Fixed-width strings

Design Goals

Currently, Tamarin-Tracing strings are UTF-8. As much as these are desirable in terms of memory consumption, index access and other functions are unbearably slow.

The design goals for the new StringObject class are

1. Fixed-width strings with fixed widths of 8, 16 and 32 bits. Widths are either automatic (based on UTF-8 input), or manual (as created).
   • 8-bit strings contain the first 256 Unicode characters. UTF-8 is not supported in strings.
   • 16-bit strings are UTF-16, including surrogate pairs.
2. String contents may be based upon static data.
3. String concatenation creates a tree structure, which is flattened when needed.
4. String APIs should be as close to SpiderMonkey string APIs as possible to make ActionMonkey implementation easy.

Tagged instance contents

Strings are immutable, but their contents may be different. String data can be 8, 16, or 32 bits wide; A string may contain a pointer to a string buffer, which may need to be deleted or not; or the string data may immediately follow the String instance in memory (by doing a raw allocate followed by an in-place constructor call); or it can hold two String references if is the result of a concat operation. Finally, a string can be the result of a substring operation. The contents may change dynamically during a flatten operation. Therefore, a String instance contains a union with a tag.

String creation

Strings may either be created with 8, 16, or 32 bit data. In addition, strings may be created with UTF-8 data, which results in the smallest width that can hold the data.

String are created using static creator functions. This allows the implementation to use raw memory allocation and in-place constructor calls to avoid having to do two memory allocations, one for the instance, and the other for the data. Strings created that way contain the data right behind the instance data.

The maximum string width determines the way strings are created. It is an optional argument to the string constructors.

1. 8 bits: If the source data contains 16 or 32 bit data, the return value is null.
2. 16 bits: If the source data contains 32 bit values, surrogate pairs are created. If a character is > 0x10FFFF, null is returned.

This allows implementers to define the maximum width of strings; they can choose to use 8, 16 or 32 bits throughout, or they can choose to go with whatever width that fits best. If they choose best-fit widths, string creation methods do not create UTF-16 surrogate pairs. If a script creates surrogate pairs, these will remain in strings, though, although a flattening operation could detect surrogate pairs and widen the flattened string to 32 bits. This should be a global setting.

Question: How are out-of-memory conditions handled? The current implementation often just assumes success. There should be some sort of exception, and the same mechanism should be used to report strings that cannot be created.

Concatenation, substrings, and flattening

It would not be a good idea to create a new, flat string every time two strings are concatenated. Consider this loop:

var s = "";
for (var i = 32; i <= 1024; i++)
  s += String.fromCharCode (i);

If a new, flat string would be created on every iteration, this would lead to a almost 1000 copy operations, with a growing string buffer. Instead, the resulting string contains two String pointers that point to the two source strings.

The above example would create a deep tree, which is also undesirable. Therefore, a String instance contains a treeDepth field that contains the deepest depth of both subtrees plus one. The concat operation will contain a threshold where a string will be flattened before it is used for concatenation. This value should be determined using various benchmarks for optimal memory/performance ration. Also, the field is limited in size (10 bits?), so at some point automatic flattening is forced.

In SM, buffer allocations are not exact, but on certain boundaries (16 bytes?), leaving spare room at the end, so appends are possible. When appended, a new String instance is created that shares its buffer with the original instance, just longer. This technique would inhibit the use of 0x00 string terminators.

Getting a substring also flattens the source string. The substring is an instance that contains a pointer to the source string, and pointer to the start of the source string buffer. The length field contains the string length. This string is already flat, although it contains a reference to another string. It may be desirable to have a separate flattening function for this case.

When a string is flattened, its two String pointers are replaced with a flat data buffer. The resulting width of the string is determined by the widths of the strings in the tree. Usually, the resulting string width is the widest of all substrings found. If desired (with an #ifdef), substrings could also be analyzed if they are wider than the containing data, if e.g. a 16-bit strings only contains 8-bit characters. This is, of course, a performance hit, but may be desired if memory footprint is important. A global setting for the maximum string width may be necessary if strings were created using UTF-8 data, because all widths may exist.

Thread safety

Since strings are immutable, they are by definition thread safe. The only unsafe operation if the flattening operation. Therefore, the flatten() should look like this:

void String::flatten() {
  if (0 != this->treeLevel) {
    ENTER_CRITICAL_SECTION;
    if (0 != this->treeLevel) {
      // do the magic
    }
    LEAVE_CRITICAL_SECTION
  }
}

SpiderMonkey compatibility

This is just the start of this section...

• Do we need to support external strings and their finalizers? External strings are strings created with a finalizer ID, which is a 32-bit integer. Finalizers for this type are registered and deregistered. The additional ID would fit into the maximum of 16 bytes for string instance data (assuming 4-byte pointers).

• How would JS_GetStringChars() and JS_GetStringBytes() work? Would they force a narrowing of the string? In that case, the corresponding method is needed (which would not be difficult to add).

• What about growable strings? They are contradictory to the immutability of Tamarin strings. They could be implemented, but there should be some safety measure so they cannot be passed in to the engine.

Sample instance data

This is a sample representation of instance data, just to illustrate the concept. In this sample, a String instance would occupy between 8+n (direct data) and 16 bytes, assuming 4-byte pointers. The first 64 bits would be the lengths and the bits-and-flags field, followed by aligned pointers.

We need to discuss whether strings should be 0-terminated. Processing a string buffer with C/C++ routines would be much easier if it did. Also, in debug mode, the display of a string is easier.

The listing uses bit fields, but I think that we should use masks and shifts to ensure the correct width and alignment of data, along with inline methods to access the fields.

class String ... {
  ...
private:
  // common data
  uint32_t length;
  struct {
    unsigned int width:2;       // 0:1, 1:2, 2:not used, 3:4
    unsigned int tag:2;         // data tag: 0:direct, 1:regular, 2:concat, 3:substr
    unsigned int dynamic:1;     // if nonzero, buffer must be deleted
    unsigned int treeDepth:x;   // to be defined (more fields may follow)
    unsigned int padding:y;     // padding to 32 bits
  } data;
  // variable data according to tag
  union {
    struct {                    // data follows directly
      union {
        unsigned char c8 [100]; // big number for debug display
        utf16_t c16 [100];      // actual size varies
        utf32_t c32 [100];
      }
    } direct;
    struct {                    // regular string (also flattened)
      union {
        unsigned char* c8;
        utf16_t* c16;
        utf32_t* c32;
      }
    } regular;
    struct {                    // concatenated
      String* left, *right;
    } concat;
    struct {                    // substring
      String* source;           // flattened substring source
      union {
        unsigned char* c8;      // buffer into source's data
        utf16_t* c16;
        utf32_t* c32;
      }
    } substr;
  }
}