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is the array literal in a loop stack or heap allocated?
Oct 11
mw
Oct 11
mw
Oct 11
ryuukk_
October 11
Gravatar of mwPosted by mwReply
mw
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Hi,

I want to confirm: in the following loop, is the array literal a vs. b stack or heap allocated? and how many times?

void main() {

int[2] a;
int[] b;

int i;
While(++i <=100) {

a = [i, i+1]; // array literal
b = [i, i+1];

}

}

Thanks.

October 10
Gravatar of H. S. TeohPosted by H. S. Teoh
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H. S. Teoh
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Posted in reply to mw


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On Wed, Oct 11, 2023 at 02:54:53AM +0000, mw via Digitalmars-d-learn wrote:
> Hi,
> 
> I want to confirm: in the following loop, is the array literal `a` vs. `b` stack or heap allocated? and how many times?
> 
> void main() {
> 
> int[2] a;

This is stack-allocated. Once per call to the function.


> int[] b;

This is an empty slice. It can refer to either stack or heap memory, depending on what's assigned to it.


> int i;
> While(++i <=100) {
> 
>   a = [i, i+1];  // array literal

`a` is overwritten in-place once per loop.


>   b = [i, i+1];
[...]

A new array consisting of 2 elements is allocated, once per loop, and assigned to b each time. Any arrays from previous iterations will be collected by the GC eventually.


T

-- 
They pretend to pay us, and we pretend to work. -- Russian saying
October 11
Gravatar of ryuukk_Posted by ryuukk_
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ryuukk_
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Posted in reply to mw


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On Wednesday, 11 October 2023 at 02:54:53 UTC, mw wrote:

>

Hi,

I want to confirm: in the following loop, is the array literal a vs. b stack or heap allocated? and how many times?

void main() {

int[2] a;
int[] b;

int i;
While(++i <=100) {

a = [i, i+1]; // array literal
b = [i, i+1];

}

}

Thanks.

a is a static array, therefore it won't allocate any, it's a memcpy

b will be heap allocated, and it'll do an allocate at each iteration

void test_b()
{
    int[] a;
    int i;
    while (++i <= 100)
    {
        a = [i, i + 1];
        printf("%p\n", a.ptr);
    }
}

You can run this, and it'll print a different address each time

If you add [] it'll do range based copy (memcpy), but since the array is not initialized, it has a length of 0, so you only need to allocate once (either with GC or with malloc)

void test_b()
{
    int[] a;
    int i;

    a.length = 2; // initialize the heap allocated array here

    // or with malloc:
    // auto ptr = malloc(int.sizeof * 2);
    // a = cast(int[]) ptr[0 .. int.sizeof * 2];

    while (++i <= 100)
    {
        a[] = [i, i + 1];
        printf("%p\n", a.ptr);
    }
}

Otherwise you'd get: core.exception.RangeError@onlineapp.d(18): Range violation

I don't use D with the GC, so my memory about it is probably foggy, but i'm pretty sure what i said is right, please anyone correct me if i'm wrong

October 11
Gravatar of mwPosted by mw
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mw
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Posted in reply to H. S. Teoh


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On Wednesday, 11 October 2023 at 03:15:30 UTC, H. S. Teoh wrote:
> On Wed, Oct 11, 2023 at 02:54:53AM +0000, mw via Digitalmars-d-learn wrote:
>> Hi,
>> 
>> I want to confirm: in the following loop, is the array literal `a` vs. `b` stack or heap allocated? and how many times?
>> 
>> void main() {
>> 
>> int[2] a;
>
> This is stack-allocated. Once per call to the function.
>
>
>> int[] b;
>
> This is an empty slice. It can refer to either stack or heap memory, depending on what's assigned to it.
>
>
>> int i;
>> While(++i <=100) {
>> 
>>   a = [i, i+1];  // array literal
>
> `a` is overwritten in-place once per loop.

How about the temporary array literal on the right hand side? It's stack / heap allocated? Or it's not in the language specification, but up to the (optimizing) compiler to decide?

>
>>   b = [i, i+1];
> [...]
>
> A new array consisting of 2 elements is allocated, once per loop, and assigned to b each time. Any arrays from previous iterations will be collected by the GC eventually.
>
>
> T
October 11
Gravatar of ImperatornPosted by Imperatorn
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On Wednesday, 11 October 2023 at 02:54:53 UTC, mw wrote:

>

Hi,

I want to confirm: in the following loop, is the array literal a vs. b stack or heap allocated? and how many times?

void main() {

int[2] a;
int[] b;

int i;
While(++i <=100) {

a = [i, i+1]; // array literal
b = [i, i+1];

}

}

Thanks.

profile=gc

October 12
Gravatar of Steven SchveighofferPosted by Steven Schveighoffer
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Steven Schveighoffer
Gravatar of Steven Schveighoffer
Posted in reply to mw


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On 10/10/23 10:54 PM, mw wrote:

>

Hi,

I want to confirm: in the following loop, is the array literal a vs. b stack or heap allocated? and how many times?

ask the compiler:

void main() @nogc {

int[2] a;
int[] b;

int i;
while(++i <=100) {

  a = [i, i+1];  // array literal
  //b = [i, i+1]; // yes, this allocates, had to comment it out

}

}

-Steve