Make undef and emptyString global imports rather than functions.

In addition to the likely performance benefit, it allows us to use
them in constant expressions, without support for the expression
builder. We use that opportunity when creating the field values in
`newDefault` functions, like we already did for static fields.

Author: sjrd

Diff for: wasm/src/main/scala/org/scalajs/linker/backend/wasmemitter/ClassEmitter.scala

@@ -42,7 +42,7 @@ class ClassEmitter(coreSpec: CoreSpec) {
       val global = WasmGlobal(
         genGlobalName.forStaticField(name.name),
         transformType(ftpe),
-        WasmExpr(List(genStaticVarInit(ftpe))),
+        WasmExpr(List(genZeroOf(ftpe))),
         isMutable = true
       )
       ctx.addGlobal(global)
@@ -67,18 +67,24 @@ class ClassEmitter(coreSpec: CoreSpec) {
     }
   }
 
-  private def genStaticVarInit(tpe: IRTypes.Type)(implicit ctx: WasmContext): WasmInstr = {
+  private def genZeroOf(tpe: IRTypes.Type)(implicit ctx: WasmContext): WasmInstr = {
     import IRTypes._
 
     tpe match {
-      case BooleanType | CharType | ByteType | ShortType | IntType => I32_CONST(0)
-      case LongType                                                => I64_CONST(0L)
-      case FloatType                                               => F32_CONST(0.0f)
-      case DoubleType                                              => F64_CONST(0.0)
-      case AnyType | ClassType(_) | ArrayType(_) | NullType        => REF_NULL(WasmHeapType.None)
-
-      case NoType | NothingType | StringType | UndefType | _: RecordType =>
-        throw new AssertionError(s"Unexpected type for static variable: ${tpe.show()}")
+      case BooleanType | CharType | ByteType | ShortType | IntType =>
+        I32_CONST(0)
+
+      case LongType   => I64_CONST(0L)
+      case FloatType  => F32_CONST(0.0f)
+      case DoubleType => F64_CONST(0.0)
+      case StringType => GLOBAL_GET(genGlobalName.emptyString)
+      case UndefType  => GLOBAL_GET(genGlobalName.undef)
+
+      case AnyType | ClassType(_) | ArrayType(_) | NullType =>
+        REF_NULL(WasmHeapType.None)
+
+      case NoType | NothingType | _: RecordType =>
+        throw new AssertionError(s"Unexpected type for field: ${tpe.show()}")
     }
   }
 
@@ -533,7 +539,7 @@ class ClassEmitter(coreSpec: CoreSpec) {
       instrs += REF_NULL(WasmHeapType(genTypeName.itables))
 
     classInfo.allFieldDefs.foreach { f =>
-      WasmExpressionBuilder.generateIRBody(IRTypes.zeroOf(f.ftpe)(clazz.pos), f.ftpe)
+      instrs += genZeroOf(f.ftpe)
     }
     instrs += STRUCT_NEW(structName)
 

Diff for: wasm/src/main/scala/org/scalajs/linker/backend/wasmemitter/CoreWasmLib.scala

@@ -168,6 +168,8 @@ object CoreWasmLib {
       )
     }
 
+    addGlobalHelperImport(genGlobalName.undef, WasmRefType.any, isMutable = false)
+    addGlobalHelperImport(genGlobalName.emptyString, WasmRefType.any, isMutable = false)
     addGlobalHelperImport(genGlobalName.idHashCodeMap, WasmRefType.extern, isMutable = false)
   }
 
@@ -264,7 +266,6 @@ object CoreWasmLib {
 
     addHelperImport(genFunctionName.is, List(anyref, anyref), List(WasmInt32))
 
-    addHelperImport(genFunctionName.undef, List(), List(WasmRefType.any))
     addHelperImport(genFunctionName.isUndef, List(anyref), List(WasmInt32))
 
     for (primRef <- List(BooleanRef, ByteRef, ShortRef, IntRef, FloatRef, DoubleRef)) {
@@ -307,7 +308,6 @@ object CoreWasmLib {
       List(WasmRefType.any)
     )
 
