diff --git a/src/builtins.ts b/src/builtins.ts
index eac6391b..d37045f4 100644
--- a/src/builtins.ts
+++ b/src/builtins.ts
@@ -1567,7 +1567,6 @@ export function compileCall(compiler: Compiler, prototype: FunctionPrototype, ty
         arg0 = compiler.compileExpression(operands[0], Type.i32, ConversionKind.NONE);
 
       type = compiler.currentType;
-      arg1 = operands.length == 2 ? compiler.compileExpression(operands[1], compiler.options.usizeType) : compiler.options.usizeType.toNativeZero(module);
       compiler.currentType = type.nonNullableType;
 
       // just return ifTrueish if assertions are disabled, or simplify if dropped anyway
@@ -1579,6 +1578,21 @@ export function compileCall(compiler: Compiler, prototype: FunctionPrototype, ty
         return arg0;
       }
 
+      var abort: ExpressionRef = module.createUnreachable();
+      var abortPrototype = compiler.program.elements.get("abort");
+      if (abortPrototype && abortPrototype.kind == ElementKind.FUNCTION_PROTOTYPE) {
+        var abortInstance = (<FunctionPrototype>abortPrototype).resolve();
+        if (abortInstance && compiler.compileFunction(abortInstance)) {
+          abort = compiler.makeCall(abortInstance, [
+            operands.length == 2 ? compiler.compileExpression(operands[1], compiler.options.usizeType) : compiler.options.usizeType.toNativeZero(module),
+            compiler.compileStaticString(reportNode.range.source.path),
+            module.createI32(reportNode.range.line),
+            module.createI32(reportNode.range.column)
+          ]);
+        }
+      }
+      compiler.currentType = type.nonNullableType;
+
       if (contextualType == Type.void) { // simplify if dropped anyway
         switch (compiler.currentType.kind) {
 
@@ -1587,7 +1601,7 @@ export function compileCall(compiler: Compiler, prototype: FunctionPrototype, ty
               module.createUnary(UnaryOp.EqzI32,
                 arg0
               ),
-              module.createUnreachable()
+              abort
             );
             break;
 
@@ -1597,7 +1611,7 @@ export function compileCall(compiler: Compiler, prototype: FunctionPrototype, ty
               module.createUnary(UnaryOp.EqzI64,
                 arg0
               ),
-              module.createUnreachable()
+              abort
             );
             break;
 
@@ -1607,7 +1621,7 @@ export function compileCall(compiler: Compiler, prototype: FunctionPrototype, ty
               module.createUnary(compiler.options.target == Target.WASM64 ? UnaryOp.EqzI64 : UnaryOp.EqzI32,
                 arg0
               ),
-              module.createUnreachable()
+              abort
             );
             break;
 
@@ -1619,7 +1633,7 @@ export function compileCall(compiler: Compiler, prototype: FunctionPrototype, ty
                 arg0,
                 module.createF32(0)
               ),
-              module.createUnreachable()
+              abort
             );
             break;
 
@@ -1629,13 +1643,13 @@ export function compileCall(compiler: Compiler, prototype: FunctionPrototype, ty
                 arg0,
                 module.createF64(0)
               ),
-              module.createUnreachable()
+              abort
             );
             break;
 
