TraceMonkey and Type Instability in JavaScript

Work on TraceMonkey continues! TraceMonkey is Mozilla’s JavaScript JIT based on Franz-Gal trace compilation. The JIT ships with Firefox 3.1 and can be enabled in the beta via about:config. This article is about how TraceMonkey was recently changed to handle type changes in JavaScript data flow.

Introduction

The greatest advantage to a trace compiling JIT rather than a method compiling JIT, for dynamic languages, is type specialization. Observe the following loop:

Select All Code:
function f(c) {
  for (var i = 0; i < 500; i++)
    c += 2;
  return c;
}

In a traditional JIT this method would need to be compiled generically enough to use all possible incoming types. The function could be invoked using either a string, object, number, or anything — and the compiled code must account for that. Optimizing further requires static analysis.

Type Specialization

TraceMonkey’s approach is remarkably different. If a hot path (loop) is encountered, the runtime behavior is observed, recorded, and compiled as a “trace tree.” If that loop is run using integers, the trace tree becomes a specialized chunk of assembly using straight integer math. Because trace compilation only compiles instructions that have been hit (for example, it may only compile one side to an if path), it’s much easier and faster to type specialize than doing aggressive static analysis.

This approach has worked very well so far. Except for type instability.

If that loop above were run a second time with c being a string instead, the original tree could not be used as the types are not compatible. This resulted in a “tree mismatch,” and excessive tree mismatches meant destroying the tree to make way for another. What if you could have multiple type-specialized trees for a given loop? Enter multi-trees.

Over the past week or so I have been working on (and now landed) my second major TraceMonkey patch — multi-trees.

Multiple Type Specializations per Path

TraceMonkey stores a cache of all trees. The bytecode location of each hot loop is mapped to a linked list of trees; the first in each list is called the “root peer.” Each tree records typemaps, which are a simple arrays describing the data type of each slot in the stack. A tree contains one entry typemap, which is the set of types the stack must have in order to execute the compiled tree.

The simplest case of multiple type specializations is the example above, where a simple loop’s typemap can alternate on entry. This is the easiest case to handle. The linked list is grown for each new combination of types. f(3) will create the root peer with an integer specialization. f('a') will link in a new tree with a string specialization.

To invoke a tree, the root tree is fetched and the list is traversed until a matching entry typemap is found. The tree is then executed. If no matching typemap exists, the list is appended and a new tree using the new typemap is recorded. This means there are multiple trees recorded for the same loop, and we pick one matching the types on the stack.

Type Instability Inside Loops

The hard cases involve type instability within the same loop. TraceMonkey relies on being able to close loops, meaning that the loop edge (tail of the loop) can jump directly back to the top. This can only happen if the entry type map is equivalent to the type map at the loop edge. For example, a number turning into a string can’t be closed because the compiled code at the top of the loop expects a double, and we never recorded a path to bridge the conversion back.

There are many reasons this can happen. Take the following dumb example:

Select All Code:
function g() {
  var q;
  for (var i = 0; i < 100; i++)
    q = 2.0;
  return q;
}

The first time we enter this loop, the entry typemap contains Boolean for q since it is undefined. When the loop reaches its first edge, q has changed to a Number. Recording starts on the second iteration, so now both the entry and exit typemaps will contain Number.

Now if g() is run again we can’t run the tree we just recorded, because it expects q to be a Number when it really starts out as Boolean. To solve this we immediately start recording a new tree. At the end we encounter a problem: in our tree, q started out as Boolean and now it’s a Number. This means the loop cannot be closed because the exit and entry points are not type compatible. Joining the ends would be incorrect.

To solve this we search for a tree X (on the same loop) whose entry types are identical to our ending types. If X exists we compile a “loop” that runs once, then immediately jumps into tree X. This resolves the type conflict by bridging the type flow to another loop. Visualization:

Figure 1

Delayed Bridging

What happens if no matching tree exists? Take this example:

Select All Code:
function f(var c) {
   var q;
   for (var i = 0; i < c; i++)
     q = 2.0;
   return q;
}
f(1);
f(1);
f(20);

The first call to f puts a counter that says “record a trace for me next time I’m reached.” The second call to f records a trace with a type instability: q flows from undefined to Number. Unlike the previous example though, there is no stable tree to complete the bridge.

To solve this, we have the top of each tree (where the entry typemap is stored) contain a list of unstable loop edge exit points for that specific tree. During the third call to f a stable loop will be compiled. We then use the following algorithm:

1. For all other peers that have compiled code,
2. For all unstable exit points in that peer,
3. If any unstable exit point has a typemap matching our entry typemap, bridge the points together and remove the unstable exit from its peer’s list.

This algorithm is invoked every time a main trace is compiled (that is, a trace that’s not tying to extend a branch off a tree). Once it runs in the above example we a diagram very similar to Figure 1.