-    addHelperImport(genFunctionName.emptyString, List(), List(WasmRefType.any))
     addHelperImport(genFunctionName.stringLength, List(WasmRefType.any), List(WasmInt32))
     addHelperImport(genFunctionName.stringCharAt, List(WasmRefType.any, WasmInt32), List(WasmInt32))
     addHelperImport(genFunctionName.jsValueToString, List(WasmRefType.any), List(WasmRefType.any))
@@ -521,7 +521,7 @@ object CoreWasmLib {
     instrs += LOCAL_SET(iLocal)
 
     // result := ""
-    instrs += CALL(genFunctionName.emptyString)
+    instrs += GLOBAL_GET(genGlobalName.emptyString)
     instrs += LOCAL_SET(resultLocal)
 
     instrs.loop() { labelLoop =>

Diff for: wasm/src/main/scala/org/scalajs/linker/backend/wasmemitter/LoaderContent.scala

@@ -72,7 +72,7 @@ const scalaJSHelpers = {
   is: Object.is,
 
   // undefined
-  undef: () => void 0,
+  undef: void 0,
   isUndef: (x) => x === (void 0),
 
   // Boxes (upcast) -- most are identity at the JS level but with different types in Wasm
@@ -118,7 +118,7 @@ const scalaJSHelpers = {
   closureRestNoData: (f, n) => ((...args) => f(...args.slice(0, n), args.slice(n))),
 
   // Strings
-  emptyString: () => "",
+  emptyString: "",
   stringLength: (s) => s.length,
   stringCharAt: (s, i) => s.charCodeAt(i),
   jsValueToString: (x) => (x === void 0) ? "undefined" : x.toString(),

Diff for: wasm/src/main/scala/org/scalajs/linker/backend/wasmemitter/VarGen.scala

@@ -39,6 +39,12 @@ object VarGen {
     def forJSPrivateField(fieldName: FieldName): WasmGlobalName =
       WasmGlobalName(s"jspfield.${fieldName.nameString}")
 
+    val undef: WasmGlobalName =
+      WasmGlobalName("undef")
+
+    val emptyString: WasmGlobalName =
+      WasmGlobalName("emptyString")
+
     val stringLiteralCache: WasmGlobalName =
       WasmGlobalName("string_literal")
 
@@ -122,7 +128,6 @@ object VarGen {
 
     val is = make("is")
 
-    val undef = make("undef")
     val isUndef = make("isUndef")
 
     def box(primRef: PrimRef): WasmFunctionName = make("b" + primRef.charCode)
@@ -138,7 +143,6 @@ object VarGen {
     val closureThisRest = make("closureThisRest")
     val closureRestNoData = make("closureRestNoData")
 
-    val emptyString = make("emptyString")
     val stringLength = make("stringLength")
     val stringCharAt = make("stringCharAt")
     val jsValueToString = make("jsValueToString") // for actual toString() call

Diff for: wasm/src/main/scala/org/scalajs/linker/backend/wasmemitter/WasmExpressionBuilder.scala

@@ -792,7 +792,7 @@ private class WasmExpressionBuilder private (
         case IRTrees.DoubleLiteral(v)  => instrs += WasmInstr.F64_CONST(v)
 
         case v: IRTrees.Undefined =>
-          instrs += CALL(genFunctionName.undef)
+          instrs += GLOBAL_GET(genGlobalName.undef)
         case v: IRTrees.Null =>
           instrs += WasmInstr.REF_NULL(Types.WasmHeapType.None)
 
@@ -1586,7 +1586,7 @@ private class WasmExpressionBuilder private (
     targetTpe match {
       case IRTypes.UndefType =>
         instrs += DROP
-        instrs += CALL(genFunctionName.undef)
+        instrs += GLOBAL_GET(genGlobalName.undef)
       case IRTypes.StringType =>
         instrs += REF_AS_NOT_NULL