Possible quick win in GC?

[Mike via Digitalmars-d]

On Monday, 29 September 2014 at 07:03:29 UTC, Abdulhaq wrote:
> On Sunday, 28 September 2014 at 20:20:29 UTC, David Nadlinger 
> wrote:
>> On Sunday, 28 September 2014 at 16:29:45 UTC, Abdulhaq wrote:
>>> I got the idea after thinking that it should be fairly simple 
>>> for the compiler to detect straightforward cases of when a 
>>> variable can be declared as going on the stack - i.e. no 
>>> references to it are retained after its enclosing function 
>>> returns.
>>
>> LDC does the "fairly simple" part of this already in a custom 
>> LLVM optimizer pass. The issue is that escape analysis is 
>> fairly hard in general, and currently even more limited 
>> because we only do it on the LLVM IR level (i.e. don't 
>> leverage any additional attributes like scope, pure, … that 
>> might be present in the D source code).
>>
>> David
>
> That's interesting, yes I guessed that the escape analysis 
> would present the harder part, but I'm hoping that the 
> algorithm can be built up incrementally, identifying the easy 
> wins first and then over time extending it to cover harder 
> cases.
>
> One way that I see it working it is to conduct a form of 
> lowering where the new operator has some information added to 
> it to indicate the 'band' that the GC should place the non-root 
> objects into (root objects go on the stack). Using the syntax 
> of C++'s placement new (but totally different semantics) code 
> could be lowered to e.g.
>
> External externalObj = new(0) External(); // 0 means use the 
> default heap
> Foo foo = new(0x1234) Foo(); // 0x1234 is the heap/band id for 
> this set of objects
> ...
> Bar bar = new (0x1234) Bar();
>
> When the GC allocates memory it does so in the indicated 
> band/heap, and then when foo (the root object of the object 
> graph) goes out of scope the relevant band/heap is destroyed en 
> bloc. The benefit of the idea is that when scanning for objects
>  that can be deleted the GC does not need to consider those 
> objects in the non default bands/heaps. For some classes of 
> programs such as compilers (it was Higgs that gave me the 
> stimulus), and with good static analysis (aye there's the rub 
> cap'n) this could represent a very substantial time saving on 
> ech GC sweep.

Sounds a little like http://wiki.dlang.org/DIP46

Mike