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April 18, 2012
GC API: What can change for precise scanning?
Now that the compiler infrastructure has been implemented, I've 
gotten busy figuring out how to make D's default GC precise.  As 
a first attempt, I think I'm going to adapt my original solution 
from http://d.puremagic.com/issues/show_bug.cgi?id=3463 since 
it's simple and it works except that there previously was no 
clean way to get the offset info into the GC.  As Walter pointed 
out in another thread, the GCInfo template is allowed to 
instantiate to data instead of a function.  IMHO unless/until 
major architectural changes to the GC are made that require a 
function pointer, there's no point in adding this indirection.

I started working on this and I ran into a roadblock.  I need to 
know what parts of the GC API are allowed to change, and discuss 
how to abstract away the implementation of it from the GC API.  I 
assume the stuff in core.memory needs to stay mostly the same, 
though I guess we would need to add a setType() function that 
takes a pointer into a block of memory and a TypeInfo object and 
changes how the GC interprets the bits in the block.

In gc.d, we define a bunch of extern(C) functions and the proxy 
thing.  Since we've given up on the idea of swapping precise GCs 
at link time, can I just rip out all this unnecesary indirection? 
 If not, is it ok to change some of these signatures?  I 
definitely want to avoid allocating (requiring the GC lock) and 
then calling a function to set the type (requiring another lock 
acquisition) so the signature of malloc(), etc. needs to change 
somewhere.

More generally, what is the intended way to get GCInfo pointers 
from TypeInfo into the guts of the GC where they can be acted on?
April 18, 2012
Re: GC API: What can change for precise scanning?
Le 18/04/2012 02:36, dsimcha a écrit :
> Now that the compiler infrastructure has been implemented, I've gotten
> busy figuring out how to make D's default GC precise. As a first
> attempt, I think I'm going to adapt my original solution from
> http://d.puremagic.com/issues/show_bug.cgi?id=3463 since it's simple and
> it works except that there previously was no clean way to get the offset
> info into the GC. As Walter pointed out in another thread, the GCInfo
> template is allowed to instantiate to data instead of a function. IMHO
> unless/until major architectural changes to the GC are made that require
> a function pointer, there's no point in adding this indirection.
>
> I started working on this and I ran into a roadblock. I need to know
> what parts of the GC API are allowed to change, and discuss how to
> abstract away the implementation of it from the GC API. I assume the
> stuff in core.memory needs to stay mostly the same, though I guess we
> would need to add a setType() function that takes a pointer into a block
> of memory and a TypeInfo object and changes how the GC interprets the
> bits in the block.
>
> In gc.d, we define a bunch of extern(C) functions and the proxy thing.
> Since we've given up on the idea of swapping precise GCs at link time,
> can I just rip out all this unnecesary indirection? If not, is it ok to
> change some of these signatures? I definitely want to avoid allocating
> (requiring the GC lock) and then calling a function to set the type
> (requiring another lock acquisition) so the signature of malloc(), etc.
> needs to change somewhere.
>
> More generally, what is the intended way to get GCInfo pointers from
> TypeInfo into the guts of the GC where they can be acted on?

I guess that the flag to indicate if some piece of memory may have 
pointer can go away.

I think you certainly can remove all indirection. Additionally, I wonder 
why most of theses functions are extern(C).
April 18, 2012
Re: GC API: What can change for precise scanning?
On 18-04-2012 11:56, deadalnix wrote:
> Le 18/04/2012 02:36, dsimcha a écrit :
>> Now that the compiler infrastructure has been implemented, I've gotten
>> busy figuring out how to make D's default GC precise. As a first
>> attempt, I think I'm going to adapt my original solution from
>> http://d.puremagic.com/issues/show_bug.cgi?id=3463 since it's simple and
>> it works except that there previously was no clean way to get the offset
>> info into the GC. As Walter pointed out in another thread, the GCInfo
>> template is allowed to instantiate to data instead of a function. IMHO
>> unless/until major architectural changes to the GC are made that require
>> a function pointer, there's no point in adding this indirection.
>>
>> I started working on this and I ran into a roadblock. I need to know
>> what parts of the GC API are allowed to change, and discuss how to
>> abstract away the implementation of it from the GC API. I assume the
>> stuff in core.memory needs to stay mostly the same, though I guess we
>> would need to add a setType() function that takes a pointer into a block
>> of memory and a TypeInfo object and changes how the GC interprets the
>> bits in the block.
>>
>> In gc.d, we define a bunch of extern(C) functions and the proxy thing.
>> Since we've given up on the idea of swapping precise GCs at link time,
>> can I just rip out all this unnecesary indirection? If not, is it ok to
>> change some of these signatures? I definitely want to avoid allocating
>> (requiring the GC lock) and then calling a function to set the type
>> (requiring another lock acquisition) so the signature of malloc(), etc.
>> needs to change somewhere.
>>
>> More generally, what is the intended way to get GCInfo pointers from
>> TypeInfo into the guts of the GC where they can be acted on?
>
> I guess that the flag to indicate if some piece of memory may have
> pointer can go away.

