v8/test/mjsunit/wasm/stringrefs-exec.js

// Copyright 2022 the V8 project authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

// Flags: --experimental-wasm-stringref

d8.file.execute("test/mjsunit/wasm/wasm-module-builder.js");

let kSig_w_ii = makeSig([kWasmI32, kWasmI32], [kWasmStringRef]);
let kSig_w_v = makeSig([], [kWasmStringRef]);
let kSig_i_w = makeSig([kWasmStringRef], [kWasmI32]);
let kSig_i_wi = makeSig([kWasmStringRef, kWasmI32], [kWasmI32]);
let kSig_i_ww = makeSig([kWasmStringRef, kWasmStringRef], [kWasmI32]);
let kSig_w_wii = makeSig([kWasmStringRef, kWasmI32, kWasmI32],
                         [kWasmStringRef]);
let kSig_v_wi = makeSig([kWasmStringRef, kWasmI32], []);
let kSig_v_wiii = makeSig([kWasmStringRef, kWasmI32, kWasmI32, kWasmI32],
                          []);
let kSig_w_ww = makeSig([kWasmStringRef, kWasmStringRef], [kWasmStringRef]);
let kSig_w_w = makeSig([kWasmStringRef], [kWasmStringRef]);

function encodeWtf8(str) {
  // String iterator coalesces surrogate pairs.
  let out = [];
  for (let codepoint of str) {
    codepoint = codepoint.codePointAt(0);
    if (codepoint <= 0x7f) {
      out.push(codepoint);
    } else if (codepoint <= 0x7ff) {
      out.push(0xc0 | (codepoint >> 6));
      out.push(0x80 | (codepoint & 0x3f));
    } else if (codepoint <= 0xffff) {
      out.push(0xe0 | (codepoint >> 12));
      out.push(0x80 | ((codepoint >> 6) & 0x3f));
      out.push(0x80 | (codepoint & 0x3f));
    } else if (codepoint <= 0x10ffff) {
      out.push(0xf0 | (codepoint >> 18));
      out.push(0x80 | ((codepoint >> 12) & 0x3f));
      out.push(0x80 | ((codepoint >> 6) & 0x3f));
      out.push(0x80 | (codepoint & 0x3f));
    } else {
      throw new Error("bad codepoint " + codepoint);
    }
  }
  return out;
}

let interestingStrings = ['',
                          'ascii',
                          'latin \xa9 1',
                          'two \ucccc byte',
                          'surrogate \ud800\udc000 pair',
                          'isolated \ud800 leading',
                          'isolated \udc00 trailing',
                          '\ud800 isolated leading at beginning',
                          '\udc00 isolated trailing at beginning',
                          'isolated leading at end \ud800',
                          'isolated trailing at end \udc00',
                          'swapped surrogate \udc00\ud800 pair'];

function IsSurrogate(codepoint) {
  return 0xD800 <= codepoint && codepoint <= 0xDFFF
}
function HasIsolatedSurrogate(str) {
  for (let codepoint of str) {
    let value = codepoint.codePointAt(0);
    if (IsSurrogate(value)) return true;
  }
  return false;
}
function ReplaceIsolatedSurrogates(str, replacement='\ufffd') {
  let replaced = '';
  for (let codepoint of str) {
    replaced +=
      IsSurrogate(codepoint.codePointAt(0)) ? replacement : codepoint;
  }
  return replaced;
}

function makeWtf8TestDataSegment() {
  let data = []
  let valid = {};
  let invalid = {};

  for (let str of interestingStrings) {
    let bytes = encodeWtf8(str);
    valid[str] = { offset: data.length, length: bytes.length };
    for (let byte of bytes) {
      data.push(byte);
    }
  }
  for (let bytes of ['trailing high byte \xa9',
                     'interstitial high \xa9 byte',
                     'invalid \xc0 byte',
                     'invalid three-byte \xed\xd0\x80',
                     'surrogate \xed\xa0\x80\xed\xb0\x80 pair']) {
    invalid[bytes] = { offset: data.length, length: bytes.length };
    for (let i = 0; i < bytes.length; i++) {
      data.push(bytes.charCodeAt(i));
    }
  }

  return { valid, invalid, data: Uint8Array.from(data) };
};

