Binary expression operator overloads for +/==; Check allocation flow in ternary expressions; Cache empty array buffers; Sealed decorator for non-derivable internals

src/compiler.ts

@@ -2672,7 +2672,18 @@ export class Compiler extends DiagnosticEmitter {
             expr = module.createBinary(BinaryOp.EqI32, leftExpr, rightExpr);
             break;
           }
-          case TypeKind.USIZE: // TODO: check operator overload
+          case TypeKind.USIZE: { // check operator overload
+            if (this.currentType.is(TypeFlags.REFERENCE)) {
+              let classInstance = assert(this.currentType.classReference);
+              let operatorName = classInstance.prototype.fnEquals;
+              if (operatorName != null) {
+                expr = this.compileOperatorOverload(classInstance, operatorName, leftExpr, rightExpr);
+                assert(this.currentType == Type.bool);
+                break;
+              }
+            }
+            // fall-through
+          }
           case TypeKind.ISIZE: {
             expr = module.createBinary(
               this.options.isWasm64
@@ -2822,7 +2833,17 @@ export class Compiler extends DiagnosticEmitter {
             expr = module.createBinary(BinaryOp.AddI32, leftExpr, rightExpr);
             break;
           }
-          case TypeKind.USIZE: // TODO: check operator overload
+          case TypeKind.USIZE: { // check operator overload
+            if (this.currentType.is(TypeFlags.REFERENCE)) {
+              let classInstance = assert(this.currentType.classReference);
+              let operatorName = classInstance.prototype.fnConcat;
+              if (operatorName != null) {
+                expr = this.compileOperatorOverload(classInstance, operatorName, leftExpr, rightExpr);
+                break;
+              }
+            }
+            // fall-through
+          }
           case TypeKind.ISIZE: {
             expr = module.createBinary(
               this.options.isWasm64
@@ -3726,6 +3747,20 @@ export class Compiler extends DiagnosticEmitter {
       : expr;
   }
 
+  compileOperatorOverload(
+    classInstance: Class,
+    operatorName: string,
+    leftExpr: ExpressionRef,
+    rightExpr: ExpressionRef
+  ): ExpressionRef {
+    var classPrototype = classInstance.prototype;
+    var operatorPrototype = assert(assert(classPrototype.members).get(operatorName));
+    assert(operatorPrototype.kind == ElementKind.FUNCTION_PROTOTYPE);
+    var operatorInstance = (<FunctionPrototype>operatorPrototype).resolve();
+    if (!operatorInstance) return this.module.createUnreachable();
+    return this.makeCallDirect(operatorInstance, [ leftExpr, rightExpr ]);
+  }
+
   compileAssignment(expression: Expression, valueExpression: Expression, contextualType: Type): ExpressionRef {
     var currentFunction = this.currentFunction;
     var resolved = this.program.resolveExpression(expression, currentFunction); // reports
@@ -4526,7 +4561,6 @@ export class Compiler extends DiagnosticEmitter {
           assert(parent.kind == ElementKind.CLASS);
           let thisType = (<Class>parent).type;
           if (currentFunction.is(CommonFlags.CONSTRUCTOR)) {
-            let nativeSizeType = this.options.nativeSizeType;
             let flow = currentFunction.flow;
             if (!flow.is(FlowFlags.ALLOCATES)) {
               flow.set(FlowFlags.ALLOCATES);
@@ -5152,8 +5186,36 @@ export class Compiler extends DiagnosticEmitter {
         : this.compileExpression(ifElse, contextualType);
     }
 
