diff --git a/.gitignore b/.gitignore
index aac1b9026..06b4db74f 100644
--- a/.gitignore
+++ b/.gitignore
@@ -60,3 +60,6 @@ typings/
 lib/
 
 package-lock.json
+
+# Generated polyfill binaries
+packages/proposal-sign-extension-ops/src/polyfills/*.wasm
diff --git a/package.json b/package.json
index 242ad4119..4d17fdf3a 100644
--- a/package.json
+++ b/package.json
@@ -31,7 +31,8 @@
     "markdown-to-html": "0.0.13",
     "mocha": "4.0.1",
     "performance-now": "2.1.0",
-    "prettier": "1.9.2"
+    "prettier": "1.9.2",
+	"assemblyscript": "github:AssemblyScript/assemblyscript"
   },
   "dependencies": {
     "babel-plugin-mamacro": "0.0.3",
diff --git a/packages/helper-wasm-bytecode/src/index.js b/packages/helper-wasm-bytecode/src/index.js
index c7f854cc3..1c17719b0 100644
--- a/packages/helper-wasm-bytecode/src/index.js
+++ b/packages/helper-wasm-bytecode/src/index.js
@@ -319,6 +319,13 @@ const symbolsByByte = {
   0xba: createSymbolObject("convert_u/i64", "f64"),
   0xbb: createSymbolObject("promote/f32", "f64"),
 