(function TestStringNewWtf8() {
  let builder = new WasmModuleBuilder();

  builder.addMemory(1, undefined, false, false);
  let data = makeWtf8TestDataSegment();
  builder.addDataSegment(0, data.data);

  builder.addFunction("string_new_utf8", kSig_w_ii)
    .exportFunc()
    .addBody([
      kExprLocalGet, 0, kExprLocalGet, 1,
      kGCPrefix, kExprStringNewWtf8, 0, kWtf8PolicyReject
    ]);

  builder.addFunction("string_new_wtf8", kSig_w_ii)
    .exportFunc()
    .addBody([
      kExprLocalGet, 0, kExprLocalGet, 1,
      kGCPrefix, kExprStringNewWtf8, 0, kWtf8PolicyAccept
    ]);

  builder.addFunction("string_new_utf8_sloppy", kSig_w_ii)
    .exportFunc()
    .addBody([
      kExprLocalGet, 0, kExprLocalGet, 1,
      kGCPrefix, kExprStringNewWtf8, 0, kWtf8PolicyReplace
    ]);

  let instance = builder.instantiate();
  for (let [str, {offset, length}] of Object.entries(data.valid)) {
    assertEquals(str, instance.exports.string_new_wtf8(offset, length));
    if (HasIsolatedSurrogate(str)) {
      assertThrows(() => instance.exports.string_new_utf8(offset, length),
                   WebAssembly.RuntimeError, "invalid UTF-8 string");

      // Isolated surrogates have the three-byte pattern ED [A0,BF]
      // [80,BF].  When the sloppy decoder gets to the second byte, it
      // will reject the sequence, and then retry parsing at the second
      // byte.  Seeing the second byte can't start a sequence, it
      // replaces the second byte and continues with the next, which
      // also can't start a sequence.  The result is that one isolated
      // surrogate is replaced by three U+FFFD codepoints.
      assertEquals(ReplaceIsolatedSurrogates(str, '\ufffd\ufffd\ufffd'),
                   instance.exports.string_new_utf8_sloppy(offset, length));
    } else {
      assertEquals(str, instance.exports.string_new_utf8(offset, length));
      assertEquals(str,
                   instance.exports.string_new_utf8_sloppy(offset, length));
    }
  }
  for (let [str, {offset, length}] of Object.entries(data.invalid)) {
    assertThrows(() => instance.exports.string_new_wtf8(offset, length),
                 WebAssembly.RuntimeError, "invalid WTF-8 string");
    assertThrows(() => instance.exports.string_new_utf8(offset, length),
                 WebAssembly.RuntimeError, "invalid UTF-8 string");
  }
})();

function encodeWtf16LE(str) {
  // String iterator coalesces surrogate pairs.
  let out = [];
  for (let i = 0; i < str.length; i++) {
    codeunit = str.charCodeAt(i);
    out.push(codeunit & 0xff)
    out.push(codeunit >> 8);
  }
  return out;
}

function makeWtf16TestDataSegment() {
  let data = []
  let valid = {};

  for (let str of interestingStrings) {
    valid[str] = { offset: data.length, length: str.length };
    for (let byte of encodeWtf16LE(str)) {
      data.push(byte);
    }
  }

  return { valid, data: Uint8Array.from(data) };
};

(function TestStringNewWtf16() {
  let builder = new WasmModuleBuilder();

  builder.addMemory(1, undefined, false, false);
  let data = makeWtf16TestDataSegment();
  builder.addDataSegment(0, data.data);

  builder.addFunction("string_new_wtf16", kSig_w_ii)
    .exportFunc()
    .addBody([
      kExprLocalGet, 0, kExprLocalGet, 1,
      kGCPrefix, kExprStringNewWtf16, 0
    ]);

  let instance = builder.instantiate();
  for (let [str, {offset, length}] of Object.entries(data.valid)) {
    assertEquals(str, instance.exports.string_new_wtf16(offset, length));
  }
})();