-    var ifThenExpr = this.compileExpression(ifThen, contextualType);
-    var ifElseExpr = this.compileExpression(ifElse, contextualType);
+    var currentFunction = this.currentFunction;
+    var ifThenExpr: ExpressionRef;
+    var ifElseExpr: ExpressionRef;
+
+    // if part of a constructor, keep track of memory allocations
+    if (currentFunction.is(CommonFlags.CONSTRUCTOR)) {
+      let flow = currentFunction.flow;
+
+      flow = flow.enterBranchOrScope();
+      currentFunction.flow = flow;
+      ifThenExpr = this.compileExpression(ifThen, contextualType);
+      let ifThenAllocates = flow.is(FlowFlags.ALLOCATES);
+      flow = flow.leaveBranchOrScope();
+      currentFunction.flow = flow;
+
+      flow = flow.enterBranchOrScope();
+      currentFunction.flow = flow;
+      ifElseExpr = this.compileExpression(ifElse, contextualType);
+      let ifElseAllocates = flow.is(FlowFlags.ALLOCATES);
+      flow = flow.leaveBranchOrScope();
+      currentFunction.flow = flow;
+
+      if (ifThenAllocates && ifElseAllocates) flow.set(FlowFlags.ALLOCATES);
+
+    // otherwise simplify
+    } else {
+      ifThenExpr = this.compileExpression(ifThen, contextualType);
+      ifElseExpr = this.compileExpression(ifElse, contextualType);
+    }
+
     return this.module.createIf(condExpr, ifThenExpr, ifElseExpr);
   }
 

src/diagnosticMessages.generated.ts

@@ -20,6 +20,7 @@ export enum DiagnosticCode {
   Structs_cannot_implement_interfaces = 208,
   Invalid_regular_expression_flags = 209,
   Implementation_0_must_match_the_signature_1 = 210,
+  Class_0_is_sealed_and_cannot_be_extended = 211,
   Unterminated_string_literal = 1002,
   Identifier_expected = 1003,
   _0_expected = 1005,
@@ -122,6 +123,7 @@ export function diagnosticCodeToString(code: DiagnosticCode): string {
     case 208: return "Structs cannot implement interfaces.";
     case 209: return "Invalid regular expression flags.";
     case 210: return "Implementation '{0}' must match the signature '{1}'.";
+    case 211: return "Class '{0}' is sealed and cannot be extended.";
     case 1002: return "Unterminated string literal.";
     case 1003: return "Identifier expected.";
     case 1005: return "'{0}' expected.";

src/diagnosticMessages.json

@@ -12,6 +12,7 @@
   "Structs cannot implement interfaces.": 208,
   "Invalid regular expression flags.": 209,
   "Implementation '{0}' must match the signature '{1}'.": 210,
+  "Class '{0}' is sealed and cannot be extended.": 211,
 
   "Unterminated string literal.": 1002,
   "Identifier expected.": 1003,

src/parser.ts

@@ -171,6 +171,10 @@ export class Parser extends DiagnosticEmitter {
           flags |= CommonFlags.UNMANAGED;
           continue;
         }
+        if (text == "sealed") {
+          flags |= CommonFlags.SEALED;
+          continue;
+        }
       }
       if (!decorators) decorators = [];
       decorators.push(decorator);

src/program.ts

@@ -2041,34 +2041,36 @@ export enum CommonFlags {
   BUILTIN = 1 << 14,
   /** Is unmanaged. */
   UNMANAGED = 1 << 15,
+  /** Is sealed. */
+  SEALED = 1 << 16,
 
   // Extended modifiers usually derived from basic modifiers or internal decorators
 
   /** Is ambient, that is either declared or nested in a declared element. */
-  AMBIENT = 1 << 16,
+  AMBIENT = 1 << 17,
   /** Is generic. */
-  GENERIC = 1 << 17,
+  GENERIC = 1 << 18,
   /** Is part of a generic context. */
-  GENERIC_CONTEXT = 1 << 18,
+  GENERIC_CONTEXT = 1 << 19,
   /** Is an instance member. */
-  INSTANCE = 1 << 19,
+  INSTANCE = 1 << 20,
   /** Is a constructor. */
-  CONSTRUCTOR = 1 << 20,
+  CONSTRUCTOR = 1 << 21,
   /** Is an arrow function. */
-  ARROW = 1 << 21,
+  ARROW = 1 << 22,
   /** Is a module export. */
-  MODULE_EXPORT = 1 << 22,
+  MODULE_EXPORT = 1 << 23,
   /** Is a module import. */
-  MODULE_IMPORT = 1 << 23,
+  MODULE_IMPORT = 1 << 24,
 