+  // Sign extension op proposal
+  0xc0: createSymbolObject("extend8_s", "i32"),
+  0xc1: createSymbolObject("extend16_s", "i32"),
+  0xc2: createSymbolObject("extend8_s", "i64"),
+  0xc3: createSymbolObject("extend16_s", "i64"),
+  0xc4: createSymbolObject("extend32_s", "i64"),
+
   0xbc: createSymbolObject("reinterpret/f32", "i32"),
   0xbd: createSymbolObject("reinterpret/f64", "i64"),
   0xbe: createSymbolObject("reinterpret/i32", "f32"),
diff --git a/packages/proposal-sign-extension-ops/README.md b/packages/proposal-sign-extension-ops/README.md
new file mode 100644
index 000000000..17c7d5f28
--- /dev/null
+++ b/packages/proposal-sign-extension-ops/README.md
@@ -0,0 +1,4 @@
+# Sign extension proposal
+
+<https://github.com/WebAssembly/sign-extension-ops>
+
diff --git a/packages/proposal-sign-extension-ops/package.json b/packages/proposal-sign-extension-ops/package.json
new file mode 100644
index 000000000..6555a2358
--- /dev/null
+++ b/packages/proposal-sign-extension-ops/package.json
@@ -0,0 +1,35 @@
+{
+  "name": "@webassemblyjs/proposal-sign-ops",
+  "version": "1.5.6",
+  "description": "",
+  "main": "lib/index.js",
+  "scripts": {
+    "test": "echo \"Error: no test specified\" && exit 1"
+  },
+  "repository": {
+    "type": "git",
+    "url": "https://github.com/xtuc/webassemblyjs.git"
+  },
+  "publishConfig": {
+    "access": "public"
+  },
+  "author": "Mauro Bringolf",
+  "license": "MIT",
+  "dependencies": {
+    "@webassemblyjs/ast": "1.5.6",
+    "@webassemblyjs/helper-buffer": "1.5.6",
+    "@webassemblyjs/helper-wasm-bytecode": "1.5.6",
+    "@webassemblyjs/helper-wasm-section": "1.5.6",
+    "@webassemblyjs/wasm-edit": "1.5.6",
+    "@webassemblyjs/wasm-gen": "1.5.6",
+    "@webassemblyjs/wasm-opt": "1.5.6",
+    "debug": "^3.1.0"
+  },
+  "devDependencies": {
+    "@webassemblyjs/helper-test-framework": "1.5.6",
+    "@webassemblyjs/wast-parser": "1.5.6",
+    "@webassemblyjs/wasm-parser": "1.5.6",
+    "@webassemblyjs/wast-printer": "1.5.6",
+    "wabt": "1.0.0"
+  }
+}
diff --git a/packages/proposal-sign-extension-ops/src/index.js b/packages/proposal-sign-extension-ops/src/index.js
new file mode 100644
index 000000000..f2811d629
--- /dev/null
+++ b/packages/proposal-sign-extension-ops/src/index.js
@@ -0,0 +1,91 @@
+import { traverse, callInstruction, numberLiteral } from "@webassemblyjs/ast";
+import { parse } from "@webassemblyjs/wast-parser";
+import { readFileSync } from "fs";
+import path from "path";
+
+const funcFromWast = sourcePath => {
+  const ast = parse(readFileSync(sourcePath, "utf8"));
+  return ast.body[0].fields.find(f => f.type === "Func");
+};
+
+class Polyfills {
+  constructor(startIndex) {
+    this.startIndex = startIndex;
+
+    this.asts = {
+      i32_extend8_s: funcFromWast(
+        path.join(__dirname, "/polyfills/i32_extend8_s.wast")
+      ),
+      i32_extend16_s: funcFromWast(
+        path.join(__dirname, "/polyfills/i32_extend16_s.wast")
+      ),
+      i64_extend8_s: funcFromWast(
+        path.join(__dirname, "/polyfills/i64_extend8_s.wast")
+      ),
+      i64_extend16_s: funcFromWast(
+        path.join(__dirname, "/polyfills/i64_extend16_s.wast")
+      ),
+      i64_extend32_s: funcFromWast(
+        path.join(__dirname, "/polyfills/i64_extend32_s.wast")
+      )
+    };
+
+    this.instructions = Object.keys(this.asts);
+
+    this.index = {};
+
+    this.instructions.forEach(instrName => {
+      this.index[instrName] = -1;
+    });
+  }
+
+  replaceWith(path, instrName) {
+    if (this.index[instrName] === -1) {
+      this.index[instrName] = this.startIndex++;
+    }
+
+    const index = this.index[instrName];
+    path.replaceWith(callInstruction(numberLiteral(index, String(index))));
+  }
+
+  matchesInstruction(instrName) {
+    return this.instructions.includes(instrName);
+  }
+
+  insertInto(ast) {
+    const funcsToPush = Object.keys(this.index)
+      .filter(instrName => this.index[instrName] >= 0)
+      .sort((x, y) => (this.index[x] > this.index[y] ? 1 : -1))
+      .map(instrName => this.asts[instrName]);
+
+    ast.body[0].fields.push(...funcsToPush);
+  }
+}
+
+export function transformAst(ast) {
+  let numberOfFuncs = 0;
+
+  const countFuncVisitor = {
+    Func() {
+      ++numberOfFuncs;
+    }
+  };
+
+  traverse(ast, countFuncVisitor);
+
+  const polyfills = new Polyfills(numberOfFuncs);
+
+  const signExtendVisitor = {
+    Instr(path) {
+      if (polyfills.matchesInstruction(path.node.id)) {
