CTFE Overhaul (Was: RFC: Thrift project proposal (draft))

[spir]

On 03/26/2011 09:57 PM, Don wrote:
>>> The basic problem with the current implementation of CTFE is that it
>>> uses copy-on-write. This means that references (including dynamic
>>> arrays) don't work properly -- they just copy a snapshot of the thing
>>> they are referencing. This is bug 1330. It also means it burns up memory
>>> like you wouldn't believe.
>>
>> Right.  IIUC there's also no way to free the memory from copies that are no
>> longer referenced.  I can see where this would leak memory like a sieve.
>
> That's not the big problem, actually. The issue is that x[7]=6;
> duplicates x, even if x has 10K elements.
> Now consider:
>    for(int i=0; i<x.length; ++i) x[i]=3;
>    // creates 100M new elements!! Should create none, or 10K at most.

Hello Don,

I don't understand your point. I have once implemented a toy dynamic language, 
using the common trick of boxed elements (à la Lisp). But I wanted to maintain 
value semantics as standard. A cheap way to do that is copy on write; it is 
actually cheap since simple, atomic, elements are never copied (since they 
cannot be changed on place), thus one just just needs to trace complex elements 
(array-lists & named tuples in my case):
	x := [1,2,3]	// create the array value, assign its ref
	y := x		// copy the ref, mark the value as shared
	x[1] := 0	// copy the value, reassign the ref, then change
But the new value is not shared, thus:
	x[1] := 1	// change only

So that in your loop example, at most one array copy happens (iff it was 
shared). This is as far as I know what is commonly called copy-on-write. There 
is no need to copy the value over and over again on every change if it is not 
multiple-referenced, and noone does that, I guess.

Side-Note: assignments of the form of "y := x" are really special, at least 
conceptually; but also practically when pointers or refs enter the game. I call 
them "symbol assignments" as the source is a symbol.

Denis
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