diff --git a/test/cctest/test-dtoa.h b/test/cctest/test-dtoa.h
new file mode 100644
index 0000000000..9a36a3bf0c
--- /dev/null
+++ b/test/cctest/test-dtoa.h
@@ -0,0 +1,91 @@
+static const int kBufferSize = 50;
+
+static double ComposeDouble(char* buffer, int sign, int length, int point) {
+  int k = point - length;
+  // Integrate exponent into buffer.
+  buffer[length] = 'e';
+  snprintf(&buffer[length+1], kBufferSize - length - 1, "%d", k);
+  double result;
+  sscanf(buffer, "%lf", &result);  // NOLINT
+  if (sign) {
+    result *= -1;
+  }
+  return result;
+}
+
+
+static bool IsCorrect(double v, char* buffer, int sign, int length, int point) {
+  return v == ComposeDouble(buffer, sign, length, point);
+}
+
+
+// The precision of long doubles is not enough to ensure the correct rounding.
+static bool IsRounded(double v, char* buffer, int sign, int length, int point) {
+  // We don't test when v is 0.
+  if (v == 0) return true;
+
+  // Simplify things by working with positive numbers.
+  if (v < 0) v = -v;
+  char correct_buffer[100];
+  snprintf(correct_buffer, sizeof(correct_buffer), "%.90e", v);
+  // Get rid of the '.'
+  correct_buffer[1] = correct_buffer[0];
+  char* correct_str = &correct_buffer[1];
+
+  int i = 0;
+  while (true) {
+    if (correct_str[i] == '\0' || correct_str[i] == 'e') {
+      // We should never need all digits.
+      return false;
+    }
+
+    if (buffer[i] == '\0' || buffer[i] == 'e') {
+      // Verify that the remaining correct digits are small enough.
+      if (correct_str[i] < '5') return true;
+      return false;  // For simplicity we assume that '5' is rounded up.
+    }
+
+    if (buffer[i] != correct_str[i]) {
+      if (buffer[i] < correct_str[i]) return false;
+      if (buffer[i] - correct_str[i] != 1) return false;
+      if (correct_str[i+1] < '5') return false;
+      return true;
+    }
+
+    // Both characters are equal
+    i++;
+  }
+}
+
+
+static bool IsShortest(double v,
+                       char* buffer,
+                       int sign,
+                       int length,
+                       int point) {
+  // Now test if a shorter version would still yield the same result.
+  // Not an exhaustive test, but better than nothing.
+
+  if (length == 1) return true;
+
+  char last_digit = buffer[length - 1];
+
+  if (buffer[length - 1] == '0') return false;
+
+  if (v == ComposeDouble(buffer, sign, length - 1, point)) {
+    return false;
+  }
+
+  bool result = true;
+  if (buffer[length-2] != '9') {
+    buffer[length - 2]++;
+    double changed_value = ComposeDouble(buffer, sign, length-1, point);
+    if (v == changed_value) {
+      printf("ROUNDED FAILED DEBUG: %s\n", buffer);
+      result = false;
+    }
+    buffer[length - 2]--;
+  }
+  buffer[length - 1] = last_digit;
+  return result;
+}