+        polyfills.replaceWith(path, path.node.id);
+      }
+    }
+  };
+
+  traverse(ast, signExtendVisitor);
+
+  polyfills.insertInto(ast);
+
+  return ast;
+}
diff --git a/packages/proposal-sign-extension-ops/src/polyfills/i32_extend16_s.wast b/packages/proposal-sign-extension-ops/src/polyfills/i32_extend16_s.wast
new file mode 100644
index 000000000..45a3fb183
--- /dev/null
+++ b/packages/proposal-sign-extension-ops/src/polyfills/i32_extend16_s.wast
@@ -0,0 +1,16 @@
+(module
+  (type (func (param i32) (result i32)))
+  (func $i32_extend16_s (param i32) (result i32)
+    (get_local 0)
+    (i32.const -32768)
+    (i32.or)
+    (get_local 0)
+    (i32.const 32767)
+    (i32.and)
+    (get_local 0)
+    (i32.const 32768)
+    (i32.and)
+    (select)
+  )
+  (export "i32_extend16_s" (func $i32_extend16_s))
+)
diff --git a/packages/proposal-sign-extension-ops/src/polyfills/i32_extend8_s.wast b/packages/proposal-sign-extension-ops/src/polyfills/i32_extend8_s.wast
new file mode 100644
index 000000000..78a7712a1
--- /dev/null
+++ b/packages/proposal-sign-extension-ops/src/polyfills/i32_extend8_s.wast
@@ -0,0 +1,16 @@
+(module
+  (type (func (param i32) (result i32)))
+  (func $i32_extend8_s (param i32) (result i32)
+    (get_local 0)
+    (i32.const -128)
+    (i32.or)
+    (get_local 0)
+    (i32.const 127)
+    (i32.and)
+    (get_local 0)
+    (i32.const 128)
+    (i32.and)
+    (select)
+  )
+  (export "i32_extend8_s" (func $i32_extend8_s))
+)
diff --git a/packages/proposal-sign-extension-ops/src/polyfills/i64_extend16_s.wast b/packages/proposal-sign-extension-ops/src/polyfills/i64_extend16_s.wast
new file mode 100644
index 000000000..ac52c000b
--- /dev/null
+++ b/packages/proposal-sign-extension-ops/src/polyfills/i64_extend16_s.wast
@@ -0,0 +1,18 @@
+(module
+  (type (func (param i64) (result i64)))
+  (func $i64_extend16_s (param i64) (result i64)
+    (get_local 0)
+    (i64.const -32768)
+    (i64.or)
+    (get_local 0)
+    (i64.const 32767)
+    (i64.and)
+    (get_local 0)
+    (i64.const 32768)
+    (i64.and)
+    (i64.const 0)
+    (i64.ne)
+    (select)
+  )
+  (export "i64_extend16_s" (func $i64_extend16_s))
+)
diff --git a/packages/proposal-sign-extension-ops/src/polyfills/i64_extend32_s.wast b/packages/proposal-sign-extension-ops/src/polyfills/i64_extend32_s.wast
new file mode 100644
index 000000000..648d6b635
--- /dev/null
+++ b/packages/proposal-sign-extension-ops/src/polyfills/i64_extend32_s.wast
@@ -0,0 +1,18 @@
+(module
+  (type (func (param i64) (result i64)))
+  (func $i64_extend32_s (param i64) (result i64)
+    (get_local 0)
+    (i64.const -2147483648)
+    (i64.or)
+    (get_local 0)
+    (i64.const 2147483647)
+    (i64.and)
+    (get_local 0)
+    (i64.const 2147483648)
+    (i64.and)
+    (i64.const 0)
+    (i64.ne)
+    (select)
+  )
+  (export "i64_extend32_s" (func $i64_extend32_s))
+)
diff --git a/packages/proposal-sign-extension-ops/src/polyfills/i64_extend8_s.wast b/packages/proposal-sign-extension-ops/src/polyfills/i64_extend8_s.wast
new file mode 100644
index 000000000..04cd1a2dc
--- /dev/null
+++ b/packages/proposal-sign-extension-ops/src/polyfills/i64_extend8_s.wast
@@ -0,0 +1,18 @@
+(module
+  (type (func (param i64) (result i64)))
+  (func $i64_extend8_s (param i64) (result i64)
+    (get_local 0)
+    (i64.const -128)
+    (i64.or)
+    (get_local 0)
+    (i64.const 127)
+    (i64.and)
+    (get_local 0)
+    (i64.const 128)
+    (i64.and)
+    (i64.const 0)
+    (i64.ne)
+    (select)
+  )
+  (export "i64_extend8_s" (func $i64_extend8_s))
+)
diff --git a/packages/proposal-sign-extension-ops/test/exec.js b/packages/proposal-sign-extension-ops/test/exec.js
new file mode 100644
index 000000000..034d85ac4
--- /dev/null
+++ b/packages/proposal-sign-extension-ops/test/exec.js
@@ -0,0 +1,54 @@
+const path = require("path");
+const wabt = require("wabt");
+const assert = require("assert");
+
+const { readFileSync } = require("fs");
+
+const polyfills = [
+  {
+    name: "i32_extend8_s",
+    fixtures: [
+      [0, 0],
+      [0x7f, 127],
+      [0x80, -128],
+      [0xff, -1],
+      [0x01234500, 0],
+      [0xfedcba80, -0x80],
+      [-1, -1]
+    ]
+  },
+  {
+    name: "i32_extend16_s",
+    fixtures: [
+      [0, 0],
+      [0x7fff, 32767],
+      [0x8000, -32768],
+      [0xffff, -1],
+      [0x01230000, 0],
+      [0xfedc8000, -0x8000],
+      [-1, -1]
+    ]
+  }
+];
+
+polyfills.forEach(p => {
