Re: Do array literals still always allocate?
On 5/14/17 7:40 AM, Nicholas Wilson wrote:
On Sunday, 14 May 2017 at 10:18:40 UTC, ag0aep6g wrote:
On 05/14/2017 01:57 AM, Nicholas Wilson wrote:
1D arrays it doesn't, 2D or higher it does.
What do you mean? This works just fine as well:
import std.random;
import std.stdio;
int[2][2] testfunc(int num) @nogc
{
return [[0, 1], [num, 3]];
}
int main()
{
int[2][2] arr = testfunc(uniform(0, 15));
writeln(arr);
return 0;
}
dynamic array literals is what I meant.
dynamic array literals allocate most of the time. The one time they
don't is the specialized case of initializing a static array, I think of
any dimension.
However, to the OP's point, they *did* allocate up until a certain
version, even when initializing a static array.
That is:
int[2] x = [1, 2];
used to allocate a dynamic array on the heap, and then copy the data
into x (and leave the allocated data for the GC). Now it just populates
x without allocating.
LDC might be smarter, and take more valid shortcuts, so that is good.
But in general you can expect array literals to allocate when not used
as static array initializers.
-Steve
Re: Do array literals still always allocate?
On Sunday, 14 May 2017 at 12:02:03 UTC, Eugene Wissner wrote:
On Sunday, 14 May 2017 at 11:45:12 UTC, ag0aep6g wrote:
On 05/14/2017 01:40 PM, Nicholas Wilson wrote:
dynamic array literals is what I meant.
I don't follow. Can you give an example in code?
void main()
{
ubyte[] arr = [ 1, 2, 3, 4, 5 ];
assert(arr == [ 1, 2, 3, 4, 5 ]);
}
Both, assignment and comparison, allocate.
LDC is able to promote those to alloca, not sure about DMD or its
interaction with @nogc.
see
https://github.com/ldc-developers/ldc/blob/master/gen/passes/GarbageCollect2Stack.cpp#L832 for the details.
Re: Do array literals still always allocate?
On 05/14/2017 02:02 PM, Eugene Wissner wrote:
void main()
{
ubyte[] arr = [ 1, 2, 3, 4, 5 ];
assert(arr == [ 1, 2, 3, 4, 5 ]);
}
Both, assignment and comparison, allocate.
Sure, but Nicholas said 1D arrays behave different from 2D arrays.
Re: Do array literals still always allocate?
On Sunday, 14 May 2017 at 11:45:12 UTC, ag0aep6g wrote:
On 05/14/2017 01:40 PM, Nicholas Wilson wrote:
dynamic array literals is what I meant.
I don't follow. Can you give an example in code?
void main()
{
ubyte[] arr = [ 1, 2, 3, 4, 5 ];
assert(arr == [ 1, 2, 3, 4, 5 ]);
}
Both, assignment and comparison, allocate.
Re: Do array literals still always allocate?
On 05/14/2017 01:40 PM, Nicholas Wilson wrote: dynamic array literals is what I meant. I don't follow. Can you give an example in code?
Re: Do array literals still always allocate?
On Sunday, 14 May 2017 at 10:18:40 UTC, ag0aep6g wrote:
On 05/14/2017 01:57 AM, Nicholas Wilson wrote:
1D arrays it doesn't, 2D or higher it does.
What do you mean? This works just fine as well:
import std.random;
import std.stdio;
int[2][2] testfunc(int num) @nogc
{
return [[0, 1], [num, 3]];
}
int main()
{
int[2][2] arr = testfunc(uniform(0, 15));
writeln(arr);
return 0;
}
dynamic array literals is what I meant.
Re: Do array literals still always allocate?
On 05/14/2017 01:57 AM, Nicholas Wilson wrote:
1D arrays it doesn't, 2D or higher it does.
What do you mean? This works just fine as well:
import std.random;
import std.stdio;
int[2][2] testfunc(int num) @nogc
{
return [[0, 1], [num, 3]];
}
int main()
{
int[2][2] arr = testfunc(uniform(0, 15));
writeln(arr);
return 0;
}
Re: Do array literals still always allocate?
On Sunday, 14 May 2017 at 01:15:03 UTC, Lewis wrote: On Saturday, 13 May 2017 at 19:22:09 UTC, Steven Schveighoffer wrote: It's just out of date. Can't remember the version, but this did use to allocate. It doesn't any more. But only for this case. In most cases it does allocate. Okay cool, that's good to hear. For reference the most recent place I remember seeing this was http://stackoverflow.com/questions/6751575/how-to-initialise-static-arrays-in-d-without-a-gc-allocation (although I've definitely seen others in the past). I'll add an answer to the SO question to clarify that this is no longer an issue. Thanks all! It may allocate if it is returned. I think it elided for one caller because it constructs into the callers stack frame but only if the length of all return path are the same length. For 2D or higher it always allocates, i think.