           case TypeKind.VOID:
             compiler.error(DiagnosticCode.Operation_not_supported, reportNode.range);
-            ret = module.createUnreachable();
+            ret = abort;
             break;
         }
         compiler.currentType = Type.void;
@@ -1648,7 +1662,7 @@ export function compileCall(compiler: Compiler, prototype: FunctionPrototype, ty
               module.createUnary(UnaryOp.EqzI32,
                 module.createTeeLocal(tempLocal0.index, arg0)
               ),
-              module.createUnreachable(),
+              abort,
               module.createGetLocal(tempLocal0.index, NativeType.I32)
             );
             break;
@@ -1660,7 +1674,7 @@ export function compileCall(compiler: Compiler, prototype: FunctionPrototype, ty
               module.createUnary(UnaryOp.EqzI64,
                 module.createTeeLocal(tempLocal0.index, arg0)
               ),
-              module.createUnreachable(),
+              abort,
               module.createGetLocal(tempLocal0.index, NativeType.I64)
             );
             break;
@@ -1672,7 +1686,7 @@ export function compileCall(compiler: Compiler, prototype: FunctionPrototype, ty
               module.createUnary(compiler.options.target == Target.WASM64 ? UnaryOp.EqzI64 : UnaryOp.EqzI32,
                 module.createTeeLocal(tempLocal0.index, arg0)
               ),
-              module.createUnreachable(),
+              abort,
               module.createGetLocal(tempLocal0.index, compiler.options.nativeSizeType)
             );
             break;
@@ -1684,7 +1698,7 @@ export function compileCall(compiler: Compiler, prototype: FunctionPrototype, ty
                 module.createTeeLocal(tempLocal0.index, arg0),
                 module.createF32(0)
               ),
-              module.createUnreachable(),
+              abort,
               module.createGetLocal(tempLocal0.index, NativeType.F32)
             );
             break;
@@ -1696,14 +1710,14 @@ export function compileCall(compiler: Compiler, prototype: FunctionPrototype, ty
                 module.createTeeLocal(tempLocal0.index, arg0),
                 module.createF64(0)
               ),
-              module.createUnreachable(),
+              abort,
               module.createGetLocal(tempLocal0.index, NativeType.F64)
             );
             break;
 
           case TypeKind.VOID:
             compiler.error(DiagnosticCode.Operation_not_supported, reportNode.range);
-            ret = module.createUnreachable();
+            ret = abort;
             break;
         }
       }
diff --git a/src/compiler.ts b/src/compiler.ts
index 1ec3f47c..f64fcfe5 100644
--- a/src/compiler.ts
+++ b/src/compiler.ts
@@ -2862,27 +2862,7 @@ export class Compiler extends DiagnosticEmitter {
         return this.module.createI32(intValue.toI32());
 
       case LiteralKind.STRING:
-        var stringValue = (<StringLiteralExpression>expression).value;
-        var stringSegment: MemorySegment | null = this.stringSegments.get(stringValue);
-        if (!stringSegment) {
-          var stringLength = stringValue.length;
-          var stringBuffer = new Uint8Array(4 + stringLength * 2);
-          stringBuffer[0] =  stringLength         & 0xff;
-          stringBuffer[1] = (stringLength >>>  8) & 0xff;
-          stringBuffer[2] = (stringLength >>> 16) & 0xff;
-          stringBuffer[3] = (stringLength >>> 24) & 0xff;
-          for (var i = 0; i < stringLength; ++i) {
-            stringBuffer[4 + i * 2] =  stringValue.charCodeAt(i)        & 0xff;
-            stringBuffer[5 + i * 2] = (stringValue.charCodeAt(i) >>> 8) & 0xff;
-          }
-          stringSegment = this.addMemorySegment(stringBuffer);
-          this.stringSegments.set(stringValue, stringSegment);
-        }
-        var stringOffset = stringSegment.offset;
-        this.currentType = this.options.usizeType;
-        return this.options.isWasm64
-          ? this.module.createI64(stringOffset.lo, stringOffset.hi)
-          : this.module.createI32(stringOffset.lo);
+        return this.compileStaticString((<StringLiteralExpression>expression).value);
 
       // case LiteralKind.OBJECT:
       // case LiteralKind.REGEXP:
@@ -2891,6 +2871,29 @@ export class Compiler extends DiagnosticEmitter {
     throw new Error("not implemented");
   }
 