+  p.path = path.join(__dirname, `../lib/polyfills/${p.name}.wast`);
+});
+
+polyfills.forEach(polyfill => {
+  describe(polyfill.path, () => {
+    const wast = readFileSync(polyfill.path, "utf8").trim();
+    const wasm = wabt
+      .parseWat(polyfill.path, wast)
+      .toBinary({ write_debug_names: false });
+
+    it("should return correct values", () => {
+      return WebAssembly.instantiate(wasm.buffer).then(result => {
+        const polyfillFn = result.instance.exports[polyfill.name];
+        polyfill.fixtures.forEach(([input, output]) => {
+          assert.equal(output, polyfillFn(input));
+        });
+      });
+    });
+  });
+});
diff --git a/packages/proposal-sign-extension-ops/test/index.js b/packages/proposal-sign-extension-ops/test/index.js
new file mode 100644
index 000000000..23a18ac31
--- /dev/null
+++ b/packages/proposal-sign-extension-ops/test/index.js
@@ -0,0 +1,41 @@
+const { parse } = require("@webassemblyjs/wast-parser");
+const { print } = require("@webassemblyjs/wast-printer");
+const path = require("path");
+
+const {
+  getFixtures,
+  compareStrings
+} = require("@webassemblyjs/helper-test-framework");
+const { readFileSync, existsSync, writeFileSync } = require("fs");
+
+const { transformAst } = require("../lib/");
+
+const testCases = getFixtures(
+  path.join(__dirname, "./wast/"),
+  "/**/input.wast"
+);
+
+testCases.forEach(testCase => {
+  describe(testCase, () => {
+    const input = readFileSync(testCase, "utf8").trim();
+
+    it("should transpile sign extension operators into functions correctly", () => {
+      const actualOutput = print(transformAst(parse(input)));
+
+      if (!existsSync(path.join(path.dirname(testCase), "output.wast"))) {
+        writeFileSync(
+          path.join(path.dirname(testCase), "output.wast"),
+          actualOutput
+        );
+        return;
+      }
+
+      const expectedOutput = readFileSync(
+        path.join(path.dirname(testCase), "output.wast"),
+        "utf8"
+      ).trim();
+
+      compareStrings(actualOutput, expectedOutput);
+    });
+  });
+});
diff --git a/packages/proposal-sign-extension-ops/test/wast/empty/input.wast b/packages/proposal-sign-extension-ops/test/wast/empty/input.wast
new file mode 100644
index 000000000..4d181e6e6
--- /dev/null
+++ b/packages/proposal-sign-extension-ops/test/wast/empty/input.wast
@@ -0,0 +1,4 @@
+(module
+  (func)
+)
+
diff --git a/packages/proposal-sign-extension-ops/test/wast/empty/output.wast b/packages/proposal-sign-extension-ops/test/wast/empty/output.wast
new file mode 100644
index 000000000..4d181e6e6
--- /dev/null
+++ b/packages/proposal-sign-extension-ops/test/wast/empty/output.wast
@@ -0,0 +1,4 @@
+(module
+  (func)
+)
+
diff --git a/packages/proposal-sign-extension-ops/test/wast/mixed/input.wast b/packages/proposal-sign-extension-ops/test/wast/mixed/input.wast
new file mode 100644
index 000000000..91c7ff015
--- /dev/null
+++ b/packages/proposal-sign-extension-ops/test/wast/mixed/input.wast
@@ -0,0 +1,15 @@
+(module
+  (func (param i32) (result i32)
+    (get_local 0)
+    (i32_extend16_s)
+    (i32_extend8_s)
+  )
+  (func (param i32) (result i32)
+    (get_local 0)
+  )
+  (func (param i32) (result i32)
+    (get_local 0)
+    (i32_extend8_s)
+  )
+)
+
diff --git a/packages/proposal-sign-extension-ops/test/wast/mixed/output.wast b/packages/proposal-sign-extension-ops/test/wast/mixed/output.wast
new file mode 100644
index 000000000..827a06c5b
--- /dev/null
+++ b/packages/proposal-sign-extension-ops/test/wast/mixed/output.wast
@@ -0,0 +1,38 @@
+(module
+  (func (param i32) (result i32)
+    (get_local 0)
+    (call 3)
+    (call 4)
+  )
+  (func (param i32) (result i32)
+    (get_local 0)
+  )
+  (func (param i32) (result i32)
+    (get_local 0)
+    (call 4)
+  )
+  (func $i32_extend16_s (param i32) (result i32)
+    (get_local 0)
+    (i32.const -32768)
+    (i32.or)
+    (get_local 0)
+    (i32.const 32767)
+    (i32.and)
+    (get_local 0)
+    (i32.const 32768)
+    (i32.and)
+    (select)
+  )
+  (func $i32_extend8_s (param i32) (result i32)
+    (get_local 0)
+    (i32.const -128)
+    (i32.or)
+    (get_local 0)
+    (i32.const 127)
+    (i32.and)
+    (get_local 0)
+    (i32.const 128)
+    (i32.and)
+    (select)
+  )
+)
diff --git a/packages/proposal-sign-extension-ops/test/wast/multiple-instruction-i32_extend16_s/input.wast b/packages/proposal-sign-extension-ops/test/wast/multiple-instruction-i32_extend16_s/input.wast