(function TestStringConst() {
  let builder = new WasmModuleBuilder();
  for (let [index, str] of interestingStrings.entries()) {
    builder.addLiteralStringRef(encodeWtf8(str));

    builder.addFunction("string_const" + index, kSig_w_v)
      .exportFunc()
      .addBody([kGCPrefix, kExprStringConst, index]);

    builder.addGlobal(kWasmStringRef, false,
                      [kGCPrefix, kExprStringConst, index])
      .exportAs("global" + index);
  }

  let instance = builder.instantiate();
  for (let [index, str] of interestingStrings.entries()) {
    assertEquals(str, instance.exports["string_const" + index]());
    assertEquals(str, instance.exports["global" + index].value);
  }
})();

(function TestStringMeasureUtf8AndWtf8() {
  let builder = new WasmModuleBuilder();

  builder.addFunction("string_measure_utf8", kSig_i_w)
    .exportFunc()
    .addBody([
      kExprLocalGet, 0,
      kGCPrefix, kExprStringMeasureWtf8, 0
    ]);

  builder.addFunction("string_measure_wtf8", kSig_i_w)
    .exportFunc()
    .addBody([
      kExprLocalGet, 0,
      kGCPrefix, kExprStringMeasureWtf8, 1
    ]);

  builder.addFunction("string_measure_wtf8_replace", kSig_i_w)
    .exportFunc()
    .addBody([
      kExprLocalGet, 0,
      kGCPrefix, kExprStringMeasureWtf8, 2
    ]);

  builder.addFunction("string_measure_utf8_null", kSig_i_v)
    .exportFunc()
    .addBody([
      kExprRefNull, kStringRefCode,
      kGCPrefix, kExprStringMeasureWtf8, 0
    ]);

  builder.addFunction("string_measure_wtf8_null", kSig_i_v)
    .exportFunc()
    .addBody([
      kExprRefNull, kStringRefCode,
      kGCPrefix, kExprStringMeasureWtf8, 1
    ]);

  builder.addFunction("string_measure_wtf8_replace_null", kSig_i_v)
    .exportFunc()
    .addBody([
      kExprRefNull, kStringRefCode,
      kGCPrefix, kExprStringMeasureWtf8, 2
    ]);

  let instance = builder.instantiate();
  for (let str of interestingStrings) {
    let wtf8 = encodeWtf8(str);
    assertEquals(wtf8.length, instance.exports.string_measure_wtf8(str));
    assertEquals(wtf8.length,
                 instance.exports.string_measure_wtf8_replace(str));
    if (HasIsolatedSurrogate(str)) {
      assertEquals(-1, instance.exports.string_measure_utf8(str));
    } else {
      assertEquals(wtf8.length, instance.exports.string_measure_utf8(str));
    }
  }

  assertThrows(() => instance.exports.string_measure_utf8_null(),
               WebAssembly.RuntimeError, "dereferencing a null pointer");
  assertThrows(() => instance.exports.string_measure_wtf8_null(),
               WebAssembly.RuntimeError, "dereferencing a null pointer");
  assertThrows(() => instance.exports.string_measure_wtf8_replace_null(),
               WebAssembly.RuntimeError, "dereferencing a null pointer");
})();

(function TestStringMeasureWtf16() {
  let builder = new WasmModuleBuilder();

  builder.addFunction("string_measure_wtf16", kSig_i_w)
    .exportFunc()
    .addBody([
      kExprLocalGet, 0,
      kGCPrefix, kExprStringMeasureWtf16
    ]);

  builder.addFunction("string_measure_wtf16_null", kSig_i_v)
    .exportFunc()
    .addBody([
      kExprRefNull, kStringRefCode,
      kGCPrefix, kExprStringMeasureWtf16
    ]);

  let instance = builder.instantiate();
  for (let str of interestingStrings) {
    assertEquals(str.length, instance.exports.string_measure_wtf16(str));
  }

  assertThrows(() => instance.exports.string_measure_wtf16_null(),
               WebAssembly.RuntimeError, "dereferencing a null pointer");
})();