   // Compilation states
 
   /** Is compiled. */
-  COMPILED = 1 << 24,
+  COMPILED = 1 << 25,
   /** Has a constant value and is therefore inlined. */
-  INLINED = 1 << 25,
+  INLINED = 1 << 26,
   /** Is scoped. */
-  SCOPED = 1 << 26
+  SCOPED = 1 << 27
 }
 
 /** Base class of all program elements. */
@@ -2873,6 +2875,13 @@ export class ClassPrototype extends Element {
         );
         return null;
       }
+      if (baseClass.is(CommonFlags.SEALED)) {
+        this.program.error(
+          DiagnosticCode.Class_0_is_sealed_and_cannot_be_extended,
+          declaration.extendsType.range, baseClass.internalName
+        );
+        return null;
+      }
       if (baseClass.prototype.is(CommonFlags.UNMANAGED) != this.is(CommonFlags.UNMANAGED)) {
         this.program.error(
           DiagnosticCode.Structs_cannot_extend_classes_and_vice_versa,

std/assembly.d.ts

@@ -354,5 +354,8 @@ declare function operator(token: string): (target: any, propertyKey: string, des
 /** Annotates a class as being unmanaged with limited capabilities. */
 declare function unmanaged(target: Function): any;
 
+/** Annotates a class as being sealed / non-derivable. */
+declare function sealed(target: Function): any;
+
 /** Annotates a class field with an explicit offset. */
 declare function offset(offset: usize): any;

std/assembly/array.ts

@@ -240,6 +240,7 @@ export class Array<T> {
 }
 
 @unmanaged
+@sealed
 export class CArray<T> {
 
   private constructor() {}

std/assembly/arraybuffer.ts

@@ -1,14 +1,12 @@
 const HEADER_SIZE: usize = sizeof<i32>();
 
-@unmanaged
+@sealed
 export class ArrayBuffer {
 
   readonly byteLength: i32;
 
   constructor(length: i32) {
-    if (<u32>length > 0x7fffffff) {
-      throw new RangeError("Invalid array buffer length");
-    }
+    if (<u32>length > 0x7fffffff) throw new RangeError("Invalid array buffer length");
     var buffer = allocate_memory(HEADER_SIZE + <usize>length);
     store<i32>(buffer, length);
     return changetype<ArrayBuffer>(buffer);
@@ -21,9 +19,21 @@ export class ArrayBuffer {
     if (end < 0) end = max(len + end, 0);
     else end = min(end, len);
     var newLen = max(end - begin, 0);
-    var buffer = allocate_memory(HEADER_SIZE + <usize>newLen);
-    store<i32>(buffer, newLen);
-    move_memory(buffer + HEADER_SIZE, changetype<usize>(this) + HEADER_SIZE + begin, newLen);
-    return changetype<ArrayBuffer>(buffer);
+    if (newLen) {
+      let buffer = allocate_memory(HEADER_SIZE + <usize>newLen);
+      store<i32>(buffer, newLen);
+      move_memory(buffer + HEADER_SIZE, changetype<usize>(this) + HEADER_SIZE + begin, newLen);
+      return changetype<ArrayBuffer>(buffer);
+    } else if (ArrayBuffer.EMPTY) {
+      return ArrayBuffer.EMPTY;
+    } else {
+      let buffer = allocate_memory(HEADER_SIZE);
+      store<i32>(buffer, 0);
+      ArrayBuffer.EMPTY = changetype<ArrayBuffer>(buffer);
+      return changetype<ArrayBuffer>(buffer);
+    }
   }
+
+  /** @internal */
+  static EMPTY: ArrayBuffer | null = null;
 }

std/assembly/string.ts

@@ -2,15 +2,16 @@
 const EMPTY: String = changetype<String>("");
 