new file mode 100644
index 000000000..ac66606a2
--- /dev/null
+++ b/packages/proposal-sign-extension-ops/test/wast/multiple-instruction-i32_extend16_s/input.wast
@@ -0,0 +1,14 @@
+(module
+  (func (param i32) (result i32)
+    (get_local 0)
+    (i32_extend16_s)
+  )
+  (func (param i32) (result i32)
+    (get_local 0)
+  )
+  (func (param i32) (result i32)
+    (get_local 0)
+    (i32_extend16_s)
+  )
+)
+
diff --git a/packages/proposal-sign-extension-ops/test/wast/multiple-instruction-i32_extend16_s/output.wast b/packages/proposal-sign-extension-ops/test/wast/multiple-instruction-i32_extend16_s/output.wast
new file mode 100644
index 000000000..13f198a7b
--- /dev/null
+++ b/packages/proposal-sign-extension-ops/test/wast/multiple-instruction-i32_extend16_s/output.wast
@@ -0,0 +1,25 @@
+(module
+  (func (param i32) (result i32)
+    (get_local 0)
+    (call 3)
+  )
+  (func (param i32) (result i32)
+    (get_local 0)
+  )
+  (func (param i32) (result i32)
+    (get_local 0)
+    (call 3)
+  )
+  (func $i32_extend16_s (param i32) (result i32)
+    (get_local 0)
+    (i32.const -32768)
+    (i32.or)
+    (get_local 0)
+    (i32.const 32767)
+    (i32.and)
+    (get_local 0)
+    (i32.const 32768)
+    (i32.and)
+    (select)
+  )
+)
diff --git a/packages/proposal-sign-extension-ops/test/wast/multiple-instruction-i32_extend8_s/input.wast b/packages/proposal-sign-extension-ops/test/wast/multiple-instruction-i32_extend8_s/input.wast
new file mode 100644
index 000000000..d05cd70dd
--- /dev/null
+++ b/packages/proposal-sign-extension-ops/test/wast/multiple-instruction-i32_extend8_s/input.wast
@@ -0,0 +1,15 @@
+(module
+  (func (param i32) (result i32)
+    (get_local 0)
+    (i32_extend8_s)
+  )
+  (func (param i32) (result i32)
+    (get_local 0)
+    (i32_extend8_s)
+  )
+  (func (param i32) (result i32)
+    (get_local 0)
+    (i32_extend8_s)
+  )
+)
+
diff --git a/packages/proposal-sign-extension-ops/test/wast/multiple-instruction-i32_extend8_s/output.wast b/packages/proposal-sign-extension-ops/test/wast/multiple-instruction-i32_extend8_s/output.wast
new file mode 100644
index 000000000..f07f0095c
--- /dev/null
+++ b/packages/proposal-sign-extension-ops/test/wast/multiple-instruction-i32_extend8_s/output.wast
@@ -0,0 +1,26 @@
+(module
+  (func (param i32) (result i32)
+    (get_local 0)
+    (call 3)
+  )
+  (func (param i32) (result i32)
+    (get_local 0)
+    (call 3)
+  )
+  (func (param i32) (result i32)
+    (get_local 0)
+    (call 3)
+  )
+  (func $i32_extend8_s (param i32) (result i32)
+    (get_local 0)
+    (i32.const -128)
+    (i32.or)
+    (get_local 0)
+    (i32.const 127)
+    (i32.and)
+    (get_local 0)
+    (i32.const 128)
+    (i32.and)
+    (select)
+  )
+)
diff --git a/packages/proposal-sign-extension-ops/test/wast/single-instruction-i32_extend16_s/input.wast b/packages/proposal-sign-extension-ops/test/wast/single-instruction-i32_extend16_s/input.wast
new file mode 100644
index 000000000..ddbb8821c
--- /dev/null
+++ b/packages/proposal-sign-extension-ops/test/wast/single-instruction-i32_extend16_s/input.wast
@@ -0,0 +1,7 @@
+(module
+  (func (param i32) (result i32)
+    (get_local 0)
+    (i32_extend16_s)
+  )
+)
+
diff --git a/packages/proposal-sign-extension-ops/test/wast/single-instruction-i32_extend16_s/output.wast b/packages/proposal-sign-extension-ops/test/wast/single-instruction-i32_extend16_s/output.wast
new file mode 100644
index 000000000..6a69176ba
--- /dev/null
+++ b/packages/proposal-sign-extension-ops/test/wast/single-instruction-i32_extend16_s/output.wast
@@ -0,0 +1,18 @@
+(module
+  (func (param i32) (result i32)
+    (get_local 0)
+    (call 1)
+  )
+  (func $i32_extend16_s (param i32) (result i32)
+    (get_local 0)
+    (i32.const -32768)
+    (i32.or)
+    (get_local 0)
+    (i32.const 32767)
+    (i32.and)
+    (get_local 0)
+    (i32.const 32768)
+    (i32.and)
+    (select)
+  )
+)
diff --git a/packages/proposal-sign-extension-ops/test/wast/single-instruction-i32_extend8_s/input.wast b/packages/proposal-sign-extension-ops/test/wast/single-instruction-i32_extend8_s/input.wast
new file mode 100644
index 000000000..ec3a4b4cd
--- /dev/null
+++ b/packages/proposal-sign-extension-ops/test/wast/single-instruction-i32_extend8_s/input.wast
@@ -0,0 +1,7 @@
+(module
+  (func (param i32) (result i32)
+    (get_local 0)
+    (i32_extend8_s)
+  )
+)
+