The important result of this is that we can bridge many types of instability together. For example, imagine a scenario where two traces are mutually unstable. This is easily bridged:

Figure 2

Other crazy situations are handled as well. For example, stable loops with unstable branch exits, or chaining multiple unstable traces together (multiple levels of mutual instability). One case I ran into was something like this:

Figure 3

Nested Type Instability

The situation gets hairy with nested trees. Consider a loop like:

Select All Code:
for (var i = 0; i < 100; i++) {
  var q;
  for (var j = 0; j < 100; j++) {
    q = 2.0;
  }
}

In this example there’s an inner tree that will quickly type stabilize. When the outer loop starts recording, the inner loop’s incoming types will not match the existing root tree. In this case the outer loop’s recording is temporarily aborted and the inner loop immediately starts recording again under the assumption that it will record a type stabilizing trace. In this loop it does, and the two trees are connected together. The outer loop can then record and directly call into the tree that starts out with unstable types.

This solves all sorts of crazy data flow stability problems. Previously SunSpider was traced as very type unstable, and loop hoisted variables (such as the q in that example) served as tracing poison. They’re now handled fine. Our least favorite test, access-fannkuch, used to have 100,000 and even 200,000 side exits (exits from the JIT to the interpreter). It now only has 200 and it runs 2.3X faster than the interpreter (over an old 1.3X). Other cases like crypto-md5 are almost completely covered by tracing.

Thin Loops

Multi-trees go hand in hand with “thin loops,” a concept Andreas landed recently. The idea is that if a loop doesn’t close because it’s too short, we close it anyway and assume the best. That works really nicely for very thin loops, especially thin nested loops which would prevent the outer from recording. Unfortunately it doesn’t give thin loops time to type stabilize, so many thin loops are thrown out. For example:

Select All Code:
function h(c, k) {
  var q = new String('');
  for (var i = 0; i < c; i++)
    q = k + q;
  return q;
}
h(1, 'a');
h(1, 'a');
h(5, 'a');

Multi-trees solves this. The first call to h tells the JIT to start recording the next time h is called. When h gets called the second time a thin loop is recorded. But the types are not stable, and the loop cannot be closed – we get another dangling trace that runs once and exits. When h gets called a third time it type stabilizes. A new loop is compiled and the two traces are bridged together.

Note: h is not immediately stable because new String returns a JSVAL_OBJECT in SpiderMonkey, whereas the addition returns a JSVAL_STRING.

Type Speculation

TraceMonkey has a very cool optimization called type speculation. Floating point values like 30.0 or -12.0 fit into an integer. When TraceMonkey starts recording a loop it speculates that such “promotable” numbers are integers for the entire loop. When the loop closes, if the number in that slot changed to a double, the loop can’t be closed and the recording is thrown out. How do you tell it to not perform integer promotion next time that loop is recorded?

The original solution was an oracle, a simple lossy hash table. If an integer->double conflict arose the oracle would be told “please blacklist stack slot N for bytecode location X, so we don’t try to promote it to an integer.” Since the hash table is lossy this led to false positives as traces filled the JIT cache.

Multitrees rids most uses of the oracle. If a loop can’t be closed because of integer->double conflicts, a new trace is immediately recorded with the same conflicts demoted to doubles on entry. This is better than compiling bridged traces (int->double and double->double), because the extra time spent compiling has very little gain.

There is one case where the oracle is still important (global variables aside), in that the oracle can help memoize type instability for nested loops. If the JIT cache is flushed the oracle will still remember which stack slots can’t be promoted, and outer loops can be compiled faster. Thus the oracle only memoizes stack slots when recording inner tree calls.

There is also a case where we build similar traces that are impossible to connect. For example, two variables going from int->int in one tree, and double->double in a peer tree. If a branch from the first goes from int->double for one the variables but not the other, the traces cannot be connected without some sort of intermediate conversion logic. Perhaps with very bushy trees it would make more sense to build such intermediate bridges. For now we simply throw out the tree that is causing the problems (the original integer tree). This is safe because integers can always be boxed into doubles, so the second trace is all we need.

Problems

It’s not perfect. There’s potential for code/trace explosion if types are too unstable. The heuristics for deeply nested trees can get messy too. If there are many inner trees that are type unstable it can take a long time to build a stable outer tree.

Compared to the original “single-tree specialization” TraceMonkey, there are cases that get a little slower (although still faster than with no JIT). This is because the original algorithm aggressively pegged incompatible values into mismatching trees. For example, undefined (Boolean) was passed as a 0 for integer slots and NaN for double slots. This was wrong. For example, ((undefined == undefined) == true) but ((NaN == NaN) == false). And ((undefined == undefined) == true) but ((undefined == 0) == false). Other operators are hurt too. NaN is a poison value and causes any expression to return false.

So there were cases where the compiled code was simply wrong and produced incorrect results (but boy was it fast!). By the time this fault was discovered it was too late. Removing this aggressive coercion corrected a small percentage of code at the price of greatly reducing performance. Trees would continually get trashed because they mismatched. Multitrees solved this, but in some cases it uncovers new paths that don’t yet trace well.

Conclusion

As TraceMonkey improves, type specialization will bring JIT’d code closer and closer to C equivalents. Multiple type specialized trees furthers us toward this goal. It solves a huge class of correctness bugs and improves performance in many scenarios where nested trees could not be compiled.

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