[M3devel] Static chains

Tue Jan 15 07:00:48 CET 2008

ps: I "like" nested functions, and even the ability for them to capture their outer context, like in Scheme:

  (define make-adder (lambda x) (lambda y) (x + y))  

  (define add5 (make-adder 5))  

  (add 5 4)  

   => 9   

but I've lived without this long enough. Capturing just the right amount of context manually is also easy. Otherwise the compiler/runtime either has to a) heap allocate and garbage collect all stack frames b) do the analysis to decide what to heap allocate.
I believe Java and C# do b. They don't necessarily have nested "functions", but nested classes with functions, same thing. Java might make you declare captured state as "final" (aka const aka readonly).

That said, heap allocated stack frames are a powerful measure in preventing stack exhaustion, which is an otherwise seemingly difficult and ignored problem. The stack is not infinite yet most programmers rarely think about its size.
 - Jay

From: jayk123 at hotmail.comTo: rodney.bates at wichita.edu; hosking at cs.purdue.eduCC: m3devel at elegosoft.comSubject: RE: [M3devel] Static chainsDate: Tue, 15 Jan 2008 05:54:05 +0000

I really don't think an optimizing compiler should be passing around unused static links just for the debugger to use. The debugger should look up the stack itself. Optimizing compilers remove locals, remove parameters, remove functions, create custom calling conventions, etc. An non-optimizing compiler, sure, I guess. Though it seems some level of optimization should be always on.It pains me to think that every little function call I write might be made, and not inlined.Microsoft compilers now do inlining across static lib boundaries.Or that constant arithmetic like:#define A 1#define B 2int c = (A + B) might be computed at runtime. Or that while(1)  ..  .. if ... break; might generate a runtime check if 1 is not zero (along with the code bloat). Microsoft has "__forceinline", and it is a warning or error if the compiler is unable to inline such a function.  - Jay> Date: Mon, 14 Jan 2008 17:24:35 -0600> From: rodney.bates at wichita.edu> To: hosking at cs.purdue.edu> CC: m3devel at elegosoft.com> Subject: Re: [M3devel] Static chains> > Think of the debugger as an interpreter for a substantial subset of the> language. When the program is stopped at some point, m3gdb can, in effect,> inject code to evaluate an arbitrary expression, typed by the debugger> user, or execute an arbitrary assignment or call statement. Although> there are a number of gaps yet in the implementation, m3gdb is approaching> fitting this description.> > The user-typed code can both create and use static link values that do> not occur in the original code that the compiler saws. The simplest> example is, when stopped in a nested procedure, the user types> 'print NonLocalVariable'. This works fine right now, unless the compiler> has not stored the SL in the current proc's activation record, which a> compiler might well do if the nested proc, as coded, has no references> to any nonlocal variables. (Or keeps the SL only in a register, etc.)> > You could say this is only a convenience, as the user could work around> it by just typing 'up' the exact number of times to get to the static> parent's frame (which is, of course, also a dynamic ancestor). But that> can slow down debugging a great deal, and I often find it a very important> convenience. Anyway, you could say that to a degree about much of what a> debugger does.> > A more complex example might be 'print Nested1(Nested2,NonlocalVar)',> where Nested1 and Nested2 are nested procedures. Here, the debugger> has create and pass the appropriate SL value in order to call Nested1,> which, unless Nested1 is one scope deeper than the current stop point,> also entails m3gdb's fetching the SL that was given to some procedure> on the current dynamic call chain. The same things are required of> m3gdb to construct the closure for Nested2, which it must pass to Nested1.> And if NonlocalVar is in an outer scope where Nested1 and/or Nested2> aren't visible, there would be no workaround possible at all.> > Tony, I haven't been able to try out your latest change anyway, as> I seem to have once again regressed to not being able to get CM3 to> link to a working version of libpthread on Mandriva. Also, I am about> to travel for a week, so may be out of this discussion for a while.> > > Tony Hosking wrote:> > Rodney,> > > > I just realised that the static chains variables are *not* always being > > set up for nested function internal use. i.e., callers will pass them, > > but nested functions that don't explicitly use them will not have a > > static chain. Is this something essential for you? Why would you need > > a static chain from within a nested function that doesn't use any outer > > scope variables? Since it doesn't use any outer scope variables they > > cannot affect its execution, so debugging the value of those variables > > seems unnecessary. This is the default behavior of gcc for static chains.> > > > -- Tony> > > > > > -- > -------------------------------------------------------------> Rodney M. Bates, retired assistant professor> Dept. of Computer Science, Wichita State University> Wichita, KS 67260-0083> 316-978-3922> rodney.bates at wichita.edu

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