall(functionCall: FunctionCall) =
             functioncallAsmGen.translateFunctionCall(functionCall)
 
+    internal fun saveXbeforeCall(functionCall: IFunctionCall)  =
+            functioncallAsmGen.saveXbeforeCall(functionCall)
+
+    internal fun restoreXafterCall(functionCall: IFunctionCall) =
+            functioncallAsmGen.restoreXafterCall(functionCall)
+
     internal fun translateNormalAssignment(assign: AsmAssignment) =
             assignmentAsmGen.translateNormalAssignment(assign)
 

compiler/src/prog8/compiler/target/c64/codegen/ExpressionsAsmGen.kt

@@ -1324,49 +1324,34 @@ internal class ExpressionsAsmGen(private val program: Program, private val asmge
 +""")
     }
 
-    private fun translateFunctionCallResultOntoStack(expression: FunctionCall) {
+    private fun translateFunctionCallResultOntoStack(call: FunctionCall) {
         // only for use in nested expression evaluation
 
-        val sub = expression.target.targetStatement(program.namespace)
+        val sub = call.target.targetStatement(program.namespace)
         if(sub is BuiltinFunctionStatementPlaceholder) {
             val builtinFunc = BuiltinFunctions.getValue(sub.name)
-            asmgen.translateBuiltinFunctionCallExpression(expression, builtinFunc, true)
+            asmgen.translateBuiltinFunctionCallExpression(call, builtinFunc, true)
         } else {
             sub as Subroutine
-            asmgen.translateFunctionCall(expression)
+            asmgen.saveXbeforeCall(call)
+            asmgen.translateFunctionCall(call)
+            if(sub.regXasResult()) {
+                // store the return value in X somewhere that we can acces again below
+                asmgen.out("  stx  P8ZP_SCRATCH_REG")
+            }
+            asmgen.restoreXafterCall(call)
+
             val returns = sub.returntypes.zip(sub.asmReturnvaluesRegisters)
             for ((_, reg) in returns) {
-                // result value in cpu or status registers, put it on the stack
+                // result value is in cpu or status registers, put it on the stack instead (as we're evaluating an expression tree)
                 if (reg.registerOrPair != null) {
                     when (reg.registerOrPair) {
                         RegisterOrPair.A -> asmgen.out("  sta  P8ESTACK_LO,x |  dex")
                         RegisterOrPair.Y -> asmgen.out("  tya |  sta  P8ESTACK_LO,x |  dex")
                         RegisterOrPair.AY -> asmgen.out("  sta  P8ESTACK_LO,x |  tya |  sta  P8ESTACK_HI,x |  dex")
-                        RegisterOrPair.X -> {
+                        RegisterOrPair.X -> asmgen.out("  lda  P8ZP_SCRATCH_REG |  sta  P8ESTACK_LO,x |  dex")
-                            // return value in X register has been discarded, just push a zero
+                        RegisterOrPair.AX -> asmgen.out("  sta  P8ESTACK_LO,x |  lda  P8ZP_SCRATCH_REG |  sta  P8ESTACK_HI,x |  dex")
-                            if(CompilationTarget.instance.machine.cpu==CpuType.CPU65c02)
+                        RegisterOrPair.XY -> asmgen.out("  tya |  sta  P8ESTACK_HI,x |  lda  P8ZP_SCRATCH_REG |  sta  P8ESTACK_LO,x |  dex")
-                                asmgen.out("  stz  P8ESTACK_LO,x")
-                            else
-                                asmgen.out("  lda  #0 |  sta  P8ESTACK_LO,x")
-                            asmgen.out("  dex")
-                        }
-                        RegisterOrPair.AX -> {
-                            // return value in X register has been discarded, just push a zero in this place
-                            asmgen.out("  sta  P8ESTACK_LO,x")
-                            if(CompilationTarget.instance.machine.cpu==CpuType.CPU65c02)
-                                asmgen.out("  stz  P8ESTACK_HI,x")
-                            else
-                                asmgen.out("  lda  #0 |  sta  P8ESTACK_HI,x")
-                            asmgen.out("  dex")
-                        }
-                        RegisterOrPair.XY -> {
-                            // return value in X register has been discarded, just push a zero in this place
-                            if(CompilationTarget.instance.machine.cpu==CpuType.CPU65c02)
-                                asmgen.out("  stz  P8ESTACK_LO,x")
-                            else
-                                asmgen.out("  lda  #0 |  sta  P8ESTACK_LO,x")
-                            asmgen.out("  tya |  sta  P8ESTACK_HI,x |  dex")
-                        }
                         RegisterOrPair.FAC1 -> asmgen.out("  jsr  floats.push_fac1")
                         RegisterOrPair.FAC2 -> asmgen.out("  jsr  floats.push_fac2")
                         RegisterOrPair.R0,

compiler/src/prog8/compiler/target/c64/codegen/FunctionCallAsmGen.kt

@@ -7,7 +7,6 @@ import prog8.ast.base.*
 import prog8.ast.expressions.*
 import prog8.ast.statements.*
 import prog8.compiler.AssemblyError
-import prog8.compiler.CompilationOptions
 import prog8.compiler.target.CompilationTarget
 import prog8.compiler.target.CpuType
 import prog8.compiler.target.c64.codegen.assignment.*
@@ -16,25 +15,44 @@ import prog8.compiler.target.c64.codegen.assignment.*
 internal class FunctionCallAsmGen(private val program: Program, private val asmgen: AsmGen) {
 
     internal fun translateFunctionCallStatement(stmt: IFunctionCall) {
+        saveXbeforeCall(stmt)
         translateFunctionCall(stmt)
-        // functioncalls no longer return results on the stack, so simply ignore the results in the registers
+        restoreXafterCall(stmt)
+        // just ignore any result values from the function call.
     }
 