+  compileStaticString(stringValue: string): ExpressionRef {
+    var stringSegment: MemorySegment | null = this.stringSegments.get(stringValue);
+    if (!stringSegment) {
+      var stringLength = stringValue.length;
+      var stringBuffer = new Uint8Array(4 + stringLength * 2);
+      stringBuffer[0] =  stringLength         & 0xff;
+      stringBuffer[1] = (stringLength >>>  8) & 0xff;
+      stringBuffer[2] = (stringLength >>> 16) & 0xff;
+      stringBuffer[3] = (stringLength >>> 24) & 0xff;
+      for (var i = 0; i < stringLength; ++i) {
+        stringBuffer[4 + i * 2] =  stringValue.charCodeAt(i)        & 0xff;
+        stringBuffer[5 + i * 2] = (stringValue.charCodeAt(i) >>> 8) & 0xff;
+      }
+      stringSegment = this.addMemorySegment(stringBuffer);
+      this.stringSegments.set(stringValue, stringSegment);
+    }
+    var stringOffset = stringSegment.offset;
+    this.currentType = this.options.usizeType;
+    return this.options.isWasm64
+      ? this.module.createI64(stringOffset.lo, stringOffset.hi)
+      : this.module.createI32(stringOffset.lo);
+  }
+
   compileNewExpression(expression: NewExpression, contextualType: Type): ExpressionRef {
     var resolved = this.program.resolveExpression(expression.expression, this.currentFunction); // reports
     if (resolved) {
diff --git a/src/diagnostics.ts b/src/diagnostics.ts
index cee2717e..b82236b9 100644
--- a/src/diagnostics.ts
+++ b/src/diagnostics.ts
@@ -121,20 +121,12 @@ export function formatDiagnosticMessage(message: DiagnosticMessage, useColors: b
       sb.push(context);
     }
     sb.push("\n");
-    var pos = range.start;
-    var line = 1;
-    var column = 1;
-    while (pos-- > 0)
-      if (text.charCodeAt(pos) == CharCode.LINEFEED)
-        line++;
-      else if (line == 1)
-        column++;
     sb.push(" in ");
     sb.push(range.source.path);
     sb.push("(");
-    sb.push(line.toString(10));
+    sb.push(range.line.toString(10));
     sb.push(",");
-    sb.push(column.toString(10));
+    sb.push(range.column.toString(10));
     sb.push(")");
   }
   return sb.join("");
diff --git a/src/tokenizer.ts b/src/tokenizer.ts
index 8cc6c8d3..4ccf3f69 100644
--- a/src/tokenizer.ts
+++ b/src/tokenizer.ts
@@ -293,6 +293,27 @@ export class Range {
   get atStart(): Range { return new Range(this.source, this.start, this.start); }
   get atEnd(): Range { return new Range(this.source, this.end, this.end); }
 
+  get line(): i32 {
+    var text = this.source.text;
+    var pos = this.start;
+    var line = 1;
+    while (pos-- > 0)
+      if (text.charCodeAt(pos) == CharCode.LINEFEED)
+        line++;
+    return line;
+  }
+
+  get column(): i32 {
+    var text = this.source.text;
+    var pos = this.start;
+    var column = 1;
+    while (pos-- > 0)
+      if (text.charCodeAt(pos) == CharCode.LINEFEED)
+        break;
+      column++;
+    return column;
+  }
+
   toString(): string {
     return this.source.text.substring(this.start, this.end);
   }
diff --git a/std/assembly/string.ts b/std/assembly/string.ts
index d3adf31c..001e044e 100644
--- a/std/assembly/string.ts
+++ b/std/assembly/string.ts
@@ -1,12 +1,14 @@
 const EMPTY: String = changetype<String>("");
 
+const HEAD: usize = 4;
+
 function allocate(length: i32): String {
-  var ptr = allocate_memory(4 + length * 2);
+  assert(<u32>length >= 1);
+  var ptr = allocate_memory(HEAD + length * 2);
   store<i32>(ptr, length);
   return changetype<String>(ptr);
 }
 
-@unmanaged
 export class String {
 
   readonly length: i32;
@@ -16,23 +18,27 @@ export class String {
     if (<u32>pos >= this.length)
       return EMPTY;
     var out = allocate(1);
-    store<u16>(changetype<usize>(out), load<u16>(changetype<usize>(this) + (<usize>pos << 1), 4), 4);
+    store<u16>(
+      changetype<usize>(out),
+      load<u16>(changetype<usize>(this) + (<usize>pos << 1), HEAD),
+      HEAD
+    );
     return out;
   }
 
   charCodeAt(pos: i32): i32 {
     if (<u32>pos >= this.length)
       return -1; // NaN
-    return load<u16>(changetype<usize>(this) + (<usize>pos << 1), 4);
+    return load<u16>(changetype<usize>(this) + (<usize>pos << 1), HEAD);
   }
 
