Re: Scheme Continuations vs Brakes Continuations was Re: Groovy support in Cocoon
Am Mo, den 05.04.2004 schrieb Christopher Oliver um 22:44: Why don't use (Object obj, Method method, Object[] args) in the constructor of the continuation object. That's a possible alternative, but makes prevents making Continuation an abstract class. Why should the continuation be abstract? The idea of the brakes people was to create something similar to java.lang.Thread Thread t = new Thread(new Runnable() { void run() { sendPage(a); wait(); sendPage(b); } }); t.start(); Continuation.SUICIDE.invoke(null); ? *you see many question marks over my head* I hope you have patience with me :) The idea of SUICIDE is that it has the effect of unwinding the stack but not executing any further code and directly returns to the entry point (i.e. it is the continuation of the end of some code ). For example (in JS): var suicide; function f() { suicide = arguments.continuation; } f(); // == since there's no code to execute after f(), invoking suicide later will have the effect of simply terminating the script. // end of script In the current Rhino implementation creating a Continuation in a top-level script has the same effect (since what comes after the end of the script is - nothing). This behavior is as in Scheme. Hmmm, not every intuitive. So your f() has the same role like Continuation.suspend() ? With call of suspend() the Continuation goes from the State restoring into State !restoring !capturing, and in cases of the state !restoring !capturing into the state capturing. Stephan.
Scheme Continuations vs Brakes Continuations was Re: Groovy support in Cocoon
First, I'm glad to hear more input ;-) Your mail took some time to parse it. Am Mo, den 05.04.2004 schrieb Christopher Oliver um 18:47: OK, IIUC there are three modes of execution of the instrumented code: 1) isCapturing During this mode the JVM call stack is unwound and its state captured in the continuation call stack. 2) isRestoring During this mode a saved call stack in a continuation is recreated on the JVM stack. 3) !isCapturing !isRestoring This is the normal execution of the JVM - no modifications are made to the JVM call stack or to the continuation call stack. Correct. This is achieved by inserting code before each invoke instruction to copy local variables from the continuation stack to the JVM stack and to reset the program counter to the value saved in the continuation stack if isRestoring is true, and by inserting code after each invoke instruction to copy the values of local variables from the JVM call stack as well as the program counter into the continuation call stack if isCapturing is true. Yes, but the code for the restore procedure is at the begining of the method not the before the invocation, but anyway.. I need some pseudo code for myself ;-) Current situation: void myFlow() { if (continuation.isRestoring()) { continuation.getStack().popLocalVariables(); continuation.getStack().popOperandStack(); continuation.getStack().popReference(); // used for the //method invocation push(null) as many null object as the method invovation need for the parameters goto continuation.getStack().popLastProcessCounter(); } [...] label 1: invoke MySubFlow.bla(); if (continuation.isCapturing() { remove the return value (is unimportant) continuation.getStack().pushLocalVariables(); continuation.getStack().pushOperandStack(); continuation.getStack().pushLastProcessCounter(label 1); continuation.getStack().pushReference(this); return null or void; } [...] } To make continuations more Scheme-like it must be possible to construct a continuation without modifying the JVM call stack (thus the current method invocation can continue as normal). To make this possible the byte-code transformer could work like this: 1) Insert code before an invoke instruction to push local vars and the pc onto the continuation stack if (!isRestoring !isCapturing) 2) Insert code after an invoke instruction to pop those values from the continuation stack if (!isRestoring !isCapturing) 3) If (isRestoring == true) the pre-invoke code would work as it does now. 4) If (isCapturing == true) the post-invoke code would simply cause the current method invocation to return to its caller without modifying the continuation stack - since it is already set up from (1). When the JVM stack is completely unwound another continuation will take the place of this one and that new continuation would be in isRestoring mode until it is recreated on the JVM stack at which point it would enter (!isRestoring !isCapturing) mode. Okay to recapitulate IIUC. Your proposal: void myFlow() { if (continuation.isRestoring()) { continuation.getStack().popLocalVariables(); continuation.getStack().popOperandStack(); goto continuation.getStack().popLastProcessCounter(); } [...] continuation.getStack().pushLocalVariables(); continuation.getStack().pushOperandStack(); continuation.getStack().pushLastProcessCounter(label 1); label 1: invoke MySubFlow.bla(); if (continuation.isCapturing() { return null or void; } else { continuation.getStack().popLocalVariables(); continuation.getStack().popOperandStack(); continuation.getStack().popLastProcessCounter(); } [...] } Yes, this has the benefit to call uninstrumented code, but the complete frame must be captured and restored for every invocation. The user interface to this might look something like this: public class Continuation { // Empty continuation for use as an escape procedure public static final Continuation SUICIDE; I didn't get the role of SUICIDE. // Invoke a captured continuation - it will replace the current continuation. I.e. the current JVM call stack will be unwound, the call stack saved in this object will be recreated on the JVM call stack, and the method invocation in which this object was created will return with returnValue. public void invoke(Object returnValue); Do you want to use the returnValue to return the WebContinuation in sendPageAndWait() ? The problem I see is that you suspend the thread at a method invocation not at the end of the method. // Get the current continuation - public static Continuation getCurrentContinuation(); // Entry point to the instrumented code (precondition: cannot be called recursively and method.getDeclaringClass() must be