-
+    internal fun saveXbeforeCall(stmt: IFunctionCall) {
-    internal fun translateFunctionCall(stmt: IFunctionCall) {
-        // output the code to setup the parameters and perform the actual call
-        // does NOT output the code to deal with the result values!
         val sub = stmt.target.targetSubroutine(program.namespace) ?: throw AssemblyError("undefined subroutine ${stmt.target}")
-        val saveX = sub.shouldSaveX()
+        if(sub.shouldSaveX()) {
             val regSaveOnStack = sub.asmAddress==null       // rom-routines don't require registers to be saved on stack, normal subroutines do because they can contain nested calls
             val (keepAonEntry: Boolean, keepAonReturn: Boolean) = sub.shouldKeepA()
-        if(saveX) {
             if(regSaveOnStack)
                 asmgen.saveRegisterStack(CpuRegister.X, keepAonEntry)
             else
                 asmgen.saveRegisterLocal(CpuRegister.X, (stmt as Node).definingSubroutine()!!)
         }
+    }
 
+    internal fun restoreXafterCall(stmt: IFunctionCall) {
+        val sub = stmt.target.targetSubroutine(program.namespace) ?: throw AssemblyError("undefined subroutine ${stmt.target}")
+        if(sub.shouldSaveX()) {
+            val regSaveOnStack = sub.asmAddress==null       // rom-routines don't require registers to be saved on stack, normal subroutines do because they can contain nested calls
+            val (keepAonEntry: Boolean, keepAonReturn: Boolean) = sub.shouldKeepA()
+
+            if(regSaveOnStack)
+                asmgen.restoreRegisterStack(CpuRegister.X, keepAonReturn)
+            else
+                asmgen.restoreRegisterLocal(CpuRegister.X)
+        }
+    }
+
+    internal fun translateFunctionCall(stmt: IFunctionCall) {
+        // Output only the code to setup the parameters and perform the actual call
+        // NOTE: does NOT output the code to deal with the result values!
+        // NOTE: does NOT output code to save/restore the X register for this call! Every caller should deal with this in their own way!!
+        //       (you can use subroutine.shouldSaveX() and saveX()/restoreX() routines as a help for this)
+
+        val sub = stmt.target.targetSubroutine(program.namespace) ?: throw AssemblyError("undefined subroutine ${stmt.target}")
         val subName = asmgen.asmSymbolName(stmt.target)
         if(stmt.args.isNotEmpty()) {
             if(sub.asmParameterRegisters.isEmpty()) {
@@ -84,12 +102,7 @@ internal class FunctionCallAsmGen(private val program: Program, private val asmg
             asmgen.out("  jsr  $subName")
         }
 
-        if(saveX) {
+        // remember: dealing with the X register and/or dealing with return values is the responsibility of the caller
-            if(regSaveOnStack)
-                asmgen.restoreRegisterStack(CpuRegister.X, keepAonReturn)
-            else
-                asmgen.restoreRegisterLocal(CpuRegister.X)
-        }
     }
 
     private fun registerArgsViaStackEvaluation(stmt: IFunctionCall, sub: Subroutine) {

compiler/src/prog8/compiler/target/c64/codegen/assignment/AssignmentAsmGen.kt

@@ -142,7 +142,10 @@ internal class AssignmentAsmGen(private val program: Program, private val asmgen
                     is FunctionCall -> {
                         when (val sub = value.target.targetStatement(program.namespace)) {
                             is Subroutine -> {
+                                asmgen.saveXbeforeCall(value)
                                 asmgen.translateFunctionCall(value)
+                                if(!sub.regXasResult())
+                                    asmgen.restoreXafterCall(value)
                                 val returnValue = sub.returntypes.zip(sub.asmReturnvaluesRegisters).singleOrNull { it.second.registerOrPair!=null } ?:
                                     sub.returntypes.zip(sub.asmReturnvaluesRegisters).single { it.second.statusflag!=null }
                                 when (returnValue.first) {
@@ -186,6 +189,8 @@ internal class AssignmentAsmGen(private val program: Program, private val asmgen
                                                     throw AssemblyError("should be just one register byte result value")
                                             }
                                         }
+                                        // we've processed the result value in the X register by now, so it's now safe to restore it:
+                                        asmgen.restoreXafterCall(value)
                                     }
                                 }
                             }

examples/test.p8

@@ -6,14 +6,16 @@
 main {
 
     sub start () {
-        uword current_time
+        uword mem
+
+        mem = 5*mem*c64.MEMTOP(0,1)
+        txt.print_uwhex(mem, 1)
+
+        mem = 5*mem*c64.CHRIN()
+        txt.print_uwhex(mem,1)
+
+        test_stack.test()
 
-        repeat 6 {
-            current_time = c64.RDTIM16()
-            txt.print_uw(current_time)
-            txt.chrout('\n')
-            sys.wait(30)
-        }
 
 
 ;        found = strfind("irmen de jong", ' ')