   codePointAt(pos: i32): i32 {
     if (<u32>pos >= this.length)
       return -1; // undefined
-    var first = <i32>load<u16>(changetype<usize>(this) + (<usize>pos << 1), 4);
+    var first = <i32>load<u16>(changetype<usize>(this) + (<usize>pos << 1), HEAD);
     if (first < 0xD800 || first > 0xDBFF || pos + 1 == this.length)
       return first;
-    var second = <i32>load<u16>(changetype<usize>(this) + ((<usize>pos + 1) << 1), 4);
+    var second = <i32>load<u16>(changetype<usize>(this) + ((<usize>pos + 1) << 1), HEAD);
     if (second < 0xDC00 || second > 0xDFFF)
       return first;
     return ((first - 0xD800) << 10) + (second - 0xDC00) + 0x10000;
@@ -45,9 +51,19 @@ export class String {
     var thisLen: isize = this.length;
     var otherLen: isize = other.length;
     var len: usize = thisLen + otherLen;
+    if (len == 0)
+      return EMPTY;
     var out = allocate(len);
-    move_memory(changetype<usize>(out) + 4, changetype<usize>(this) + 4, thisLen << 1);
-    move_memory(changetype<usize>(out) + 4 + (thisLen << 1), changetype<usize>(other) + 4, otherLen << 1);
+    move_memory(
+      changetype<usize>(out) + HEAD,
+      changetype<usize>(this) + HEAD,
+      thisLen << 1
+    );
+    move_memory(
+      changetype<usize>(out) + HEAD + (thisLen << 1),
+      changetype<usize>(other) + HEAD,
+      otherLen << 1
+    );
     return out;
   }
 
@@ -58,7 +74,11 @@ export class String {
     var start: isize = end - searchLength;
     if (start < 0)
       return false;
-    return !compare_memory(changetype<usize>(this) + 4 + (start << 1), changetype<usize>(searchString) + 4, searchLength << 1);
+    return !compare_memory(
+      changetype<usize>(this) + HEAD + (start << 1),
+      changetype<usize>(searchString) + HEAD,
+      searchLength << 1
+    );
   }
 
   @operator("==")
@@ -69,7 +89,11 @@ export class String {
       return false;
     if (this.length != other.length)
       return false;
-    return !compare_memory(changetype<usize>(this) + 4, changetype<usize>(other) + 4, <usize>(this.length << 1));
+    return !compare_memory(
+      changetype<usize>(this) + HEAD,
+      changetype<usize>(other) + HEAD,
+      <usize>(this.length << 1)
+    );
   }
 
   includes(searchString: String, position: i32 = 0): bool {
@@ -83,7 +107,11 @@ export class String {
     var start: isize = min<isize>(max<isize>(pos, 0), len);
     var searchLen: isize = searchString.length;
     for (var k: usize = start; <isize>k + searchLen <= len; ++k)
-      if (!compare_memory(changetype<usize>(this) + 4 + (k << 1), changetype<usize>(searchString) + 4, searchLen << 1))
+      if (!compare_memory(
+        changetype<usize>(this) + HEAD + (k << 1),
+        changetype<usize>(searchString) + HEAD,
+        searchLen << 1)
+      )
         return <i32>k;
     return -1;
   }
@@ -97,7 +125,11 @@ export class String {
     var searchLength: isize = searchString.length;
     if (searchLength + start > len)
       return false;
-    return !compare_memory(changetype<usize>(this) + 4 + (start << 1), changetype<usize>(searchString) + 4, searchLength << 1);
+    return !compare_memory(
+      changetype<usize>(this) + HEAD + (start << 1),
+      changetype<usize>(searchString) + HEAD,
+      searchLength << 1
+    );
   }
 
   substr(start: i32, length: i32 = i32.MAX_VALUE): String {
@@ -108,10 +140,14 @@ export class String {
     if (intStart < 0)
       intStart = max<isize>(size + intStart, 0);
     var resultLength: isize = min<isize>(max<isize>(end, 0), size - intStart);
-    if (resultLength < 0)
+    if (resultLength <= 0)
       return EMPTY;
     var out = allocate(resultLength);
-    move_memory(changetype<usize>(out) + 4, changetype<usize>(this) + 4 + (intStart << 1), <usize>resultLength << 1);
+    move_memory(
+      changetype<usize>(out) + HEAD,
+      changetype<usize>(this) + HEAD + (intStart << 1),
+      <usize>resultLength << 1
+    );
     return out;
   }
 