(function TestStringEncodeWtf8() {
  let builder = new WasmModuleBuilder();

  builder.addMemory(1, undefined, true /* exported */, false);

  for (let [policy, name] of ["utf8", "wtf8", "replace"].entries()) {
    builder.addFunction("encode_" + name, kSig_v_wi)
      .exportFunc()
      .addBody([
        kExprLocalGet, 0,
        kExprLocalGet, 1,
        kGCPrefix, kExprStringEncodeWtf8, 0, policy,
      ]);
  }

  builder.addFunction("encode_null", kSig_v_v)
    .exportFunc()
    .addBody([
        kExprRefNull, kStringRefCode,
        kExprI32Const, 42,
        kGCPrefix, kExprStringEncodeWtf8, 0, 0,
      ]);

  let instance = builder.instantiate();
  let memory = new Uint8Array(instance.exports.memory.buffer);
  function clearMemory(low, high) {
    for (let i = low; i < high; i++) {
      memory[i] = 0;
    }
  }
  function assertMemoryBytesZero(low, high) {
    for (let i = low; i < high; i++) {
      assertEquals(0, memory[i]);
    }
  }
  function checkMemory(offset, bytes) {
    let slop = 64;
    assertMemoryBytesZero(Math.max(0, offset - slop), offset);
    for (let i = 0; i < bytes.length; i++) {
      assertEquals(bytes[i], memory[offset + i]);
    }
    assertMemoryBytesZero(offset + bytes.length,
                          Math.min(memory.length,
                                   offset + bytes.length + slop));
  }

  for (let str of interestingStrings) {
    let wtf8 = encodeWtf8(str);
    let offset = memory.length - wtf8.length;
    instance.exports.encode_wtf8(str, offset);
    checkMemory(offset, wtf8);
    clearMemory(offset, offset + wtf8.length);
  }

  for (let str of interestingStrings) {
    let offset = 0;
    if (HasIsolatedSurrogate(str)) {
      assertThrows(() => instance.exports.encode_utf8(str, offset),
          WebAssembly.RuntimeError,
          "Failed to encode string as UTF-8: contains unpaired surrogate");
    } else {
      let wtf8 = encodeWtf8(str);
      instance.exports.encode_utf8(str, offset);
      checkMemory(offset, wtf8);
      clearMemory(offset, offset + wtf8.length);
    }
  }

  for (let str of interestingStrings) {
    let offset = 42;
    instance.exports.encode_replace(str, offset);
    let replaced = ReplaceIsolatedSurrogates(str);
    if (!HasIsolatedSurrogate(str)) assertEquals(str, replaced);
    let wtf8 = encodeWtf8(replaced);
    checkMemory(offset, wtf8);
    clearMemory(offset, offset + wtf8.length);
  }

  assertThrows(() => instance.exports.encode_null(),
               WebAssembly.RuntimeError, "dereferencing a null pointer");

  checkMemory(memory.length - 10, []);

  for (let str of interestingStrings) {
    let wtf8 = encodeWtf8(str);
    let offset = memory.length - wtf8.length + 1;
    assertThrows(() => instance.exports.encode_wtf8(str, offset),
                 WebAssembly.RuntimeError, "memory access out of bounds");
    assertThrows(() => instance.exports.encode_utf8(str, offset),
                 WebAssembly.RuntimeError, "memory access out of bounds");
    assertThrows(() => instance.exports.encode_replace(str, offset),
                 WebAssembly.RuntimeError, "memory access out of bounds");
    checkMemory(offset - 1, []);
  }
})();

