@oid said:

@ddw_music said:

In Pd, wires are procedure calls, not gotos.

Are they actually procedures or just conceptually?

The distinction I'm thinking of is that gotos have no history, while procedure calls do. When a chain of objects runs out of connections, pd and max do backtrack, so there must be a call stack for the objects that have been traversed so far (this, I'm certain, is not conceptual in pd -- there are stack overflow errors, for one thing). A goto is just a jump, with no memory of how it got there.

hjh

@ddw_music All that stuff about instruction pointers and the like was me getting turned around and falling back onto Fourth because that is where my mind goes when I think about stacks. In pd there is no instruction pointer so never any need to return or backtrack or goto. In pd's core, every internal and external object is a literal OOP object and the inlets and outlets are methods of that object. When we create a new object pd creates a new C object of that type and makes an entry for it in glist with its pointer and UI details; when we connect a wire to its outlet pd retrieves the pointers to those objects from the glist and then goes through the outlet method. I am not quite sure how it deals with reaching the end of the branch, I suspect that the last object always has its outlet method run to point it at the branch point and when we append a new object pd just passes that to the new object when it updates the previous end of chain object's outlet method to be the new object.

Some details there might be a tad off, I am not quite good enough with C pointers to really follow pd's source yet, but I am fairly sure that is the mechanics of it. I have no idea about how Max handles this stuff but I would assume it is quite similar.

@FFW said:

By the way, it's also the order the process stack is filled because the full tree is flatten before it computes

Is there a process stack? Can you tell me where to look in the source for it? What is actually on this stack? Is my above reasoning regarding the patch just being a chain of objects incorrect? The patch being a chain of C objects explains why we can't turn off message rate and what happens when pd is running out of cpu but I have not quite grasped how pd's scheduler actually works. I'm guessing my previous post is difficult for a non-native English speaker since my grasp is not great and my explanation is probably not the clearest and I am not sure how well you know pd's internals either. I am almost at the point where I will post on the mailing list to get a response from the devs but I want to get enough of it that my post is not just a very verbose way of saying "I don't get this, explain the entire inner workings of pd for me!"

@oid "Process stack" is my own term to describe how orders arrive at the processor. I'm thinking at low level, below the C one.
Let's consider this tree and translate it in pseudo-assembly or pseudo-forth:

: BANG
push bang to stack

: MESSAGE_One
pop stack if bang push "One"

: MESSAGE_Two
pop stack if bang push "Two"

: PRINT
pop stack, send to console


: RUN
BANG MESSAGE_One PRINT BANG MESSAGE_Two PRINT

RUN

Trigger is unused at run time, it's only used at "flatten" time to put the methods in the right order. the :RUN content is the process stack I talk about.

However audio tree is flatten to create a DSP chain but maybe the control tree is not…

@FFW I sort of thought you meant something like that but wasn't sure. I don't think message rate stuff is flattened and can't find anything in the source but I don't really know, I just don't see much advantage in doing it with how slow and simple message rate is. Thanks for the clarification.

@oid said:

@ddw_music In pd there is no instruction pointer so never any need to return or backtrack or goto.

You absolutely must backtrack in the case of any object that has multiple outputs. Backtracking is most straightforwardly done with a stack.

A stack is only a data structure that defines two operations: add something new at the end (push) and take the last item off of the end (pop) -- last in, first out. "Stack" doesn't define what it should be used for. It may be data, as in FORTH; it may be execution frames; can be anything.

If necessary, I could find it in the source code later, but let's assume that Miller Puckette is no fool, and he knows that tree traversal is best done with a stack, and that he isn't going to waste time trying to implement some alternative to a stack because of a preconception that stacks are for x or y but never z (and further, that he wouldn't post "stack overflow" errors to the console if that was misleading).

I am not quite sure how it deals with reaching the end of the branch...

I'm also not sure of the implementation details in C (don't really need to be), but if each object gets pushed onto a message-passing stack, then it's easy: when this object is done, pop an object off the stack and see if it has outputs that haven't been issued yet; then issue them, or if there are none, pop again and repeat until the stack is empty. Because a stack is LIFO, this is always (literally!) backtracking.

If you've ever used Max's graphical debug mode: it literally shows you the message-passing stack in a window. I'll admit that I don't have the line-number evidence to show exactly how pd does it in C, but I'm not making this up...

Trigger is unused at run time, it's only used at "flatten" time... However audio tree is flatten to create a DSP chain but maybe the control tree is not…

I'm quite sure there's no flattening for control objects. (See Max's debugger, or I bet pd's trace mode will show the same.)

Edit:

trace: bang 
trigger: bang      -- not flattened
message: One 
print: One
trigger: bang      -- and, after doing "One," it backtracked to here
message: Two 
print: Two
backtrace:  -- what happened before the [trace] object?
bng: bang 

Note that you don't know "what happened before the [trace] object" without a stack

Again, if I'm really saying anything controversial, when time permits I can go diving into pd's sources and find the evidence. But IMO the [trace] output is enough.

hjh