Re: Scheme Continuations vs Brakes Continuations was Re: Groovy support in Cocoon
Stephan Michels wrote: First, I'm glad to hear more input ;-) Your mail took some time to parse it. Am Mo, den 05.04.2004 schrieb Christopher Oliver um 18:47: OK, IIUC there are three modes of execution of the instrumented code: 1) isCapturing During this mode the JVM call stack is unwound and its state captured in the continuation call stack. 2) isRestoring During this mode a saved call stack in a continuation is recreated on the JVM stack. 3) !isCapturing !isRestoring This is the normal execution of the JVM - no modifications are made to the JVM call stack or to the continuation call stack. Correct. This is achieved by inserting code before each invoke instruction to copy local variables from the continuation stack to the JVM stack and to reset the program counter to the value saved in the continuation stack if isRestoring is true, and by inserting code after each invoke instruction to copy the values of local variables from the JVM call stack as well as the program counter into the continuation call stack if isCapturing is true. Yes, but the code for the restore procedure is at the begining of the method not the before the invocation, but anyway.. Yes, of course. Sorry for not being more accurate. I need some pseudo code for myself ;-) Current situation: void myFlow() { if (continuation.isRestoring()) { continuation.getStack().popLocalVariables(); continuation.getStack().popOperandStack(); continuation.getStack().popReference(); // used for the //method invocation push(null) as many null object as the method invovation need for the parameters goto continuation.getStack().popLastProcessCounter(); } [...] label 1: invoke MySubFlow.bla(); if (continuation.isCapturing() { remove the return value (is unimportant) continuation.getStack().pushLocalVariables(); continuation.getStack().pushOperandStack(); continuation.getStack().pushLastProcessCounter(label 1); continuation.getStack().pushReference(this); return null or void; } [...] } OK. To make continuations more Scheme-like it must be possible to construct a continuation without modifying the JVM call stack (thus the current method invocation can continue as normal). To make this possible the byte-code transformer could work like this: 1) Insert code before an invoke instruction to push local vars and the pc onto the continuation stack if (!isRestoring !isCapturing) 2) Insert code after an invoke instruction to pop those values from the continuation stack if (!isRestoring !isCapturing) 3) If (isRestoring == true) the pre-invoke code would work as it does now. 4) If (isCapturing == true) the post-invoke code would simply cause the current method invocation to return to its caller without modifying the continuation stack - since it is already set up from (1). When the JVM stack is completely unwound another continuation will take the place of this one and that new continuation would be in isRestoring mode until it is recreated on the JVM stack at which point it would enter (!isRestoring !isCapturing) mode. Okay to recapitulate IIUC. Your proposal: void myFlow() { if (continuation.isRestoring()) { continuation.getStack().popLocalVariables(); continuation.getStack().popOperandStack(); goto continuation.getStack().popLastProcessCounter(); } [...] continuation.getStack().pushLocalVariables(); continuation.getStack().pushOperandStack(); continuation.getStack().pushLastProcessCounter(label 1); label 1: invoke MySubFlow.bla(); if (continuation.isCapturing() { return null or void; } else { continuation.getStack().popLocalVariables(); continuation.getStack().popOperandStack(); continuation.getStack().popLastProcessCounter(); } [...] } Yes, this has the benefit to call uninstrumented code, but the complete frame must be captured and restored for every invocation. See my second message regarding this. The user interface to this might look something like this: public class Continuation { // Empty continuation for use as an escape procedure public static final Continuation SUICIDE; I didn't get the role of SUICIDE. // Invoke a captured continuation - it will replace the current continuation. I.e. the current JVM call stack will be unwound, the call stack saved in this object will be recreated on the JVM call stack, and the method invocation in which this object was created will return with returnValue. public void invoke(Object returnValue); Do you want to use the returnValue to return the WebContinuation in sendPageAndWait() ? The problem I see is that you suspend the thread at a method invocation not at the end of the method. // Get the current continuation - public static Continuation getCurrentContinuation(); // Entry point to the instrumented code (precondition: cannot be called