• John Burton (28/28) Sep 08 2017 I wrote this simple program to test out my understanding of
• =?UTF-8?Q?Ali_=c3=87ehreli?= (40/58) Sep 08 2017 Although I responded below, I'm curious on what others think about this

John Burton <john.burton jbmail.com> writes:

I wrote this simple program to test out my understanding of 
memory allocation :-

     import std.stdio;

     void main()
     {
         int [] array = new int[250];

         writeln(array.length, " elements ", array);

         // Append one value to the array
         array ~= 123;
         writeln(array.length, " elements ", array);
     }

I compiled it with 'dmd test.d -profile=gc'

After running it, the profile report was :-

     bytes allocated, allocations, type, function, file:line
                2000               1 int[] D main test.d:5
                   4               1 int[] D main test.d:10

This is not how I expected it to be. I would have expected that 
the runtime either did not have to allocate at all at line 10 to 
add a new element because there was already space or it would 
have to allocate space for the new enlarged array and copy the 
array to it, in which case I'd expect it to allocate 2004 bytes 
(or more) to copy the enlarged array in to.

I would not expect that it could have allocated 4 bytes to add an 
item separably from the original 2000. Is there some way that the 
runtime can grown the original allocation by 4 bytes and that's 
what I'm seeing? If so, is there a limit to how much it can do 
this?

Can anyone help me understand what is going on here?

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

Although I responded below, I'm curious on what others think about this 
question.

On 09/08/2017 02:13 AM, John Burton wrote:
 I wrote this simple program to test out my understanding of memory
 allocation :-

I changed it to display the location and capacity:

import std.stdio;

void info(A)(A array) {
     writefln("ptr: %s, capacity: %s, length: %s",
              array.ptr, array.capacity, array.length);
}

void main()
{
     int [] array = new int[250];

     info(array);

     // Append one value to the array
     array ~= 123;
     info(array);
}

Sample output:

ptr: 7FD9B63CD000, capacity: 255, length: 250
ptr: 7FD9B63CD000, capacity: 255, length: 251

 I compiled it with 'dmd test.d -profile=gc'

 After running it, the profile report was :-

     bytes allocated, allocations, type, function, file:line
                2000               1 int[] D main test.d:5
                   4               1 int[] D main test.d:10

Here's my output with v2.076.0-dirty (that really is the version :o)):

            4000	              1	int[] D main deneme.d:412
              72	              1	std.format.FormatException 
std.exception.enforceEx!(FormatException).enforceEx!bool.enforceEx 
/usr/include/dmd/phobos/std/exception.d:615
              64	              2	std.array.Appender!string.Appender.Data 
std.array.Appender!string.Appender.this 
/usr/include/dmd/phobos/std/array.d:2817
              32	              1 
std.array.Appender!(char[]).Appender.Data 
std.array.Appender!(char[]).Appender.this 
/usr/include/dmd/phobos/std/array.d:2817
               4	              1	int[] D main deneme.d:417

And why is mine 4000 bytes? I guess it's the size of the underlying buffer.

 This is not how I expected it to be. I would have expected that the
 runtime either did not have to allocate at all at line 10 to add a new
 element because there was already space or it would have to allocate
 space for the new enlarged array and copy the array to it, in which case
 I'd expect it to allocate 2004 bytes (or more) to copy the enlarged
 array in to.

 I would not expect that it could have allocated 4 bytes to add an item
 separably from the original 2000. Is there some way that the runtime can
 grown the original allocation by 4 bytes and that's what I'm seeing? If
 so, is there a limit to how much it can do this?

 Can anyone help me understand what is going on here?

I think you're right. Perhaps more accurate information is not possible 
with the current implementation. Perhaps a higher-layer function counts 
"potential" allocations without being sure whether the space could be 
used for this slice or not. The "D Slices" article has related information:

   https://dlang.org/d-array-article.html

Ali