Elide total unused Lua tuple components (#1283)

Author: Frotty

de.peeeq.wurstscript/src/main/java/de/peeeq/wurstscript/translation/imtranslation/EliminateTuples.java

@@ -272,7 +272,7 @@ private static void retainDiscardedValue(ImExpr value, ImStmts stmts, ImTranslat
             }
             return;
         }
-        if (isTriviallyDiscardable(value)) {
+        if (isSafelyDiscardable(value)) {
             return;
         }
         if (SideEffectAnalyzer.quickcheckHasSideeffects(value)) {
@@ -284,14 +284,34 @@ private static void retainDiscardedValue(ImExpr value, ImStmts stmts, ImTranslat
         }
     }
 
-    private static boolean isTriviallyDiscardable(ImExpr value) {
-        return value instanceof ImBoolVal
+    private static boolean isSafelyDiscardable(ImExpr value) {
+        if (value instanceof ImBoolVal
             || value instanceof ImIntVal
             || value instanceof ImRealVal
             || value instanceof ImStringVal
             || value instanceof ImNull
             || value instanceof ImVarAccess
-            || value instanceof ImFuncRef;
+            || value instanceof ImFuncRef) {
+            return true;
+        }
+        if (value instanceof ImOperatorCall operatorCall
+            && isTotalOperator(operatorCall.getOp())) {
+            return operatorCall.getArguments().stream()
+                .allMatch(EliminateTuples::isSafelyDiscardable);
+        }
+        return false;
+    }
+
+    /**
+     * Operators other than integer division and modulo are total for well-typed scalar IM values.
+     * Keeping their unused results in a Lua discard sink only repeats pure scalar work and prevents
+     * tuple-component DCE. Division and modulo stay conservative because a zero divisor can fail.
+     */
+    private static boolean isTotalOperator(WurstOperator operator) {
+        return switch (operator) {
+            case DIV_INT, MOD_INT, MOD_REAL, JASS_MOD_INT -> false;
+            default -> true;
+        };
     }
 
     /**
@@ -1006,7 +1026,7 @@ private static void lowerBundleInto(ImExpr expression, ImFunction f, boolean cap
 
         value.setParent(null);
         boolean requiresEagerEvaluation = capturePotentiallyFailingValues
-            && !isTriviallyDiscardable(value)
+            && !isSafelyDiscardable(value)
             && !SideEffectAnalyzer.quickcheckHasSideeffects(value);
         if ((capture || requiresEagerEvaluation) && !isImmutableValue(value)) {
             result.values.add(captureValue(value, temporaryName, result.prelude, f));

de.peeeq.wurstscript/src/test/java/tests/wurstscript/tests/LuaBackendAuditTests.java

@@ -731,6 +731,46 @@ public void tupleSelectionPreservesLeftToRightEvaluation() throws IOException {
         assertFalse(compiled.contains("tupleCopy"));
     }
 
+    @Test
+    public void unusedTotalTupleComponentDoesNotSurviveLuaLowering() throws IOException {
+        test().testLua(true).executeProg().lines(
+            "package Test",
+            "native testSuccess()",
+            "tuple pair(int x, int y)",
+            "function selectFirst(int left, int right) returns int",
+            "    return pair(left + 1, right + 2).x",
+            "init",
+            "    if selectFirst(4, 10) == 5",
+            "        testSuccess()"
+        );
+
+        String compiled = compiledLua("unusedTotalTupleComponentDoesNotSurviveLuaLowering");
+        assertFalse("unused total tuple components should not require a Lua discard call",
+            compiled.contains("__wurst_tuple_discard_"));
+        assertFalse("unused total tuple components should not be evaluated",
+            compiled.contains("right + 2"));
+    }
+
+    @Test
+    public void unreadTupleReturnSlotsAreRemoved() throws IOException {
+        test().testLua(true).executeProg().lines(
+            "package Test",
+            "native testSuccess()",
+            "tuple pair(int x, int y)",
+            "@noinline function makePair(int left, int right) returns pair",
+            "    return pair(left + 1, right + 2)",
+            "init",
+            "    if makePair(4, 10).x == 5",
+            "        testSuccess()"
+        );
+
+        String compiled = compiledLua("unreadTupleReturnSlotsAreRemoved");
+        assertFalse("unread scalar return slots should be removed after tuple lowering",
+            compiled.contains("makePair_return_y"));
+        assertFalse("the producer for an unread scalar return slot should be removed",
+            compiled.contains("right + 2"));
+    }
+
     @Test
     public void discardedTupleComponentsThatCanFailAreStillEvaluated() {
         String compiled = compileLuaWithRunArgs(