+1. This is useless if we're going to use bitmaps or similar.

>
> I think you certainly can remove all indirection. Additionally, I wonder
> why most of theses functions are extern(C).


-- 
- Alex
April 18, 2012
Re: GC API: What can change for precise scanning?
On 18-04-2012 02:36, dsimcha wrote:
> Now that the compiler infrastructure has been implemented, I've gotten
> busy figuring out how to make D's default GC precise. As a first
> attempt, I think I'm going to adapt my original solution from
> http://d.puremagic.com/issues/show_bug.cgi?id=3463 since it's simple and
> it works except that there previously was no clean way to get the offset
> info into the GC. As Walter pointed out in another thread, the GCInfo
> template is allowed to instantiate to data instead of a function. IMHO
> unless/until major architectural changes to the GC are made that require
> a function pointer, there's no point in adding this indirection.
>
> I started working on this and I ran into a roadblock. I need to know
> what parts of the GC API are allowed to change, and discuss how to
> abstract away the implementation of it from the GC API. I assume the
> stuff in core.memory needs to stay mostly the same, though I guess we
> would need to add a setType() function that takes a pointer into a block
> of memory and a TypeInfo object and changes how the GC interprets the
> bits in the block.
>
> In gc.d, we define a bunch of extern(C) functions and the proxy thing.
> Since we've given up on the idea of swapping precise GCs at link time,
> can I just rip out all this unnecesary indirection? If not, is it ok to
> change some of these signatures? I definitely want to avoid allocating
> (requiring the GC lock) and then calling a function to set the type
> (requiring another lock acquisition) so the signature of malloc(), etc.
> needs to change somewhere.
>
> More generally, what is the intended way to get GCInfo pointers from
> TypeInfo into the guts of the GC where they can be acted on?

This is not specifically an answer to your question, but my opinion on 
this subject is that altering the GC API without a deprecation process 
or similar is fine. This is a core component of the runtime, and I don't 
think expecting a stable API for something like a garbage collector of 
all things is reasonable in the first place.

That said, I do think functionality has to be maintained. That is, if 
the GC can do something in a previous version, it should be able to do 
it in the subsequent version too. *How* that something is achieved in 
the subsequent version isn't so important, as long as the functionality 
is *there* in some way.

-- 
- Alex
April 18, 2012
Re: GC API: What can change for precise scanning?
On 4/18/2012 2:56 AM, deadalnix wrote:
> I think you certainly can remove all indirection. Additionally, I wonder why
> most of theses functions are extern(C).

The purpose of the indirection is so that DLLs in Windows can share a gc 
instance, rather than having two instances fight each other.
April 18, 2012
Re: GC API: What can change for precise scanning?
On 4/17/2012 5:36 PM, dsimcha wrote:
> Now that the compiler infrastructure has been implemented, I've gotten busy
> figuring out how to make D's default GC precise. As a first attempt, I think I'm
> going to adapt my original solution from
> http://d.puremagic.com/issues/show_bug.cgi?id=3463 since it's simple and it
> works except that there previously was no clean way to get the offset info into
> the GC. As Walter pointed out in another thread, the GCInfo template is allowed
> to instantiate to data instead of a function. IMHO unless/until major
> architectural changes to the GC are made that require a function pointer,
> there's no point in adding this indirection.
>
> I started working on this and I ran into a roadblock. I need to know what parts
> of the GC API are allowed to change, and discuss how to abstract away the
> implementation of it from the GC API. I assume the stuff in core.memory needs to
> stay mostly the same, though I guess we would need to add a setType() function
> that takes a pointer into a block of memory and a TypeInfo object and changes
> how the GC interprets the bits in the block.
>
> In gc.d, we define a bunch of extern(C) functions and the proxy thing. Since
> we've given up on the idea of swapping precise GCs at link time, can I just rip
> out all this unnecesary indirection? If not, is it ok to change some of these
> signatures? I definitely want to avoid allocating (requiring the GC lock) and
> then calling a function to set the type (requiring another lock acquisition) so
> the signature of malloc(), etc. needs to change somewhere.
>
> More generally, what is the intended way to get GCInfo pointers from TypeInfo
> into the guts of the GC where they can be acted on?

