Suboptimal array copy in druntime?

[via Digitalmars-d]

On Sunday, 16 April 2017 at 11:58:11 UTC, Guillaume Chatelet 
wrote:
> On Sunday, 16 April 2017 at 11:25:15 UTC, Nicholas Wilson wrote:
>> On Sunday, 16 April 2017 at 10:33:01 UTC, Stefan Koch wrote:
>>> On Sunday, 16 April 2017 at 10:08:22 UTC, Guillaume Chatelet 
>>> wrote:
>>>> I was looking at the _d_arrayassign family functions in 
>>>> druntime:
>>>> https://github.com/dlang/druntime/blob/master/src/rt/arrayassign.d#L47
>>>> https://github.com/dlang/druntime/blob/master/src/rt/arrayassign.d#L139
>>>>
>>>> [...]
>>>
>>> Nope.
>>> Those are valid points.
>>>
>>> Templatizing the code is the way to go.
>>
>> Indeed. See also http://dconf.org/2017/talks/cojocaru.html
>
> Sweet! Glad to see this is being worked on :)

Specifically, see these pull requests as an example how the rest 
of druntime can be turned into templates:

https://github.com/dlang/dmd/pull/6597
https://github.com/dlang/druntime/pull/1781
https://github.com/dlang/dmd/pull/6634
https://github.com/dlang/druntime/pull/1792

I tested the array comparison lowering a couple of weeks ago and 
the results looked promising with regards to reducing link-time 
dependencies:

main.cpp:

#include <array>
#include <cstdio>

int compareArrays(const int *p1, size_t len1, const int *p2, 
size_t len2);

int main()
{
     std::array<int, 3> arr1 = {1, 2, 3};
     std::array<int, 3> arr2 = {1, 2, 4};

     int res = compareArrays(arr1.begin(), arr1.size(), 
arr2.begin(), arr2.size());

     printf("%d\n", res);
}
compare.d:

extern(C++) pure nothrow @nogc
int compareArrays(scope const(int)* p1, size_t len1, scope 
const(int)* p2, size_t len2)
{
     return p1[0 .. len1] < p2[0 .. len2];
}

extern(C) void _d_dso_registry() {}
$ ~/dlang/install.sh install dmd-nightly
Downloading and unpacking 
http://nightlies.dlang.org/dmd-master-2017-03-28/dmd.master.linux.tar.xz
######################################################################## 100.0%
dub-1.2.1 already installed

Run `source ~/dlang/dmd-master-2017-03-28/activate` in your shell 
to use dmd-master-2017-03-28.
This will setup PATH, LIBRARY_PATH, LD_LIBRARY_PATH, DMD, DC, and 
PS1.
Run `deactivate` later on to restore your environment.

$ source ~/dlang/dmd-master-2017-03-28/activate

$ g++ -std=c++11 -c main.cpp && \
   dmd -O -betterC -c compare.d && \
   g++ main.o compare.o -o d_array_compare

$ ./d_array_compare
1

$ nm compare.o
0000000000000000 t
0000000000000000 W _D6object12__T5__cmpTiZ5__cmpFNaNbNiNexAixAiZi
0000000000000000 T _d_dso_registry
                  U _d_dso_registry
                  U _GLOBAL_OFFSET_TABLE_
                  U __start_minfo
                  U __stop_minfo
0000000000000000 T _Z13compareArraysPKimS0_m

So choose your favorite under-performing runtime hook 
(https://wiki.dlang.org/Runtime_Hooks; 
https://github.com/dlang/dmd/blob/v2.074.0/src/ddmd/backend/rtlsym.h#L42 - definitive list) and turn it into a template :P