reland thread preemption

context.ts

@@ -22,8 +22,9 @@ module J2ME {
     Load  = 0x10,
     JIT   = 0x20,
     Code  = 0x40,
+    Thread = 0x80,
 
-    All   = Trace | Link | Init | Perf | Load | JIT | Code
+    All   = Trace | Link | Init | Perf | Load | JIT | Code | Thread
   }
 
   /**
@@ -382,6 +383,7 @@ module J2ME {
       jitWriter = writers & WriterFlags.JIT ? writer : null;
       codeWriter = writers & WriterFlags.Code ? writer : null;
       initWriter = writers & WriterFlags.Init ? writer : null;
+      threadWriter = writers & WriterFlags.Thread ? writer : null;
       loadWriter = writers & WriterFlags.Load ? writer : null;
     }
 
@@ -562,17 +564,6 @@ module J2ME {
             this.bailoutFrames = [];
           }
           var frames = this.frames;
-          if (windingWriter) {
-            windingWriter.enter("Unwound");
-            frames.map(function (f) {
-              if (Frame.isMarker(f)) {
-                windingWriter.writeLn("- marker -");
-              } else {
-                windingWriter.writeLn((f.methodInfo.state === MethodState.Compiled ? "C" : "I") + " " + f.toString());
-              }
-            });
-            windingWriter.leave("");
-          }
           switch (U) {
             case VMState.Yielding:
               this.resume();

jit/baseline.ts

@@ -440,6 +440,12 @@ module J2ME {
         this.methodInfo.isStatic ? this.runtimeClassObject(this.methodInfo.classInfo) : this.getLocal(0)
         : "null";
 
+      // Insert a preemption check at the top of the method. We can only
+      // do this if the method has the necessary unwinding code.
+      if (canYield(this.methodInfo)) {
+        this.emitPreemptionCheck(this.bodyEmitter, 0);
+      }
+
       this.emitEntryPoints();
     }
 
@@ -852,6 +858,11 @@ module J2ME {
       this.emitUnwind(emitter, this.pc, nextPC);
     }
 
+    private emitPreemptionCheck(emitter: Emitter, nextPC: number) {
+      emitter.writeLn("CP();");
+      this.emitUnwind(emitter, nextPC, nextPC);
+    }
+
     private emitMonitorExit(emitter: Emitter, object: string) {
       emitter.writeLn("MX(" + object + ");");
     }

main.js

@@ -254,6 +254,10 @@ window.onload = function() {
 
     var el = document.getElementById("unwindCount");
     el.textContent = numberWithCommas(J2ME.unwindCount);
+
+    var el = document.getElementById("preemptionCount");
+    el.textContent = numberWithCommas(J2ME.preemptionCount);
+
   }, 500);
 
   function dumpCounters() {

native.js

@@ -611,7 +611,7 @@ Native["java/lang/Thread.sleep.(J)V"] = function(delay) {
 };
 
 Native["java/lang/Thread.yield.()V"] = function() {
-    $.yield();
+    $.yield("Thread.yield");
 };
 
 Native["java/lang/Thread.activeCount.()I"] = function() {

runtime.ts

@@ -88,6 +88,11 @@ module J2ME {
    */
   export var initWriter = null;
 
+  /**
+   * Traces thread execution.
+   */
+  export var threadWriter = null;
+
   /**
    * Traces generated code.
    */
@@ -724,16 +729,16 @@ module J2ME {
       $.ctx.bailout(methodInfo, bci, nextBCI, local, stack, lockObject);
     }
 
-    yield() {
+    yield(reason: string) {
-      windingWriter && windingWriter.writeLn("yielding");
       unwindCount ++;
-      runtimeCounter && runtimeCounter.count("yielding");
+      threadWriter && threadWriter.writeLn("yielding " + reason);
+      runtimeCounter && runtimeCounter.count("yielding " + reason);
       U = VMState.Yielding;
     }
 
     pause(reason: string) {
-      windingWriter && windingWriter.writeLn("pausing");
       unwindCount ++;
+      threadWriter && threadWriter.writeLn("pausing " + reason);
       runtimeCounter && runtimeCounter.count("pausing " + reason);
       U = VMState.Pausing;
     }
@@ -1787,6 +1792,58 @@ module J2ME {
       return;
     }
   }
+
+  /**
+   * Last time we preempted a thread.
+   */
+  var lastPreemption = 0;
+
+  /**
+   * The preemption check should be quick. We don't always want to measure
+   * time so we use a quick counter and mask to determine when to do the
+   * more expensive preemption check.
+   */
+  var preemptionSamples = 0;
+  var preemptionSampleMask = 0xFF;
+
+  /**
+   * Number of ms between preemptions, chosen arbitrarily.
+   */
+  var preemptionInterval = 100;
+
+  /**
+   * Number of preemptions thus far.
+   */
+  export var preemptionCount = 0;
+
+  /**
+   * Periodically preempts the current thread. Calls to this method are inserted in
+   * methods that can yield.
+   */
+  export function checkPreemption() {
+    preemptionSamples ++;
+    if ((preemptionSamples & preemptionSampleMask) === 0) {
+      preempt();
+    }
+  }
+
+  /**
+   * TODO: We will almost always preempt the next time we call this if the application
+   * has been idle. Figure out a better heurisitc here, maybe measure the frequency at
+   * at which |checkPreemption| is invoked and ony preempt if the frequency is sustained
+   * for a longer period of time *and* the time since we last preempted is above the
+   * |preemptionInterval|.
+   */
+  function preempt() {
+    var now = performance.now();
+    var elapsed = now - lastPreemption;
+    if (elapsed > preemptionInterval) {
+      lastPreemption = now;
+      preemptionCount ++;
+      threadWriter && threadWriter.writeLn("Preemption timeout: " + elapsed.toFixed(2) + " ms, samples: " + preemptionSamples + ", count: " + preemptionCount);
+      $.yield("preemption");
+    }
+  }
 }
 
 var Runtime = J2ME.Runtime;
@@ -1827,3 +1884,5 @@ var CAS = J2ME.checkArrayStore;
 var ME = J2ME.monitorEnter;
 var MX = J2ME.monitorExit;
 var TE = J2ME.translateException;
+
+var CP = J2ME.checkPreemption;