@@ -128,17 +164,21 @@ export class String {
     if (!from && to == this.length)
       return this;
     var out = allocate(len);
-    move_memory(changetype<usize>(out) + 4, changetype<usize>(this) + 4 + (from << 1), len << 1);
+    move_memory(
+      changetype<usize>(out) + HEAD,
+      changetype<usize>(this) + HEAD + (from << 1),
+      len << 1
+    );
     return out;
   }
 
   trim(): String {
     assert(this != null);
     var length: usize = this.length;
-    while (length && isWhiteSpaceOrLineTerminator(load<u16>(changetype<usize>(this) + (length << 1), 4)))
+    while (length && isWhiteSpaceOrLineTerminator(load<u16>(changetype<usize>(this) + (length << 1), HEAD)))
       --length;
     var start: usize = 0;
-    while (start < length && isWhiteSpaceOrLineTerminator(load<u16>(changetype<usize>(this) + (start << 1), 4))) {
+    while (start < length && isWhiteSpaceOrLineTerminator(load<u16>(changetype<usize>(this) + (start << 1), HEAD))) {
       ++start; --length;
     }
     if (!length)
@@ -146,7 +186,11 @@ export class String {
     if (!start && length == this.length)
       return this;
     var out = allocate(length);
-    move_memory(changetype<usize>(out) + 4, changetype<usize>(this) + 4 + (start << 1), length << 1);
+    move_memory(
+      changetype<usize>(out) + HEAD,
+      changetype<usize>(this) + HEAD + (start << 1),
+      length << 1
+    );
     return out;
   }
 
@@ -154,27 +198,37 @@ export class String {
     assert(this != null);
     var start: isize = 0;
     var len: isize = this.length;
-    while (start < len && isWhiteSpaceOrLineTerminator(load<u16>(changetype<usize>(this) + (start << 1), 4)))
+    while (start < len && isWhiteSpaceOrLineTerminator(load<u16>(changetype<usize>(this) + (start << 1), HEAD)))
       ++start;
     if (!start)
       return this;
     var outLen = len - start;
+    if (!outLen)
+      return EMPTY;
     var out = allocate(outLen);
-    move_memory(changetype<usize>(out) + 4, changetype<usize>(this) + 4 + (start << 1), outLen << 1);
+    move_memory(
+      changetype<usize>(out) + HEAD,
+      changetype<usize>(this) + HEAD + (start << 1),
+      outLen << 1
+    );
     return out;
   }
 
   trimRight(): String {
     assert(this != null);
     var len: isize = this.length;
-    while (len > 0 && isWhiteSpaceOrLineTerminator(load<u16>(changetype<usize>(this) + (len << 1), 4)))
+    while (len > 0 && isWhiteSpaceOrLineTerminator(load<u16>(changetype<usize>(this) + (len << 1), HEAD)))
       --len;
     if (len <= 0)
       return EMPTY;
     if (<i32>len == this.length)
       return this;
     var out = allocate(len);
-    move_memory(changetype<usize>(out) + 4, changetype<usize>(this) + 4, len << 1);
+    move_memory(
+      changetype<usize>(out) + HEAD,
+      changetype<usize>(this) + HEAD,
+      len << 1
+    );
     return out;
   }
 }
diff --git a/tests/compiler/std/abort.ts b/tests/compiler/std/abort.ts
new file mode 100644
index 00000000..d29a3602
--- /dev/null
+++ b/tests/compiler/std/abort.ts
@@ -0,0 +1,11 @@
+var abortCalled = false;
+
+@global
+function abort(message: string | null, filename: string, line: i32, column: i32): void {
+  abortCalled = true;
+}
+
+assert(false, "this is ok");
+
+if (!abortCalled)
+  unreachable();