Re: Do array literals still always allocate?
On Sunday, 14 May 2017 at 01:15:03 UTC, Lewis wrote:
On Saturday, 13 May 2017 at 19:22:09 UTC, Steven Schveighoffer
wrote:
It's just out of date. Can't remember the version, but this
did use to allocate. It doesn't any more. But only for this
case. In most cases it does allocate.
Okay cool, that's good to hear. For reference the most recent
place I remember seeing this was
http://stackoverflow.com/questions/6751575/how-to-initialise-static-arrays-in-d-without-a-gc-allocation (although I've definitely seen others in the past). I'll add an answer to the SO question to clarify that this is no longer an issue.
Thanks all!
You could also use something like this:
auto s(T, size_t n)(T[n] arr)
{
return arr;
}
auto is = [1, 2, 3].s; // stack allocated
I use it whenever I work with D.
Re: Do array literals still always allocate?
On Saturday, 13 May 2017 at 19:22:09 UTC, Steven Schveighoffer wrote: It's just out of date. Can't remember the version, but this did use to allocate. It doesn't any more. But only for this case. In most cases it does allocate. Okay cool, that's good to hear. For reference the most recent place I remember seeing this was http://stackoverflow.com/questions/6751575/how-to-initialise-static-arrays-in-d-without-a-gc-allocation (although I've definitely seen others in the past). I'll add an answer to the SO question to clarify that this is no longer an issue. Thanks all!
Re: Do array literals still always allocate?
On Saturday, 13 May 2017 at 18:32:16 UTC, Lewis wrote:
import std.random;
import std.stdio;
int[4] testfunc(int num) @nogc
{
return [0, 1, num, 3];
}
int main()
{
int[4] arr = testfunc(uniform(0, 15));
writeln(arr);
return 0;
}
I've read a bunch of stuff that seems to indicate that array
literals are always heap-allocated, even when being used to
populate a static array. However, testfunc() above compiles as
@nogc. This would indicate to me that D does the smart thing
and avoids a heap allocation for an array literal being used to
populate a static array. Is all the old stuff I was reading
just out-of-date now?
1D arrays it doesn't, 2D or higher it does.
Re: Do array literals still always allocate?
On Saturday, 13 May 2017 at 18:32:16 UTC, Lewis wrote:
import std.random;
import std.stdio;
int[4] testfunc(int num) @nogc
{
return [0, 1, num, 3];
}
int main()
{
int[4] arr = testfunc(uniform(0, 15));
writeln(arr);
return 0;
}
I've read a bunch of stuff that seems to indicate that array
literals are always heap-allocated, even when being used to
populate a static array. However, testfunc() above compiles as
@nogc. This would indicate to me that D does the smart thing
and avoids a heap allocation for an array literal being used to
populate a static array. Is all the old stuff I was reading
just out-of-date now?
It's just out of date. Can't remember the version, but this did
use to allocate. It doesn't any more. But only for this case. In
most cases it does allocate.
-Steve
Re: Do array literals still always allocate?
On Saturday, 13 May 2017 at 18:32:16 UTC, Lewis wrote:
import std.random;
import std.stdio;
int[4] testfunc(int num) @nogc
{
return [0, 1, num, 3];
}
int main()
{
int[4] arr = testfunc(uniform(0, 15));
writeln(arr);
return 0;
}
I've read a bunch of stuff that seems to indicate that array
literals are always heap-allocated, even when being used to
populate a static array.
On the contrary, when initializing static arrays, allocation is
not needed.
Note that what's *conceptually* happening in testfunc is this:
int[4] testfunc(int num) @nogc
{
typeof(return) result = [0, 1, num, 3];
return result;
}
i.e. the type and size of the storage is known beforehand, all
there is to do is copy the elements:
0044e528 <@nogc int[4] test.testfunc(int)>:
44e528: 55 push %rbp
44e529: 48 8b ecmov%rsp,%rbp
44e52c: 48 83 ec 10 sub$0x10,%rsp
44e530: 48 89 7d f8 mov%rdi,-0x8(%rbp)
44e534: c7 07 00 00 00 00 movl $0x0,(%rdi) #
0 goes at offset 0
44e53a: c7 47 04 01 00 00 00movl $0x1,0x4(%rdi) #
1 at offset 4
44e541: 89 77 08mov%esi,0x8(%rdi) #
parameter at offset 8
44e544: c7 47 0c 03 00 00 00movl $0x3,0xc(%rdi) #
3 at offset 12
44e54b: 48 8b 45 f8 mov-0x8(%rbp),%rax
44e54f: c9 leaveq
44e550: c3 retq
Is all the old stuff I was reading just out-of-date now?
Where exactly did you read that initialization of a static array
requires an allocation? That source should be abolished...
errrm... corrected!