diff --git a/packages/proposal-sign-extension-ops/test/wast/single-instruction-i32_extend8_s/output.wast b/packages/proposal-sign-extension-ops/test/wast/single-instruction-i32_extend8_s/output.wast
new file mode 100644
index 000000000..ffb292095
--- /dev/null
+++ b/packages/proposal-sign-extension-ops/test/wast/single-instruction-i32_extend8_s/output.wast
@@ -0,0 +1,18 @@
+(module
+  (func (param i32) (result i32)
+    (get_local 0)
+    (call 1)
+  )
+  (func $i32_extend8_s (param i32) (result i32)
+    (get_local 0)
+    (i32.const -128)
+    (i32.or)
+    (get_local 0)
+    (i32.const 127)
+    (i32.and)
+    (get_local 0)
+    (i32.const 128)
+    (i32.and)
+    (select)
+  )
+)
diff --git a/packages/proposal-sign-extension-ops/test/wast/single-instruction-i64_extend16_s/input.wast b/packages/proposal-sign-extension-ops/test/wast/single-instruction-i64_extend16_s/input.wast
new file mode 100644
index 000000000..5870a7742
--- /dev/null
+++ b/packages/proposal-sign-extension-ops/test/wast/single-instruction-i64_extend16_s/input.wast
@@ -0,0 +1,7 @@
+(module
+  (func (param i64) (result i64)
+    (get_local 0)
+    (i64_extend16_s)
+  )
+)
+
diff --git a/packages/proposal-sign-extension-ops/test/wast/single-instruction-i64_extend16_s/output.wast b/packages/proposal-sign-extension-ops/test/wast/single-instruction-i64_extend16_s/output.wast
new file mode 100644
index 000000000..8bcfd45c4
--- /dev/null
+++ b/packages/proposal-sign-extension-ops/test/wast/single-instruction-i64_extend16_s/output.wast
@@ -0,0 +1,20 @@
+(module
+  (func (param i64) (result i64)
+    (get_local 0)
+    (call 1)
+  )
+  (func $i64_extend16_s (param i64) (result i64)
+    (get_local 0)
+    (i64.const -32768)
+    (i64.or)
+    (get_local 0)
+    (i64.const 32767)
+    (i64.and)
+    (get_local 0)
+    (i64.const 32768)
+    (i64.and)
+    (i64.const 0)
+    (i64.ne)
+    (select)
+  )
+)
diff --git a/packages/proposal-sign-extension-ops/test/wast/single-instruction-i64_extend32_s/input.wast b/packages/proposal-sign-extension-ops/test/wast/single-instruction-i64_extend32_s/input.wast
new file mode 100644
index 000000000..bc8aa90eb
--- /dev/null
+++ b/packages/proposal-sign-extension-ops/test/wast/single-instruction-i64_extend32_s/input.wast
@@ -0,0 +1,7 @@
+(module
+  (func (param i64) (result i64)
+    (get_local 0)
+    (i64_extend32_s)
+  )
+)
+
diff --git a/packages/proposal-sign-extension-ops/test/wast/single-instruction-i64_extend32_s/output.wast b/packages/proposal-sign-extension-ops/test/wast/single-instruction-i64_extend32_s/output.wast
new file mode 100644
index 000000000..455693bc5
--- /dev/null
+++ b/packages/proposal-sign-extension-ops/test/wast/single-instruction-i64_extend32_s/output.wast
@@ -0,0 +1,20 @@
+(module
+  (func (param i64) (result i64)
+    (get_local 0)
+    (call 1)
+  )
+  (func $i64_extend32_s (param i64) (result i64)
+    (get_local 0)
+    (i64.const -2147483648)
+    (i64.or)
+    (get_local 0)
+    (i64.const 2147483647)
+    (i64.and)
+    (get_local 0)
+    (i64.const 2147483648)
+    (i64.and)
+    (i64.const 0)
+    (i64.ne)
+    (select)
+  )
+)
diff --git a/packages/proposal-sign-extension-ops/test/wast/single-instruction-i64_extend8_s/input.wast b/packages/proposal-sign-extension-ops/test/wast/single-instruction-i64_extend8_s/input.wast
new file mode 100644
index 000000000..044ce51d9
--- /dev/null
+++ b/packages/proposal-sign-extension-ops/test/wast/single-instruction-i64_extend8_s/input.wast
@@ -0,0 +1,7 @@
+(module
+  (func (param i64) (result i64)
+    (get_local 0)
+    (i64_extend8_s)
+  )
+)
+
diff --git a/packages/proposal-sign-extension-ops/test/wast/single-instruction-i64_extend8_s/output.wast b/packages/proposal-sign-extension-ops/test/wast/single-instruction-i64_extend8_s/output.wast
new file mode 100644
index 000000000..91c2628c0
--- /dev/null
+++ b/packages/proposal-sign-extension-ops/test/wast/single-instruction-i64_extend8_s/output.wast
@@ -0,0 +1,20 @@
+(module
+  (func (param i64) (result i64)
+    (get_local 0)
+    (call 1)
+  )
+  (func $i64_extend8_s (param i64) (result i64)
+    (get_local 0)
+    (i64.const -128)
+    (i64.or)
+    (get_local 0)
+    (i64.const 127)
+    (i64.and)
+    (get_local 0)
+    (i64.const 128)
+    (i64.and)
+    (i64.const 0)
+    (i64.ne)
+    (select)
+  )
+)
diff --git a/scripts/build.sh b/scripts/build.sh
index 651fec628..99f964870 100755
--- a/scripts/build.sh
+++ b/scripts/build.sh
@@ -29,6 +29,8 @@ for D in ./packages/*; do
     $OPTS &
 done
 