(function TestStringEncodeWtf16() {
  let builder = new WasmModuleBuilder();

  builder.addMemory(1, undefined, true /* exported */, false);

  builder.addFunction("encode_wtf16", kSig_v_wi)
    .exportFunc()
    .addBody([
      kExprLocalGet, 0,
      kExprLocalGet, 1,
      kGCPrefix, kExprStringEncodeWtf16, 0,
    ]);

  builder.addFunction("encode_null", kSig_v_v)
    .exportFunc()
    .addBody([
        kExprRefNull, kStringRefCode,
        kExprI32Const, 42,
        kGCPrefix, kExprStringEncodeWtf16, 0,
      ]);

  let instance = builder.instantiate();
  let memory = new Uint8Array(instance.exports.memory.buffer);
  function clearMemory(low, high) {
    for (let i = low; i < high; i++) {
      memory[i] = 0;
    }
  }
  function assertMemoryBytesZero(low, high) {
    for (let i = low; i < high; i++) {
      assertEquals(0, memory[i]);
    }
  }
  function checkMemory(offset, bytes) {
    let slop = 64;
    assertMemoryBytesZero(Math.max(0, offset - slop), offset);
    for (let i = 0; i < bytes.length; i++) {
      assertEquals(bytes[i], memory[offset + i]);
    }
    assertMemoryBytesZero(offset + bytes.length,
                          Math.min(memory.length,
                                   offset + bytes.length + slop));
  }

  for (let str of interestingStrings) {
    let wtf16 = encodeWtf16LE(str);
    let offset = memory.length - wtf16.length;
    instance.exports.encode_wtf16(str, offset);
    checkMemory(offset, wtf16);
    clearMemory(offset, offset + wtf16.length);
  }

  for (let str of interestingStrings) {
    let wtf16 = encodeWtf16LE(str);
    let offset = 0;
    instance.exports.encode_wtf16(str, offset);
    checkMemory(offset, wtf16);
    clearMemory(offset, offset + wtf16.length);
  }

  assertThrows(() => instance.exports.encode_null(),
               WebAssembly.RuntimeError, "dereferencing a null pointer");

  checkMemory(memory.length - 10, []);

  for (let str of interestingStrings) {
    let offset = 1;
    assertThrows(() => instance.exports.encode_wtf16(str, offset),
                 WebAssembly.RuntimeError,
                 "operation does not support unaligned accesses");
  }

  for (let str of interestingStrings) {
    let wtf16 = encodeWtf16LE(str);
    let offset = memory.length - wtf16.length + 2;
    assertThrows(() => instance.exports.encode_wtf16(str, offset),
                 WebAssembly.RuntimeError, "memory access out of bounds");
    checkMemory(offset - 2, []);
  }
})();

(function TestStringConcat() {
  let builder = new WasmModuleBuilder();

  builder.addFunction("concat", kSig_w_ww)
    .exportFunc()
    .addBody([
      kExprLocalGet, 0,
      kExprLocalGet, 1,
      kGCPrefix, kExprStringConcat
    ]);

  builder.addFunction("concat_null_head", kSig_w_w)
    .exportFunc()
    .addBody([
      kExprRefNull, kStringRefCode,
      kExprLocalGet, 0,
      kGCPrefix, kExprStringConcat
    ]);
  builder.addFunction("concat_null_tail", kSig_w_w)
    .exportFunc()
    .addBody([
      kExprLocalGet, 0,
      kExprRefNull, kStringRefCode,
      kGCPrefix, kExprStringConcat
    ]);

  let instance = builder.instantiate();

  for (let head of interestingStrings) {
    for (let tail of interestingStrings) {
      assertEquals(head + tail, instance.exports.concat(head, tail));
    }
  }

  assertThrows(() => instance.exports.concat_null_head("hey"),
               WebAssembly.RuntimeError, "dereferencing a null pointer");
  assertThrows(() => instance.exports.concat_null_tail("hey"),
               WebAssembly.RuntimeError, "dereferencing a null pointer");
})();