1. I would not try to redesign everything and do precise gc at the same time.

2. The purpose of the indirection is to support DLLs so that the different DLLs 
can share an instance.

3. The reason for function pointers for marking is so that the marking code can 
be customized and directly inlined, rather than decoding a table. It costs one 
code indirection, but after that it cannot be beaten for speed.
April 18, 2012
Re: GC API: What can change for precise scanning?
On Apr 18, 2012, at 2:56 AM, deadalnix wrote:
> 
> I think you certainly can remove all indirection. Additionally, I wonder why most of theses functions are extern(C).

So the GC implementation is opaque and the GC can therefore be chosen at link-time.  Similar to how the compiler runtime code hides behind a raft of extern© functions.
April 18, 2012
Re: GC API: What can change for precise scanning?
On Apr 17, 2012, at 5:36 PM, dsimcha wrote:
> 
> More generally, what is the intended way to get GCInfo pointers from TypeInfo into the guts of the GC where they can be acted on?

I just resurrected an old thread titled "Proposed changes to the GC interface" in the Druntime mailing list.  Perhaps we could pick up discussion there?
April 18, 2012
Re: GC API: What can change for precise scanning?
On Apr 18, 2012, at 1:13 PM, Walter Bright wrote:

> On 4/17/2012 5:36 PM, dsimcha wrote:
>> Now that the compiler infrastructure has been implemented, I've gotten busy
>> figuring out how to make D's default GC precise. As a first attempt, I think I'm
>> going to adapt my original solution from
>> http://d.puremagic.com/issues/show_bug.cgi?id=3463 since it's simple and it
>> works except that there previously was no clean way to get the offset info into
>> the GC. As Walter pointed out in another thread, the GCInfo template is allowed
>> to instantiate to data instead of a function. IMHO unless/until major
>> architectural changes to the GC are made that require a function pointer,
>> there's no point in adding this indirection.
>> 
>> I started working on this and I ran into a roadblock. I need to know what parts
>> of the GC API are allowed to change, and discuss how to abstract away the
>> implementation of it from the GC API. I assume the stuff in core.memory needs to
>> stay mostly the same, though I guess we would need to add a setType() function
>> that takes a pointer into a block of memory and a TypeInfo object and changes
>> how the GC interprets the bits in the block.
>> 
>> In gc.d, we define a bunch of extern(C) functions and the proxy thing. Since
>> we've given up on the idea of swapping precise GCs at link time, can I just rip
>> out all this unnecesary indirection? If not, is it ok to change some of these
>> signatures? I definitely want to avoid allocating (requiring the GC lock) and
>> then calling a function to set the type (requiring another lock acquisition) so
>> the signature of malloc(), etc. needs to change somewhere.
>> 
>> More generally, what is the intended way to get GCInfo pointers from TypeInfo
>> into the guts of the GC where they can be acted on?
> 
> 1. I would not try to redesign everything and do precise gc at the same time.
> 
> 2. The purpose of the indirection is to support DLLs so that the different DLLs can share an instance.
> 
> 3. The reason for function pointers for marking is so that the marking code can be customized and directly inlined, rather than decoding a table. It costs one code indirection, but after that it cannot be beaten for speed.

Leandro's GC (CDGC) is already set up to support precise scanning.  It's in the Druntime git repository, but lacks the features added to the Druntime GC compared to the Tango GC on which CDGC is based.  Still, it may be easier to update CDGC based on a diff between the Druntime and Tango GC than it would to add precise scanning to the GC Druntime currently uses.  Worth a look if anyone is interested anyway.
April 18, 2012
Re: GC API: What can change for precise scanning?
Le 18/04/2012 20:53, Sean Kelly a écrit :
> On Apr 18, 2012, at 2:56 AM, deadalnix wrote:
>>
>> I think you certainly can remove all indirection. Additionally, I wonder why most of theses functions are extern(C).
>
> So the GC implementation is opaque and the GC can therefore be chosen at link-time.  Similar to how the compiler runtime code hides behind a raft of extern© functions.

I know, but this is now impossible anyway because of the modification of 
TypeInfo anyway.
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