+
 wait
 
+cp -r packages/proposal-sign-extension-ops/src/polyfills packages/proposal-sign-extension-ops/lib/
 cp -v packages/dce/src/libwabt.js packages/dce/lib/libwabt.js
diff --git a/yarn.lock b/yarn.lock
index 6e082d90b..261b58854 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -554,6 +554,10 @@
     lodash "^4.17.5"
     to-fast-properties "^2.0.0"
 
+"@protobufjs/utf8@^1.1.0":
+  version "1.1.0"
+  resolved "https://registry.yarnpkg.com/@protobufjs/utf8/-/utf8-1.1.0.tgz#a777360b5b39a1a2e5106f8e858f2fd2d060c570"
+
 "@yarnpkg/lockfile@^1.0.0":
   version "1.0.0"
   resolved "https://registry.yarnpkg.com/@yarnpkg/lockfile/-/lockfile-1.0.0.tgz#33d1dbb659a23b81f87f048762b35a446172add3"
@@ -707,6 +711,17 @@ asn1@~0.2.3:
   version "0.2.3"
   resolved "https://registry.yarnpkg.com/asn1/-/asn1-0.2.3.tgz#dac8787713c9966849fc8180777ebe9c1ddf3b86"
 
+"assemblyscript@github:AssemblyScript/assemblyscript":
+  version "0.5.0"
+  resolved "https://codeload.github.com/AssemblyScript/assemblyscript/tar.gz/9d25f78fc15d61cc180e55aa6d53636ec7350e6e"
+  dependencies:
+    "@protobufjs/utf8" "^1.1.0"
+    binaryen "48.0.0-nightly.20180520"
+    glob "^7.1.2"
+    long "^4.0.0"
+    minimist "^1.2.0"
+    ts-node "^6.0.3"
+
 assert-plus@1.0.0, assert-plus@^1.0.0:
   version "1.0.0"
   resolved "https://registry.yarnpkg.com/assert-plus/-/assert-plus-1.0.0.tgz#f12e0f3c5d77b0b1cdd9146942e4e96c1e4dd525"
@@ -807,6 +822,10 @@ binary-extensions@^1.0.0:
     buffers "~0.1.1"
     chainsaw "~0.1.0"
 
+binaryen@48.0.0-nightly.20180520:
+  version "48.0.0-nightly.20180520"
+  resolved "https://registry.yarnpkg.com/binaryen/-/binaryen-48.0.0-nightly.20180520.tgz#24ce2172da6c6e44af97889fce5b1d96542db596"
+
 bl@~0.4.1:
   version "0.4.2"
   resolved "https://registry.yarnpkg.com/bl/-/bl-0.4.2.tgz#5db31d72f038c54e68adc39578125fe3b0addc96"
@@ -954,7 +973,7 @@ chalk@^1.1.3:
     strip-ansi "^3.0.0"
     supports-color "^2.0.0"
 
-chalk@^2.0.0, chalk@^2.0.1, chalk@^2.1.0:
+chalk@^2.0.0, chalk@^2.0.1, chalk@^2.1.0, chalk@^2.3.0:
   version "2.4.1"
   resolved "https://registry.yarnpkg.com/chalk/-/chalk-2.4.1.tgz#18c49ab16a037b6eb0152cc83e3471338215b66e"
   dependencies:
@@ -1410,7 +1429,7 @@ diff@3.3.1:
   version "3.3.1"
   resolved "https://registry.yarnpkg.com/diff/-/diff-3.3.1.tgz#aa8567a6eed03c531fc89d3f711cd0e5259dec75"
 