(function TestStringEq() {
  let builder = new WasmModuleBuilder();

  builder.addFunction("eq", kSig_i_ww)
    .exportFunc()
    .addBody([
      kExprLocalGet, 0,
      kExprLocalGet, 1,
      kGCPrefix, kExprStringEq
    ]);

  builder.addFunction("eq_null_a", kSig_i_w)
    .exportFunc()
    .addBody([
      kExprRefNull, kStringRefCode,
      kExprLocalGet, 0,
      kGCPrefix, kExprStringEq
    ]);
  builder.addFunction("eq_null_b", kSig_i_w)
    .exportFunc()
    .addBody([
      kExprLocalGet, 0,
      kExprRefNull, kStringRefCode,
      kGCPrefix, kExprStringEq
    ]);

  let instance = builder.instantiate();

  for (let head of interestingStrings) {
    for (let tail of interestingStrings) {
      let result = (head == tail)|0;
      assertEquals(result, instance.exports.eq(head, tail));
      assertEquals(result, instance.exports.eq(head + head, tail + tail));
    }
  }

  assertThrows(() => instance.exports.eq_null_a("hey"),
               WebAssembly.RuntimeError, "dereferencing a null pointer");
  assertThrows(() => instance.exports.eq_null_b("hey"),
               WebAssembly.RuntimeError, "dereferencing a null pointer");
})();

(function TestStringIsUSVSequence() {
  let builder = new WasmModuleBuilder();

  builder.addFunction("is_usv_sequence", kSig_i_w)
    .exportFunc()
    .addBody([
      kExprLocalGet, 0,
      kGCPrefix, kExprStringIsUsvSequence
    ]);

  builder.addFunction("is_usv_sequence_null", kSig_i_v)
    .exportFunc()
    .addBody([
      kExprRefNull, kStringRefCode,
      kGCPrefix, kExprStringIsUsvSequence
    ]);

  let instance = builder.instantiate();

  for (let str of interestingStrings) {
    assertEquals(HasIsolatedSurrogate(str) ? 0 : 1,
                 instance.exports.is_usv_sequence(str));
  }

  assertThrows(() => instance.exports.is_usv_sequence_null(),
               WebAssembly.RuntimeError, "dereferencing a null pointer");
})();

(function TestStringViewWtf16() {
  let builder = new WasmModuleBuilder();

  builder.addMemory(1, undefined, true /* exported */, false);

  builder.addFunction("length", kSig_i_w)
    .exportFunc()
    .addBody([
      kExprLocalGet, 0,
      kGCPrefix, kExprStringAsWtf16,
      kGCPrefix, kExprStringViewWtf16Length
    ]);

  builder.addFunction("length_null", kSig_i_v)
    .exportFunc()
    .addBody([
      kExprRefNull, kStringViewWtf16Code,
      kGCPrefix, kExprStringViewWtf16Length
    ]);

  builder.addFunction("get_codeunit", kSig_i_wi)
    .exportFunc()
    .addBody([
      kExprLocalGet, 0,
      kGCPrefix, kExprStringAsWtf16,
      kExprLocalGet, 1,
      kGCPrefix, kExprStringViewWtf16GetCodeunit
    ]);

  builder.addFunction("get_codeunit_null", kSig_i_v)
    .exportFunc()
    .addBody([
      kExprRefNull, kStringViewWtf16Code,
      kExprI32Const, 0,
      kGCPrefix, kExprStringViewWtf16GetCodeunit
    ]);

  builder.addFunction("encode", kSig_v_wiii)
    .exportFunc()
    .addBody([
      kExprLocalGet, 0,
      kGCPrefix, kExprStringAsWtf16,
      kExprLocalGet, 1,
      kExprLocalGet, 2,
      kExprLocalGet, 3,
      kGCPrefix, kExprStringViewWtf16Encode, 0
    ]);

  builder.addFunction("encode_null", kSig_v_v)
    .exportFunc()
    .addBody([
      kExprRefNull, kStringViewWtf16Code,
      kExprI32Const, 0,
      kExprI32Const, 0,
      kExprI32Const, 0,
      kGCPrefix, kExprStringViewWtf16Encode, 0
    ]);

  builder.addFunction("slice", kSig_w_wii)
    .exportFunc()
    .addBody([
      kExprLocalGet, 0,
      kGCPrefix, kExprStringAsWtf16,
      kExprLocalGet, 1,
      kExprLocalGet, 2,
      kGCPrefix, kExprStringViewWtf16Slice
    ]);

  builder.addFunction("slice_null", kSig_w_v)
    .exportFunc()
    .addBody([
      kExprRefNull, kStringViewWtf16Code,
      kExprI32Const, 0,
      kExprI32Const, 0,
      kGCPrefix, kExprStringViewWtf16Slice
    ]);