 // number of bytes preceeding string data
-const HEAD: usize = 4;
+const HEADER_SIZE: usize = 4;
 
 function allocate(length: i32): String {
   assert(length > 0); // 0 -> EMPTY
-  var ptr = allocate_memory(HEAD + (<usize>length << 1));
+  var ptr = allocate_memory(HEADER_SIZE + (<usize>length << 1));
   store<i32>(ptr, length);
   return changetype<String>(ptr);
 }
 
+@sealed
 export class String {
 
   readonly length: i32; // capped to [0, 0x7fffffff]
@@ -28,9 +29,9 @@ export class String {
       changetype<usize>(out),
       load<u16>(
         changetype<usize>(this) + (<usize>pos << 1),
-        HEAD
+        HEADER_SIZE
       ),
-      HEAD
+      HEADER_SIZE
     );
     return out;
   }
@@ -42,7 +43,7 @@ export class String {
     }
     return load<u16>(
       changetype<usize>(this) + (<usize>pos << 1),
-      HEAD
+      HEADER_SIZE
     );
   }
 
@@ -53,14 +54,14 @@ export class String {
     }
     var first = <i32>load<u16>(
       changetype<usize>(this) + (<usize>pos << 1),
-      HEAD
+      HEADER_SIZE
     );
     if (first < 0xD800 || first > 0xDBFF || pos + 1 == this.length) {
       return first;
     }
     var second = <i32>load<u16>(
       changetype<usize>(this) + ((<usize>pos + 1) << 1),
-      HEAD
+      HEADER_SIZE
     );
     if (second < 0xDC00 || second > 0xDFFF) {
       return first;
@@ -70,32 +71,26 @@ export class String {
 
   @operator("+")
   private static __concat(left: String, right: String): String {
-    if (left == null) {
-      left = changetype<String>("null");
-    }
+    if (!changetype<usize>(left)) left = changetype<String>("null");
     return left.concat(right);
   }
 
   concat(other: String): String {
     assert(this != null);
-    if (other == null) {
-      other = changetype<String>("null");
-    }
+    if (other == null) other = changetype<String>("null");
     var thisLen: isize = this.length;
     var otherLen: isize = other.length;
     var outLen: usize = thisLen + otherLen;
-    if (outLen == 0) {
-      return EMPTY;
-    }
+    if (outLen == 0) return EMPTY;
     var out = allocate(outLen);
     move_memory(
-      changetype<usize>(out) + HEAD,
-      changetype<usize>(this) + HEAD,
+      changetype<usize>(out) + HEADER_SIZE,
+      changetype<usize>(this) + HEADER_SIZE,
       thisLen << 1
     );
     move_memory(
-      changetype<usize>(out) + HEAD + (thisLen << 1),
-      changetype<usize>(other) + HEAD,
+      changetype<usize>(out) + HEADER_SIZE + (thisLen << 1),
+      changetype<usize>(other) + HEADER_SIZE,
       otherLen << 1
     );
     return out;
@@ -113,26 +108,21 @@ export class String {
       return false;
     }
     return !compare_memory(
-      changetype<usize>(this) + HEAD + (start << 1),
-      changetype<usize>(searchString) + HEAD,
+      changetype<usize>(this) + HEADER_SIZE + (start << 1),
+      changetype<usize>(searchString) + HEADER_SIZE,
       searchLength << 1
     );
   }
 