-diff@^3.2.0:
+diff@^3.1.0, diff@^3.2.0:
   version "3.5.0"
   resolved "https://registry.yarnpkg.com/diff/-/diff-3.5.0.tgz#800c0dd1e0a8bfbc95835c202ad220fe317e5a12"
 
@@ -2587,6 +2606,10 @@ lodash@^4.15.0, lodash@^4.17.4, lodash@^4.17.5, lodash@^4.2.0, lodash@^4.2.1, lo
   version "4.17.10"
   resolved "https://registry.yarnpkg.com/lodash/-/lodash-4.17.10.tgz#1b7793cf7259ea38fb3661d4d38b3260af8ae4e7"
 
+long@^4.0.0:
+  version "4.0.0"
+  resolved "https://registry.yarnpkg.com/long/-/long-4.0.0.tgz#9a7b71cfb7d361a194ea555241c92f7468d5bf28"
+
 longest@^1.0.1:
   version "1.0.1"
   resolved "https://registry.yarnpkg.com/longest/-/longest-1.0.1.tgz#30a0b2da38f73770e8294a0d22e6625ed77d0097"
@@ -2621,6 +2644,10 @@ make-dir@^1.0.0:
   dependencies:
     pify "^3.0.0"
 
+make-error@^1.1.1:
+  version "1.3.4"
+  resolved "https://registry.yarnpkg.com/make-error/-/make-error-1.3.4.tgz#19978ed575f9e9545d2ff8c13e33b5d18a67d535"
+
 map-obj@^1.0.0, map-obj@^1.0.1:
   version "1.0.1"
   resolved "https://registry.yarnpkg.com/map-obj/-/map-obj-1.0.1.tgz#d933ceb9205d82bdcf4886f6742bdc2b4dea146d"
@@ -3609,6 +3636,13 @@ sort-keys@^2.0.0:
   dependencies:
     is-plain-obj "^1.0.0"
 
+source-map-support@^0.5.3:
+  version "0.5.6"
+  resolved "https://registry.yarnpkg.com/source-map-support/-/source-map-support-0.5.6.tgz#4435cee46b1aab62b8e8610ce60f788091c51c13"
+  dependencies:
+    buffer-from "^1.0.0"
+    source-map "^0.6.0"
+
 source-map@^0.4.4:
   version "0.4.4"
   resolved "https://registry.yarnpkg.com/source-map/-/source-map-0.4.4.tgz#eba4f5da9c0dc999de68032d8b4f76173652036b"
@@ -3619,6 +3653,10 @@ source-map@^0.5.0, source-map@~0.5.1:
   version "0.5.7"
   resolved "https://registry.yarnpkg.com/source-map/-/source-map-0.5.7.tgz#8a039d2d1021d22d1ea14c80d8ea468ba2ef3fcc"
 
+source-map@^0.6.0:
+  version "0.6.1"
+  resolved "https://registry.yarnpkg.com/source-map/-/source-map-0.6.1.tgz#74722af32e9614e9c287a8d0bbde48b5e2f1a263"
+
 spdx-correct@^3.0.0:
   version "3.0.0"
   resolved "https://registry.yarnpkg.com/spdx-correct/-/spdx-correct-3.0.0.tgz#05a5b4d7153a195bc92c3c425b69f3b2a9524c82"
@@ -3891,6 +3929,19 @@ trim-right@^1.0.1:
   version "1.0.1"
   resolved "https://registry.yarnpkg.com/trim-right/-/trim-right-1.0.1.tgz#cb2e1203067e0c8de1f614094b9fe45704ea6003"
 
+ts-node@^6.0.3:
+  version "6.0.5"
+  resolved "https://registry.yarnpkg.com/ts-node/-/ts-node-6.0.5.tgz#977c1c931da7a2b09ae2930101f0104a5c2271e9"
+  dependencies:
+    arrify "^1.0.0"
+    chalk "^2.3.0"
+    diff "^3.1.0"
+    make-error "^1.1.1"
+    minimist "^1.2.0"
+    mkdirp "^0.5.1"
+    source-map-support "^0.5.3"
+    yn "^2.0.0"
+
 tunnel-agent@^0.6.0:
   version "0.6.0"
   resolved "https://registry.yarnpkg.com/tunnel-agent/-/tunnel-agent-0.6.0.tgz#27a5dea06b36b04a0a9966774b290868f0fc40fd"
@@ -4192,3 +4243,7 @@ yargs@~3.10.0:
     cliui "^2.1.0"
     decamelize "^1.0.0"
     window-size "0.1.0"
+
+yn@^2.0.0:
+  version "2.0.0"
+  resolved "https://registry.yarnpkg.com/yn/-/yn-2.0.0.tgz#e5adabc8acf408f6385fc76495684c88e6af689a"