memcpy in D

[Steven Schveighoffer]

On Sun, 30 Jun 2013 07:40:31 -0400, monarch_dodra <monarchdodra at gmail.com>  
wrote:

> On Sunday, 30 June 2013 at 11:07:24 UTC, Tyro[17] wrote:
>> What is the equivalent of memcpy
>>
>> module memcopy;
>>
>> immutable ADDRESS_BUS_SIZE = 20; // 2^20 address bus
>> byte memory[1 << ADDRESS_BUS_SIZE];
>>
>> void main()
>> {
>> 	ushort val = 12345;
>>
>> 	for (int i = 0x12340; i < 0x1234A; i+= 2) {
>> 		memcpy (&memory[i], &val, sizeof val); // D way???
>> 		val++;
>> 	}
>>
>> 	for (int i = 0x12340; i < 0x1234A; i+= 2) {
>> 		memcpy (&val, &memory[i], sizeof val); // D way???
>> 		writefln("%x", val);
>> 	}
>> }
>>
>> achieved in D? I am trying not to use memcpy or any function from the C  
>> API.
>>
>> Thanks,
>
> You could do it with ubyte a vector copy:
>
> --------
> void * dmemcpy ( void * destination, const void * source, size_t num )  
> pure nothrow
> {
>      (cast(ubyte*)destination)[0 .. num][]=(cast(const(ubyte)*)source)[0  
> .. num];
>      return destination;
> }
> --------
>
> Doing it this way has the advantage of being CTFE-able, and  
> (potentially) faster, as everything I ever read about D's memcpy is that  
> it is slow.

D's memcpy is C's memcpy.  So I don't know why it would be slower.  Note  
that with DMD on windows 32 bit, it uses DMC, which may vary in  
performance from MSVC memcpy.

Using vector assignment may or may not use memcpy.  It may be the slower  
one, but I don't know.  If it can be inlined, it certainly would be faster  
for small values.

-Steve