   @operator("==")
   private static __eq(left: String, right: String): bool {
-    if (left == null) {
-      return right == null;
-    } else if (right == null) {
-      return false;
-    }
+    if (!changetype<usize>(left)) return !changetype<usize>(right);
+    else if (!changetype<usize>(right)) return false;
     var leftLength = left.length;
-    if (leftLength != right.length) {
-      return false;
-    }
+    if (leftLength != right.length) return false;
     return !compare_memory(
-      changetype<usize>(left) + HEAD,
-      changetype<usize>(right) + HEAD,
+      changetype<usize>(left) + HEADER_SIZE,
+      changetype<usize>(right) + HEADER_SIZE,
       (<usize>leftLength << 1)
     );
   }
@@ -154,8 +144,8 @@ export class String {
     // TODO: two-way, multiple char codes
     for (let k: usize = start; <isize>k + searchLen <= len; ++k) {
       if (!compare_memory(
-        changetype<usize>(this) + HEAD + (k << 1),
-        changetype<usize>(searchString) + HEAD,
+        changetype<usize>(this) + HEADER_SIZE + (k << 1),
+        changetype<usize>(searchString) + HEADER_SIZE,
         searchLen << 1)
       ) {
         return <i32>k;
@@ -177,8 +167,8 @@ export class String {
       return false;
     }
     return !compare_memory(
-      changetype<usize>(this) + HEAD + (start << 1),
-      changetype<usize>(searchString) + HEAD,
+      changetype<usize>(this) + HEADER_SIZE + (start << 1),
+      changetype<usize>(searchString) + HEADER_SIZE,
       searchLength << 1
     );
   }
@@ -197,8 +187,8 @@ export class String {
     }
     var out = allocate(resultLength);
     move_memory(
-      changetype<usize>(out) + HEAD,
-      changetype<usize>(this) + HEAD + (intStart << 1),
+      changetype<usize>(out) + HEADER_SIZE,
+      changetype<usize>(this) + HEADER_SIZE + (intStart << 1),
       <usize>resultLength << 1
     );
     return out;
@@ -220,8 +210,8 @@ export class String {
     }
     var out = allocate(len);
     move_memory(
-      changetype<usize>(out) + HEAD,
-      changetype<usize>(this) + HEAD + (from << 1),
+      changetype<usize>(out) + HEADER_SIZE,
+      changetype<usize>(this) + HEADER_SIZE + (from << 1),
       len << 1
     );
     return out;
@@ -233,7 +223,7 @@ export class String {
     while (
       length &&
       isWhiteSpaceOrLineTerminator(
-        load<u16>(changetype<usize>(this) + (length << 1), HEAD)
+        load<u16>(changetype<usize>(this) + (length << 1), HEADER_SIZE)
       )
     ) {
       --length;
@@ -242,7 +232,7 @@ export class String {
     while (
       start < length &&
       isWhiteSpaceOrLineTerminator(
-        load<u16>(changetype<usize>(this) + (start << 1), HEAD)
+        load<u16>(changetype<usize>(this) + (start << 1), HEADER_SIZE)
       )
     ) {
       ++start, --length;
@@ -255,8 +245,8 @@ export class String {
     }
     var out = allocate(length);
     move_memory(
-      changetype<usize>(out) + HEAD,
-      changetype<usize>(this) + HEAD + (start << 1),
+      changetype<usize>(out) + HEADER_SIZE,
+      changetype<usize>(this) + HEADER_SIZE + (start << 1),
       length << 1
     );
     return out;
@@ -269,7 +259,7 @@ export class String {
     while (
       start < len &&
       isWhiteSpaceOrLineTerminator(
-        load<u16>(changetype<usize>(this) + (start << 1), HEAD)
+        load<u16>(changetype<usize>(this) + (start << 1), HEADER_SIZE)
       )
     ) {
       ++start;
@@ -283,8 +273,8 @@ export class String {
     }
     var out = allocate(outLen);
     move_memory(
-      changetype<usize>(out) + HEAD,
-      changetype<usize>(this) + HEAD + (start << 1),
+      changetype<usize>(out) + HEADER_SIZE,
+      changetype<usize>(this) + HEADER_SIZE + (start << 1),
       outLen << 1
     );
     return out;
@@ -296,7 +286,7 @@ export class String {
     while (
       len > 0 &&
       isWhiteSpaceOrLineTerminator(
-        load<u16>(changetype<usize>(this) + (len << 1), HEAD)
+        load<u16>(changetype<usize>(this) + (len << 1), HEADER_SIZE)
       )
     ) {
       --len;
@@ -309,8 +299,8 @@ export class String {
     }
     var out = allocate(len);
     move_memory(
-      changetype<usize>(out) + HEAD,
-      changetype<usize>(this) + HEAD,
+      changetype<usize>(out) + HEADER_SIZE,
+      changetype<usize>(this) + HEADER_SIZE,
       len << 1
     );
     return out;
@@ -381,7 +371,7 @@ function parse<T>(str: String, radix: i32 = 0): T {
     return <T>NaN;
   }
   var ptr = changetype<usize>(str) /* + HEAD -> offset */;
-  var code = <i32>load<u16>(ptr, HEAD);
+  var code = <i32>load<u16>(ptr, HEADER_SIZE);
 
