[WASM] Fold redundant patterns with drop instruction on a wasm level

compiler/ir/backend.wasm/src/org/jetbrains/kotlin/backend/wasm/ir2wasm/BodyGenerator.kt

@@ -35,7 +35,7 @@ class BodyGenerator(val context: WasmFunctionCodegenContext) : IrElementVisitorV
 
     private val unitGetInstance by lazy { backendContext.findUnitGetInstanceFunction() }
     fun WasmExpressionBuilder.buildGetUnit() {
-        buildCall(context.referenceFunction(unitGetInstance.symbol))
+        buildInstr(WasmOp.GET_UNIT, WasmImmediate.FuncIdx(context.referenceFunction(unitGetInstance.symbol)))
     }
 
     // Generates code for the given IR element and *always* leaves something on the stack
@@ -264,6 +264,12 @@ class BodyGenerator(val context: WasmFunctionCodegenContext) : IrElementVisitorV
             return
         }
 
+        // Get unit is a special case because it is the only function which returns the real unit instance.
+        if (call.symbol == unitGetInstance.symbol) {
+            body.buildGetUnit()
+            return
+        }
+
         call.dispatchReceiver?.let { generateExpression(it) }
         call.extensionReceiver?.let { generateExpression(it) }
         for (i in 0 until call.valueArgumentsCount) {
@@ -319,7 +325,7 @@ class BodyGenerator(val context: WasmFunctionCodegenContext) : IrElementVisitorV
         }
 
         // Unit types don't cross function boundaries
-        if (function.returnType == irBuiltIns.unitType && function.symbol != unitGetInstance.symbol)
+        if (function.returnType == irBuiltIns.unitType)
             body.buildGetUnit()
     }
 

wasm/wasm.ir/src/org/jetbrains/kotlin/wasm/ir/Operators.kt

@@ -367,6 +367,9 @@ enum class WasmOp(
     REF_CAST("ref.cast", 0xFB_41),
 
     BR_ON_CAST("br_on_cast", 0xFB_42, listOf(LABEL_IDX)),
+
+    // Pseudo-instruction, just alias for a normal call. It's used to easily spot get_unit on the wasm level.
+    GET_UNIT("call", 0x10, FUNC_IDX)
     ;
 
     constructor(mnemonic: String, opcode: Int, vararg immediates: WasmImmediateKind) : this(mnemonic, opcode, immediates.toList())

wasm/wasm.ir/src/org/jetbrains/kotlin/wasm/ir/Utils.kt

@@ -42,6 +42,16 @@ fun foldWasmInstructions(prev: WasmInstr?, next: WasmInstr): List<WasmInstr>? {
     if (next.operator == WasmOp.UNREACHABLE && prev.operator in listOf(WasmOp.UNREACHABLE, WasmOp.RETURN))
         return listOf(prev)
 
+    if (next.operator == WasmOp.DROP) {
+        // simple pure instruction + drop -> nothing
+        if (prev.operator == WasmOp.GET_UNIT || prev.operator == WasmOp.REF_NULL)
+            return listOf()
+
+        // return + drop -> return
+        if (prev.operator == WasmOp.RETURN)
+            return listOf(prev)
+    }
+
     return null
 }