  let instance = builder.instantiate();
  let memory = new Uint8Array(instance.exports.memory.buffer);
  for (let str of interestingStrings) {
    assertEquals(str.length, instance.exports.length(str));
    for (let i = 0; i < str.length; i++) {
      assertEquals(str.charCodeAt(i),
                   instance.exports.get_codeunit(str, i));
    }
    assertEquals(str, instance.exports.slice(str, 0, -1));
  }

  function checkEncoding(str, slice, start, length) {
    let bytes = encodeWtf16LE(slice);
    function clearMemory(low, high) {
      for (let i = low; i < high; i++) {
        memory[i] = 0;
      }
    }
    function assertMemoryBytesZero(low, high) {
      for (let i = low; i < high; i++) {
        assertEquals(0, memory[i]);
      }
    }
    function checkMemory(offset, bytes) {
      let slop = 64;
      assertMemoryBytesZero(Math.max(0, offset - slop), offset);
      for (let i = 0; i < bytes.length; i++) {
        assertEquals(bytes[i], memory[offset + i]);
      }
      assertMemoryBytesZero(offset + bytes.length,
                            Math.min(memory.length,
                                     offset + bytes.length + slop));
    }

    for (let offset of [0, 42, memory.length - bytes.length]) {
      instance.exports.encode(str, offset, start, length);
      checkMemory(offset, bytes);
      clearMemory(offset, offset + bytes.length);
    }

    assertThrows(() => instance.exports.encode(str, 1, start, length),
                 WebAssembly.RuntimeError,
                 "operation does not support unaligned accesses");
    assertThrows(
        () => instance.exports.encode(str, memory.length - bytes.length + 2,
                                      start, length),
        WebAssembly.RuntimeError, "memory access out of bounds");
    checkMemory(memory.length - bytes.length - 2, []);
  }
  checkEncoding("fox", "f", 0, 1);
  checkEncoding("fox", "fo", 0, 2);
  checkEncoding("fox", "fox", 0, 3);
  checkEncoding("fox", "fox", 0, 300);
  checkEncoding("fox", "", 1, 0);
  checkEncoding("fox", "o", 1, 1);
  checkEncoding("fox", "ox", 1, 2);
  checkEncoding("fox", "ox", 1, 200);
  checkEncoding("fox", "", 2, 0);
  checkEncoding("fox", "x", 2, 1);
  checkEncoding("fox", "x", 2, 2);
  checkEncoding("fox", "", 3, 0);
  checkEncoding("fox", "", 3, 1_000_000_000);
  checkEncoding("fox", "", 1_000_000_000, 1_000_000_000);
  checkEncoding("fox", "", 100, 100);
  // Bounds checks before alignment checks.
  assertThrows(() => instance.exports.encode("foo", memory.length - 1, 0, 3),
               WebAssembly.RuntimeError, "memory access out of bounds");

  assertEquals("f", instance.exports.slice("foo", 0, 0));
  assertEquals("fo", instance.exports.slice("foo", 0, 1));
  assertEquals("foo", instance.exports.slice("foo", 0, 2));
  assertEquals("oo", instance.exports.slice("foo", 1, 2));
  assertEquals("oo", instance.exports.slice("foo", 1, 100));
  assertEquals("", instance.exports.slice("foo", 1, 0));

  assertThrows(() => instance.exports.length_null(),
               WebAssembly.RuntimeError, "dereferencing a null pointer");
  assertThrows(() => instance.exports.get_codeunit_null(),
               WebAssembly.RuntimeError, "dereferencing a null pointer");
  assertThrows(() => instance.exports.get_codeunit("", 0),
               WebAssembly.RuntimeError, "string offset out of bounds");
  assertThrows(() => instance.exports.encode_null(),
               WebAssembly.RuntimeError, "dereferencing a null pointer");
  assertThrows(() => instance.exports.slice_null(),
               WebAssembly.RuntimeError, "dereferencing a null pointer");
})();