• bearophile (13/13) Jun 25 2014 Is it possible and a good idea to change the D ABI to make code
• bearophile (12/15) Jun 25 2014 I meant, letting the compiler rewrite that code like this:
• John Colvin (4/19) Jun 25 2014 perhaps i'm misunderstanding things, but isn't this exactly what
• bearophile (4/7) Jun 26 2014 So do you know why D is not using that design?
• John Colvin (5/12) Jun 26 2014 It is, but a copy is made after the function returns anyway, for
• bearophile (7/12) Jun 26 2014 As far as I know, D is using that as an optimization, not as an
• Tobias Pankrath (2/15) Jun 25 2014 Isn't this a case for Named Return Value Optimization?
• bearophile (6/7) Jun 25 2014 Perhaps, but then why aren't dmd/ldc doing it? And the question

"bearophile" <bearophileHUGS lycos.com> writes:

Is it possible and a good idea to change the D ABI to make code 
like this avoid an array copy in 100% of the cases (without 
inlining)?


ubyte[1000] foo() nothrow  safe {
     typeof(return) data;
     // some code here.
     return data;
}
void main() nothrow {
     immutable data = foo();
}


Bye,
bearophile

"bearophile" <bearophileHUGS lycos.com> writes:

 Is it possible and a good idea to change the D ABI to make code 
 like this avoid an array copy in 100% of the cases (without 
 inlining)?

I meant, letting the compiler rewrite that code like this:


void foo(ref ubyte[1000] __data) nothrow  safe {
     __data[] = 0;
     // some code here.
}
void main() nothrow {
     ubyte[1000] __data = void;
     foo(__data);
     immutable data = cast(immutable ubyte[1000])__data;
}


Bye,
bearophile

"John Colvin" <john.loughran.colvin gmail.com> writes:

On Wednesday, 25 June 2014 at 20:59:29 UTC, bearophile wrote:
 Is it possible and a good idea to change the D ABI to make 
 code like this avoid an array copy in 100% of the cases 
 (without inlining)?

 I meant, letting the compiler rewrite that code like this:


 void foo(ref ubyte[1000] __data) nothrow  safe {
     __data[] = 0;
     // some code here.
 }
 void main() nothrow {
     ubyte[1000] __data = void;
     foo(__data);
     immutable data = cast(immutable ubyte[1000])__data;
 }


 Bye,
 bearophile

perhaps i'm misunderstanding things, but isn't this exactly what 
the System V ABI specifies anyway? Large aggregate returns are 
allocated on the calling stack, passed by hidden pointer.

"bearophile" <bearophileHUGS lycos.com> writes:

John Colvin:

 isn't this exactly what the System V ABI specifies anyway? 
 Large aggregate returns are allocated on the calling stack, 
 passed by hidden pointer.

So do you know why D is not using that design?

Bye,
bearophile

"John Colvin" <john.loughran.colvin gmail.com> writes:

On Thursday, 26 June 2014 at 12:06:34 UTC, bearophile wrote:
 John Colvin:

 isn't this exactly what the System V ABI specifies anyway? 
 Large aggregate returns are allocated on the calling stack, 
 passed by hidden pointer.

 So do you know why D is not using that design?

 Bye,
 bearophile

It is, but a copy is made after the function returns anyway, for 
reasons unknown to me. For some reason the optimizer can't elide 
it (ldc2 -O5 -release, with a pragma(LDC_never_inline); inside 
foo).

"bearophile" <bearophileHUGS lycos.com> writes:

John Colvin:

 It is,

As far as I know, D is using that as an optimization, not as an 
ABI default that must happen at all optimization levels.


 but a copy is made after the function returns anyway, for 
 reasons unknown to me. For some reason the optimizer can't 
 elide it (ldc2 -O5 -release, with a pragma(LDC_never_inline); 
 inside foo).

Right. perhaps I'll have to ask this again in the main D 
newsgroup (but perhaps better to do it Monday).

Bye,
bearophile

"Tobias Pankrath" <tobias pankrath.net> writes:

On Wednesday, 25 June 2014 at 20:55:47 UTC, bearophile wrote:
 Is it possible and a good idea to change the D ABI to make code 
 like this avoid an array copy in 100% of the cases (without 
 inlining)?


 ubyte[1000] foo() nothrow  safe {
     typeof(return) data;
     // some code here.
     return data;
 }
 void main() nothrow {
     immutable data = foo();
 }


 Bye,
 bearophile

Isn't this a case for Named Return Value Optimization?

"bearophile" <bearophileHUGS lycos.com> writes:

Tobias Pankrath:

 Isn't this a case for Named Return Value Optimization?

Perhaps, but then why aren't dmd/ldc doing it? And the question 
was if that's possible in the ABI, so it always happens (unless 
the array is very small).

Bye,
bearophile