   // determine sign
   var sign: T;
@@ -389,13 +379,13 @@ function parse<T>(str: String, radix: i32 = 0): T {
     if (!--len) {
       return <T>NaN;
     }
-    code = <i32>load<u16>(ptr += 2, HEAD);
+    code = <i32>load<u16>(ptr += 2, HEADER_SIZE);
     sign = -1;
   } else if (code == CharCode.PLUS) {
     if (!--len) {
       return <T>NaN;
     }
-    code = <i32>load<u16>(ptr += 2, HEAD);
+    code = <i32>load<u16>(ptr += 2, HEADER_SIZE);
     sign = 1;
   } else {
     sign = 1;
@@ -404,7 +394,7 @@ function parse<T>(str: String, radix: i32 = 0): T {
   // determine radix
   if (!radix) {
     if (code == CharCode._0 && len > 2) {
-      switch (<i32>load<u16>(ptr + 2, HEAD)) {
+      switch (<i32>load<u16>(ptr + 2, HEADER_SIZE)) {
         case CharCode.B:
         case CharCode.b: {
           ptr += 4; len -= 2;
@@ -435,7 +425,7 @@ function parse<T>(str: String, radix: i32 = 0): T {
   // calculate value
   var num: T = 0;
   while (len--) {
-    code = <i32>load<u16>(ptr, HEAD);
+    code = <i32>load<u16>(ptr, HEADER_SIZE);
     if (code >= CharCode._0 && code <= CharCode._9) {
       code -= CharCode._0;
     } else if (code >= CharCode.A && code <= CharCode.Z) {
@@ -460,7 +450,7 @@ export function parseFloat(str: String): f64 {
     return NaN;
   }
   var ptr = changetype<usize>(str) /* + HEAD -> offset */;
-  var code = <i32>load<u16>(ptr, HEAD);
+  var code = <i32>load<u16>(ptr, HEADER_SIZE);
 
   // determine sign
   var sign: f64;
@@ -468,13 +458,13 @@ export function parseFloat(str: String): f64 {
     if (!--len) {
       return NaN;
     }
-    code = <i32>load<u16>(ptr += 2, HEAD);
+    code = <i32>load<u16>(ptr += 2, HEADER_SIZE);
     sign = -1;
   } else if (code == CharCode.PLUS) {
     if (!--len) {
       return NaN;
     }
-    code = <i32>load<u16>(ptr += 2, HEAD);
+    code = <i32>load<u16>(ptr += 2, HEADER_SIZE);
     sign = 1;
   } else {
     sign = 1;
@@ -483,12 +473,12 @@ export function parseFloat(str: String): f64 {
   // calculate value
   var num: f64 = 0;
   while (len--) {
-    code = <i32>load<u16>(ptr, HEAD);
+    code = <i32>load<u16>(ptr, HEADER_SIZE);
     if (code == CharCode.DOT) {
       ptr += 2;
       let fac: f64 = 0.1; // precision :(
       while (len--) {
-        code = <i32>load<u16>(ptr, HEAD);
+        code = <i32>load<u16>(ptr, HEADER_SIZE);
         if (code == CharCode.E || code == CharCode.e) {
           assert(false); // TODO
         }

