Re: Struct Constructor Lazy
On Wednesday, 12 July 2017 at 11:18:08 UTC, Biotronic wrote: The traditional solution is static opCall: That's bug city... the dummy argument is better, or a named static factory function. Foo foo = Foo; Foo foo = Foo(); those are different with static opCall. It also conflicts with constructors and non-static opCall.
Re: Struct Constructor Lazy
On Wednesday, 12 July 2017 at 11:57:33 UTC, Biotronic wrote: Debug = no optimization. That's not really true, you can have an optimized debug build. dmd's -debug, -g, and -O switches are all independent. -debug turns on blocks labeled with the `debug` switch in code. -g adds info for a debugger to read. -O turns on optimizations. You can use any combination of those. Looking at the generated assembly in a debug build is worthless. I don't agree - looking at the generated assembly would put to rest all these questions. But before doing that, you should show that the performance is actually bad. I can't imagine sending a zero argument (whether in a register or pushed to the stack) would ever be a serious problem.
Re: Struct Constructor Lazy
On Wednesday, 12 July 2017 at 11:00:54 UTC, Jiyan wrote:
when A(0) is called I would want here optimal performance, so
there doesnt even need to be a value pushed on the stack (i=0),
what would be like having a constructor with zero arguments (i
is never used!).
This is so, so irrelevant. Even if it isn't optimized out (which
it probably is), pushing a zero to the stack is so extremely
cheap that you'd probably not notice it, especially compared to
reading something from a file.
Im pretty new to D, can somebody tell me how i would do this?
Is this(lazy int i){ ... a solution?
That's actually more expensive than just sending the zero!
Re: Struct Constructor Lazy
On Wednesday, 12 July 2017 at 12:02:37 UTC, Jiyan wrote: Thank you, one last question: If i declare the parameter as ref i, then there shouldnt be any overhead wouldnt it? Thanks :) That would be basically the exact equivalent - instead of passing an int, you'll be passing a pointer. -- Biotronic
Re: Struct Constructor Lazy
Thank you, one last question: If i declare the parameter as ref i, then there shouldnt be any overhead wouldnt it? Thanks :)
Re: Struct Constructor Lazy
On Wednesday, 12 July 2017 at 11:34:45 UTC, Jiyan wrote:
Hey,
yes i did but to be honest i used dmd in debug version.
The thing about the static one, is that it creates a local
object A isnt that a performance issue itself - or am i wrong -
im confused actually :P?
Debug = no optimization. Looking at the generated assembly in a
debug build is worthless.
You raise a valid point. In a debug build, you're probably right
- it will need to copy the temporary to the target. With
optimizations enabled, NRVO[0] will populate the target directly,
resulting in roughly the equivalent of this code:
struct A {
int field;
static void opCall(A* p) {
p.field = getDataFromFile("file.txt");
}
}
A a;
A();
If the function is inlined, the whole problem is of course moot.
There are probably other optimizations that can interfere with
what I've described.
--
Biotronic
[0]: https://en.wikipedia.org/wiki/Return_value_optimization
Re: Struct Constructor Lazy
On Wednesday, 12 July 2017 at 11:18:08 UTC, Biotronic wrote:
On Wednesday, 12 July 2017 at 11:00:54 UTC, Jiyan wrote:
[...]
The traditional solution is static opCall:
struct A {
int field;
static A opCall() {
A result;
result.field = getDataFromFile("file.txt");
return result;
}
}
A instance = A();
I believe I've heard this is frowned upon these days, but I
don't know of a better solution.
For optimal speed you might also want to skip default
initialization of result, by writing A result = void;.
I would be surprised if the optimizer wasn't able to optimize
away the useless parameter though - have you looked at the
generated assembly?
--
Biotronic
Hey,
yes i did but to be honest i used dmd in debug version.
The thing about the static one, is that it creates a local object
A isnt that a performance issue itself - or am i wrong - im
confused actually :P?
Re: Struct Constructor Lazy
On Wednesday, 12 July 2017 at 11:00:54 UTC, Jiyan wrote:
Hey there:)
i want to know whether the following is somehow possible:
structs dont have default constructors, i know so:
struct A
{
int field;
this(int i){field = getDataFromFile("file.txt");}
}
A instance = A(0);
Here comes my issue:
when A(0) is called I would want here optimal performance, so
there doesnt even need to be a value pushed on the stack (i=0),
what would be like having a constructor with zero arguments (i
is never used!).
Im pretty new to D, can somebody tell me how i would do this?
Is this(lazy int i){ ... a solution?
The traditional solution is static opCall:
struct A {
int field;
static A opCall() {
A result;
result.field = getDataFromFile("file.txt");
return result;
}
}
A instance = A();
I believe I've heard this is frowned upon these days, but I don't
know of a better solution.
For optimal speed you might also want to skip default
initialization of result, by writing A result = void;.
I would be surprised if the optimizer wasn't able to optimize
away the useless parameter though - have you looked at the
generated assembly?
--
Biotronic
Struct Constructor Lazy
Hey there:)
i want to know whether the following is somehow possible:
structs dont have default constructors, i know so:
struct A
{
int field;
this(int i){field = getDataFromFile("file.txt");}
}
A instance = A(0);
Here comes my issue:
when A(0) is called I would want here optimal performance, so
there doesnt even need to be a value pushed on the stack (i=0),
what would be like having a constructor with zero arguments (i is
never used!).
Im pretty new to D, can somebody tell me how i would do this?
Is this(lazy int i){ ... a solution?
