• Nicholas Wilson (7/7) May 06 2016 In Dicebot's DConf talk he mentioned that it is possible to use
• Benjamin Thaut (6/13) May 06 2016 The problem with this is, that its not future proof. Once the top
• =?UTF-8?Q?Ali_=c3=87ehreli?= (3/4) May 07 2016 That was deadalnix. :)
• Nicholas Wilson (2/6) May 07 2016 Touché.
• Marco Leise (11/20) May 07 2016 Too many d's for the day. :) I agree with Benjamin, that we
• deadalnix (6/24) May 08 2016 You could have allocated an array, so that's a problem, you can
• Claude (6/13) May 09 2016 I would not rely on it *at all* actually, even now. I do not

Nicholas Wilson <iamthewilsonator hotmail.com> writes:

In Dicebot's DConf talk he mentioned that it is possible to use 
the top 16 bits for tagging pointers and the associated risks.

Regarding the GC not seeing a pointer if the top 16 bits are 
used, whats to stop us from changing the GC to (or adding an 
option to) ignore those bits when checking?

I imagine that it would cause a lot more false pointers, but when 
we get precise GC that problem would go away.

Benjamin Thaut <code benjamin-thaut.de> writes:

On Saturday, 7 May 2016 at 06:08:01 UTC, Nicholas Wilson wrote:
 In Dicebot's DConf talk he mentioned that it is possible to use 
 the top 16 bits for tagging pointers and the associated risks.

 Regarding the GC not seeing a pointer if the top 16 bits are 
 used, whats to stop us from changing the GC to (or adding an 
 option to) ignore those bits when checking?

 I imagine that it would cause a lot more false pointers, but 
 when we get precise GC that problem would go away.

The problem with this is, that its not future proof. Once the top 
16-bits of a pointer get used D programs will suddenly stop 
working. So i think we shouldn't support that officially. If you 
really want to do that you can simply hack the scan routine of 
the gc in druntime yourself and ship with the hacked version.

=?UTF-8?Q?Ali_=c3=87ehreli?= <acehreli yahoo.com> writes:

On 05/06/2016 11:08 PM, Nicholas Wilson wrote:
 In Dicebot's DConf talk

That was deadalnix. :)

Ali

Nicholas Wilson <iamthewilsonator hotmail.com> writes:

On Saturday, 7 May 2016 at 09:01:11 UTC, Ali Çehreli wrote:
 On 05/06/2016 11:08 PM, Nicholas Wilson wrote:
 In Dicebot's DConf talk

 That was deadalnix. :)

 Ali

Touché.

Marco Leise <Marco.Leise gmx.de> writes:

Am Sat, 07 May 2016 12:02:07 +0000
schrieb Nicholas Wilson <iamthewilsonator hotmail.com>:

 On Saturday, 7 May 2016 at 09:01:11 UTC, Ali =C3=87ehreli wrote:
 On 05/06/2016 11:08 PM, Nicholas Wilson wrote: =20
 In Dicebot's DConf talk =20

 That was deadalnix. :)

 Ali =20

=20
 Touch=C3=A9.

Too many d's for the day. :) I agree with Benjamin, that we
can't rely on the 16-bits being reserved forever.

I have a question about the lower 3 bits OTOH. The minimal
GC allocation size is >=3D 8 bytes, right? I.e. if we set the
lower 3 bits to 0b111, and the pointer points to a 4 byte
structure, it will still be recognized as an internal pointer
due to the overallocation.

--=20
Marco

deadalnix <deadalnix gmail.com> writes:

On Saturday, 7 May 2016 at 12:32:53 UTC, Marco Leise wrote:
 Am Sat, 07 May 2016 12:02:07 +0000
 schrieb Nicholas Wilson <iamthewilsonator hotmail.com>:

 On Saturday, 7 May 2016 at 09:01:11 UTC, Ali Çehreli wrote:
 On 05/06/2016 11:08 PM, Nicholas Wilson wrote:
 In Dicebot's DConf talk

 That was deadalnix. :)

 Ali

 
 Touché.

 Too many d's for the day. :) I agree with Benjamin, that we 
 can't rely on the 16-bits being reserved forever.

 I have a question about the lower 3 bits OTOH. The minimal GC 
 allocation size is >= 8 bytes, right? I.e. if we set the lower 
 3 bits to 0b111, and the pointer points to a 4 byte structure, 
 it will still be recognized as an internal pointer due to the 
 overallocation.

You could have allocated an array, so that's a problem, you can 
only use 2 bits. taggedPointer will check for this at compile 
time.

But, yes, LSB are safe to use is you wrap it properly because it 
will be seen as an interior pointer.

Claude <no no.no> writes:

On Saturday, 7 May 2016 at 06:08:01 UTC, Nicholas Wilson wrote:
 In Dicebot's DConf talk he mentioned that it is possible to use 
 the top 16 bits for tagging pointers and the associated risks.

 Regarding the GC not seeing a pointer if the top 16 bits are 
 used, whats to stop us from changing the GC to (or adding an 
 option to) ignore those bits when checking?

 I imagine that it would cause a lot more false pointers, but 
 when we get precise GC that problem would go away.

I would not rely on it *at all* actually, even now. I do not 
think it would make D portable to every system (let's not only 
consider x86 architecture).
We don't know how a system would choose to map physical memory 
and hardware devices to virtual memory.