I was messing around with clang codegen and noticed that sometime it optimize structs using the tail pad, and sometime it doesn't. It ended up in the following stack overflow question :

http://stackoverflow.com/questions/24110347/when-extending-a-padded-struct-why-cant-extra-fields-be-placed-in-the-tail-pad

It seems that C++ will use the padding left in a struct to pack more stuff in there. I was wonering what position we want to have with D.

Packing elements efficiently is a major importance for performance. For now, D uses C style packing (ie: nothing in the tail pad) as far as I know. This is missed opportunities to pack more (I have many places in SDC where it could be beneficial from using tail pad).

If we are gonna interoperable with C++, then we need to understand the tail pad optimization anyway. So here is what I propose:

extern(D) do tail pad optimize.
extern(C) struct do not tail pad optimize.
extern(C++) do tail pad with C++ rules:
do not tail pad if (otherwise tail pad):
1. has no non-static data members that aren't standard-layout
2. has no virtual functions and no virtual base classes
3. has the same access control for all non-static data members
4. has no base classes that aren't standard-layout
5. either has no base class with non-static data members or has no non-static data members in the most derived class and only one base with them
6. has no base classes of the same type as the first non-static data member

thought ?

deadalnix:

> thought ?

I think for D there is a lower handing fruit: the D specs allow to reorder class instance fields, but I think the D front-end is not yet doing that.

Bye,
bearophile

On Tuesday, 10 June 2014 at 08:57:26 UTC, bearophile wrote:
> deadalnix:
>
>> thought ?
>
> I think for D there is a lower handing fruit: the D specs allow to reorder class instance fields, but I think the D front-end is not yet doing that.

But only for classes, not for structs.

10-Jun-2014 11:30, deadalnix пишет:
[snip]
> extern(D) do tail pad optimize.
> extern(C) struct do not tail pad optimize.
> extern(C++) do tail pad with C++ rules:
> do not tail pad if (otherwise tail pad):
> 1. has no non-static data members that aren't standard-layout
> 2. has no virtual functions and no virtual base classes
> 3. has the same access control for all non-static data members
> 4. has no base classes that aren't standard-layout
> 5. either has no base class with non-static data members or has no
> non-static data members in the most derived class and only one base with
> them
> 6. has no base classes of the same type as the first non-static data member
>
> thought ?

Would be useful, but as proposed it might break some of current C bindings silently (unless it's extern(C): at the top of file).


-- 
Dmitry Olshansky

On 6/10/2014 12:30 AM, deadalnix wrote:
> thought ?

This does not apply to D structs because D structs do not inherit.

C doesn't have classes, so no issues there.

extern(C++) class should match the C++ ABI for this.

extern(D) class we are free to innovate with the layout.

I suggest turning this into a bugzilla enhancement request.

On Tuesday, 10 June 2014 at 20:50:46 UTC, Walter Bright wrote:
> On 6/10/2014 12:30 AM, deadalnix wrote:
>> thought ?
>
> This does not apply to D structs because D structs do not inherit.
>
> C doesn't have classes, so no issues there.
>
> extern(C++) class should match the C++ ABI for this.
>
> extern(D) class we are free to innovate with the layout.
>
> I suggest turning this into a bugzilla enhancement request.

I'm talking about structs, not classes.

On 6/10/2014 5:27 PM, deadalnix wrote:
> I'm talking about structs, not classes.

Ok, but since D structs do not inherit, how does tail pad optimization apply?

On Wednesday, 11 June 2014 at 02:10:18 UTC, Walter Bright wrote:
> On 6/10/2014 5:27 PM, deadalnix wrote:
>> I'm talking about structs, not classes.
>
> Ok, but since D structs do not inherit, how does tail pad optimization apply?

struct S1 {
     int a;
     byte b;
}

struct S2 {
     S1 s1;
     char c;
}

Sé could be 8 byte long (some variation of this are in C++ for
instance if I make a public and b private).

On 6/10/2014 7:44 PM, deadalnix wrote:
> On Wednesday, 11 June 2014 at 02:10:18 UTC, Walter Bright wrote:
>> On 6/10/2014 5:27 PM, deadalnix wrote:
>>> I'm talking about structs, not classes.
>>
>> Ok, but since D structs do not inherit, how does tail pad optimization apply?
>
> struct S1 {
>       int a;
>       byte b;
> }
>
> struct S2 {
>       S1 s1;
>       char c;
> }
>
> Sé could be 8 byte long (some variation of this are in C++ for
> instance if I make a public and b private).

Do any C++ compilers do this for this case, or just for the inheritance one?

On 6/10/2014 7:44 PM, deadalnix wrote:
> On Wednesday, 11 June 2014 at 02:10:18 UTC, Walter Bright wrote:
>> On 6/10/2014 5:27 PM, deadalnix wrote:
>>> I'm talking about structs, not classes.
>>
>> Ok, but since D structs do not inherit, how does tail pad optimization apply?
>
> struct S1 {
>       int a;
>       byte b;
> }
>
> struct S2 {
>       S1 s1;
>       char c;
> }

Hmm, this could have serious problems with the following:

S1 s1;
S2 s2;
s2.c = 3;
memcpy(&s2.s1, &s1, sizeof(S1)); // Oops! stomped on s2.c