tests/compiler/std/arraybuffer.optimized.wat

@@ -10,6 +10,7 @@
  (global "$(lib)/allocator/arena/startOffset" (mut i32) (i32.const 0))
  (global "$(lib)/allocator/arena/offset" (mut i32) (i32.const 0))
  (global $std/arraybuffer/buffer (mut i32) (i32.const 0))
+ (global "$(lib)/arraybuffer/ArrayBuffer.EMPTY" (mut i32) (i32.const 0))
  (global $std/arraybuffer/sliced (mut i32) (i32.const 0))
  (global $HEAP_BASE i32 (i32.const 44))
  (memory $0 1)
@@ -2028,82 +2029,105 @@
     )
    )
   )
-  (i32.store
-   (tee_local $3
-    (call "$(lib)/allocator/arena/allocate_memory"
-     (i32.add
-      (tee_local $2
-       (select
-        (tee_local $3
-         (i32.sub
-          (tee_local $2
-           (if (result i32)
-            (i32.lt_s
-             (get_local $2)
-             (i32.const 0)
-            )
-            (select
-             (tee_local $3
-              (i32.add
-               (get_local $5)
-               (get_local $2)
-              )
-             )
-             (tee_local $4
-              (i32.const 0)
-             )
-             (i32.gt_s
-              (get_local $3)
-              (get_local $4)
-             )
-            )
-            (select
-             (tee_local $3
-              (get_local $2)
-             )
-             (tee_local $4
-              (get_local $5)
-             )
-             (i32.lt_s
-              (get_local $3)
-              (get_local $4)
-             )
-            )
+  (if (result i32)
+   (tee_local $2
+    (select
+     (tee_local $3
+      (i32.sub
+       (tee_local $2
+        (if (result i32)
+         (i32.lt_s
+          (get_local $2)
+          (i32.const 0)
+         )
+         (select
+          (tee_local $3
+           (i32.add
+            (get_local $5)
+            (get_local $2)
            )
           )
-          (get_local $1)
+          (tee_local $4
+           (i32.const 0)
+          )
+          (i32.gt_s
+           (get_local $3)
+           (get_local $4)
+          )
+         )
+         (select
+          (tee_local $3
+           (get_local $2)
+          )
+          (tee_local $4
+           (get_local $5)
+          )
+          (i32.lt_s
+           (get_local $3)
+           (get_local $4)
+          )
          )
         )
-        (tee_local $4
-         (i32.const 0)
-        )
-        (i32.gt_s
-         (get_local $3)
-         (get_local $4)
-        )
        )
+       (get_local $1)
       )
-      (i32.const 4)
+     )
+     (tee_local $4
+      (i32.const 0)
+     )
+     (i32.gt_s
+      (get_local $3)
+      (get_local $4)
      )
     )
    )
-   (get_local $2)
-  )
-  (call "$(lib)/memory/move_memory"
-   (i32.add
-    (get_local $3)
-    (i32.const 4)
-   )
-   (i32.add
-    (i32.add
-     (get_local $0)
-     (i32.const 4)
+   (block (result i32)
+    (i32.store
+     (tee_local $3
+      (call "$(lib)/allocator/arena/allocate_memory"
+       (i32.add
+        (get_local $2)
+        (i32.const 4)
+       )
+      )
+     )
+     (get_local $2)
+    )
+    (call "$(lib)/memory/move_memory"
+     (i32.add
+      (get_local $3)
+      (i32.const 4)
+     )
+     (i32.add
+      (i32.add
+       (get_local $0)
+       (i32.const 4)
+      )
+      (get_local $1)
+     )
+     (get_local $2)
+    )
+    (get_local $3)
+   )
+   (if (result i32)
+    (get_global "$(lib)/arraybuffer/ArrayBuffer.EMPTY")
+    (get_global "$(lib)/arraybuffer/ArrayBuffer.EMPTY")
+    (block (result i32)
+     (i32.store
+      (tee_local $3
+       (call "$(lib)/allocator/arena/allocate_memory"
+        (i32.const 4)
+       )
+      )
+      (i32.const 0)
+     )
+     (set_global "$(lib)/arraybuffer/ArrayBuffer.EMPTY"
+      (get_local $3)
+     )
+     (get_local $3)
     )
-    (get_local $1)
    )
-   (get_local $2)
   )
-  (get_local $3)
  )
  (func "$(lib)/arraybuffer/ArrayBuffer#slice|trampoline" (; 6 ;) (type $iiiii) (param $0 i32) (param $1 i32) (param $2 i32) (param $3 i32) (result i32)
   (block $N=2

