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digitalmars.D.learn - A few questions regarding GC.malloc

reply "Gary Willoughby" <dev nomad.so> writes:
A few questions regarding GC.malloc.

When requesting a chunk of memory from GC.malloc am i right in 
assuming that this chunk is scanned for pointers to other GC 
resources in order to make decisions whether to collect them or 
not?

What does BlkAttr.FINALIZE do when used in the GC.malloc call?
Sep 25 2014
parent reply "monarch_dodra" <monarchdodra gmail.com> writes:
On Thursday, 25 September 2014 at 20:58:29 UTC, Gary Willoughby 
wrote:

 A few questions regarding GC.malloc.

 When requesting a chunk of memory from GC.malloc am i right in 
 assuming that this chunk is scanned for pointers to other GC 
 resources in order to make decisions whether to collect them or 
 not?


By default, yes, but you can use BlkAttr.NO_SCAN if you do not 
want that (eg, if you want to store integers). As a rule of 
thumb, you can use hasIndirections!T to know if or if not to scan 
(that's what most of phobos relies on).


 What does BlkAttr.FINALIZE do when used in the GC.malloc call?


I have no idea. I think its for classes though, since we 
(currently) don't finalize structs anyways.
Sep 25 2014
parent reply "Sean Kelly" <sean invisibleduck.org> writes:
On Thursday, 25 September 2014 at 21:43:53 UTC, monarch_dodra 
wrote:

 On Thursday, 25 September 2014 at 20:58:29 UTC, Gary Willoughby 
 wrote:

 A few questions regarding GC.malloc.

 When requesting a chunk of memory from GC.malloc am i right in 
 assuming that this chunk is scanned for pointers to other GC 
 resources in order to make decisions whether to collect them 
 or not?


 By default, yes, but you can use BlkAttr.NO_SCAN if you do not 
 want that (eg, if you want to store integers). As a rule of 
 thumb, you can use hasIndirections!T to know if or if not to 
 scan (that's what most of phobos relies on).


Yep.  It's generally just easier to do a "new T[]" when you want 
a block to ensure that the proper flags are set, as GC.malloc is 
conservative in terms of the flags it sets.


 What does BlkAttr.FINALIZE do when used in the GC.malloc call?


 I have no idea. I think its for classes though, since we 
 (currently) don't finalize structs anyways.


Yes it's for memory blocks containing class instances.  It 
basically tells the GC to call Object.~this() when collecting the 
block.
Sep 25 2014
parent reply Steven Schveighoffer <schveiguy yahoo.com> writes:
On 9/25/14 6:03 PM, Sean Kelly wrote:

 On Thursday, 25 September 2014 at 21:43:53 UTC, monarch_dodra wrote:

 On Thursday, 25 September 2014 at 20:58:29 UTC, Gary Willoughby wrote:

 What does BlkAttr.FINALIZE do when used in the GC.malloc call?


 I have no idea. I think its for classes though, since we (currently)
 don't finalize structs anyways.


 Yes it's for memory blocks containing class instances.  It basically
 tells the GC to call Object.~this() when collecting the block.


Just to add to Sean's statement, don't use this flag. It will crash the 
runtime, unless you have properly set up the classinfo pointer :) It 
does NOT work on structs, though I think there is a PR in the works to 
have structs destroyed from the GC.

-Steve
Sep 26 2014
parent reply "monarch_dodra" <monarchdodra gmail.com> writes:
On Friday, 26 September 2014 at 18:03:40 UTC, Steven 
Schveighoffer wrote:

 On 9/25/14 6:03 PM, Sean Kelly wrote:

 On Thursday, 25 September 2014 at 21:43:53 UTC, monarch_dodra 
 wrote:

 On Thursday, 25 September 2014 at 20:58:29 UTC, Gary 
 Willoughby wrote:

 What does BlkAttr.FINALIZE do when used in the GC.malloc 
 call?


 I have no idea. I think its for classes though, since we 
 (currently)
 don't finalize structs anyways.


 Yes it's for memory blocks containing class instances.  It 
 basically
 tells the GC to call Object.~this() when collecting the block.


 Just to add to Sean's statement, don't use this flag. It will 
 crash the runtime, unless you have properly set up the 
 classinfo pointer :) It does NOT work on structs, though I 
 think there is a PR in the works to have structs destroyed from 
 the GC.

 -Steve


Kind of like APPENDABLE I guess, since it only works if you 
correctly setup the appendable data, and correctly slice at the 
correct offset, both of which are implementation defined.
Sep 26 2014
parent Steven Schveighoffer <schveiguy yahoo.com> writes:
On 9/26/14 3:24 PM, monarch_dodra wrote:

 On Friday, 26 September 2014 at 18:03:40 UTC, Steven Schveighoffer wrote:

 On 9/25/14 6:03 PM, Sean Kelly wrote:

 On Thursday, 25 September 2014 at 21:43:53 UTC, monarch_dodra wrote:

 On Thursday, 25 September 2014 at 20:58:29 UTC, Gary Willoughby wrote:

 What does BlkAttr.FINALIZE do when used in the GC.malloc call?


 I have no idea. I think its for classes though, since we (currently)
 don't finalize structs anyways.


 Yes it's for memory blocks containing class instances.  It basically
 tells the GC to call Object.~this() when collecting the block.


 Just to add to Sean's statement, don't use this flag. It will crash
 the runtime, unless you have properly set up the classinfo pointer :)
 It does NOT work on structs, though I think there is a PR in the works
 to have structs destroyed from the GC.


 Kind of like APPENDABLE I guess, since it only works if you correctly
 setup the appendable data, and correctly slice at the correct offset,
 both of which are implementation defined.


Well, kind of yes.

But the difference here is that if APPENDABLE is set, and you never 
actually append to an array in that block, then you are going to be 
fine. Even if you didn't set it up, it's most likely that you will not 
encounter a problem, because whatever is there is likely not the right 
size (and that simply results in a reallocation without touching 
anything). Even if you do manage to use appending on that block, and the 
data that happens to be where the allocated size is matches the current 
slice end, it will corrupt some data (or not, the smaller block sizes 
keep the append data at the end of the block). This could potentially 
crash, or it could do nothing.

If FINALIZE is set, when the GC collects the memory it WILL crash if you 
didn't set up some sort of mock classinfo.

I don't recommend using either, but FINALIZE is a much worse outcome IMO :)

-Steve
Sep 26 2014