[MLton] stack-allocated references

[Matthew Fluet]

skaller wrote:
> On Wed, 2007-02-28 at 21:56 -0500, Jake Donham wrote:
> 
>> Is there any limit to the size of a stack frame? 
> 
> Of course there is a bound per program, which a whole
> program analyser like Mlton can no doubt be coaxed into calculating,
> and of course there is no universal bound for all programs.

Indeed, MLton does calculate the maximum frame size 
(gcState.maxFrameSize) for every program.

While there is no universal bound on the live data at a program point, 
one could fix a maximum frame size (for all programs) by rewriting 
functions to aggregate live data that exceeded the stack size and 
allocate it on the heap.  I.e., rather than spill to a mutable stack 
slot, spill (multiple) temporaries to an immutable heap object, and use 
just one stack slot to point to the object.

This trades more allocation of immutable objects for a smaller mutable 
stack frame, which might be the right trade off in a real-time setting.
(Though I doubt that any reasonable ML program yields stack frames of 
such size as to make a significant change in the real-time bound.)