tests/compiler/std/arraybuffer.untouched.wat

@@ -15,6 +15,7 @@
  (global "$(lib)/allocator/arena/offset" (mut i32) (i32.const 0))
  (global "$(lib)/arraybuffer/HEADER_SIZE" i32 (i32.const 4))
  (global $std/arraybuffer/buffer (mut i32) (i32.const 0))
+ (global "$(lib)/arraybuffer/ArrayBuffer.EMPTY" (mut i32) (i32.const 0))
  (global $std/arraybuffer/sliced (mut i32) (i32.const 0))
  (global $HEAP_BASE i32 (i32.const 44))
  (memory $0 1)
@@ -2280,7 +2281,6 @@
   (local $4 i32)
   (local $5 i32)
   (local $6 i32)
-  (local $7 i32)
   (set_local $3
    (i32.load
     (get_local $0)
@@ -2377,34 +2377,62 @@
     )
    )
   )
-  (set_local $7
-   (call "$(lib)/allocator/arena/allocate_memory"
-    (i32.add
-     (i32.const 4)
+  (if
+   (get_local $6)
+   (block
+    (set_local $4
+     (call "$(lib)/allocator/arena/allocate_memory"
+      (i32.add
+       (i32.const 4)
+       (get_local $6)
+      )
+     )
+    )
+    (i32.store
+     (get_local $4)
      (get_local $6)
     )
-   )
-  )
-  (i32.store
-   (get_local $7)
-   (get_local $6)
-  )
-  (call "$(lib)/memory/move_memory"
-   (i32.add
-    (get_local $7)
-    (i32.const 4)
-   )
-   (i32.add
-    (i32.add
-     (get_local $0)
-     (i32.const 4)
+    (call "$(lib)/memory/move_memory"
+     (i32.add
+      (get_local $4)
+      (i32.const 4)
+     )
+     (i32.add
+      (i32.add
+       (get_local $0)
+       (i32.const 4)
+      )
+      (get_local $1)
+     )
+     (get_local $6)
+    )
+    (return
+     (get_local $4)
+    )
+   )
+   (if
+    (get_global "$(lib)/arraybuffer/ArrayBuffer.EMPTY")
+    (return
+     (get_global "$(lib)/arraybuffer/ArrayBuffer.EMPTY")
+    )
+    (block
+     (set_local $4
+      (call "$(lib)/allocator/arena/allocate_memory"
+       (i32.const 4)
+      )
+     )
+     (i32.store
+      (get_local $4)
+      (i32.const 0)
+     )
+     (set_global "$(lib)/arraybuffer/ArrayBuffer.EMPTY"
+      (get_local $4)
+     )
+     (return
+      (get_local $4)
+     )
     )
-    (get_local $1)
    )
-   (get_local $6)
-  )
-  (return
-   (get_local $7)
   )
  )
  (func "$(lib)/arraybuffer/ArrayBuffer#slice|trampoline" (; 6 ;) (type $iiiii) (param $0 i32) (param $1 i32) (param $2 i32) (param $3 i32) (result i32)

tests/compiler/std/string.ts

@@ -1,3 +1,5 @@
+import "allocator/arena";
+
 // preliminary
 
 var str: string = "hi, I'm a string";
@@ -31,3 +33,6 @@ assert(parseFloat("1") == 1);
 assert(parseFloat("0.1") == 0.1);
 assert(parseFloat(".25") == 0.25);
 assert(parseFloat(".1foobar") == 0.1);
+
+var c = "a" + "b";
+assert(c == "ab");