• bearophile <bearophileHUGS lycos.com> May 27 2009
• Frits van Bommel <fvbommel REMwOVExCAPSs.nl> May 27 2009

bearophile <bearophileHUGS lycos.com> writes:

While this code:

    typedef int Int2 = 2;
    auto a = cast(int[])(new Int2[1000]);
    
Produces:

	pushl	%esi
	subl	$8, %esp
	movl	$1000, 4(%esp)
	movl	$_D20TypeInfo_ATmain4Int26__initZ, (%esp)
	xorl	%esi, %esi
	call	_d_newarrayiT
	movl	%esi, %ecx
	
(I haven't taken the running time, so I don't know if this idiom is more
efficient with LDC).

Bye,
bearophile

Frits van Bommel <fvbommel REMwOVExCAPSs.nl> writes:

bearophile wrote:
 While this code:
 
     typedef int Int2 = 2;
     auto a = cast(int[])(new Int2[1000]);
     
 Produces:
 
 	pushl	%esi
 	subl	$8, %esp
 	movl	$1000, 4(%esp)
 	movl	$_D20TypeInfo_ATmain4Int26__initZ, (%esp)
 	xorl	%esi, %esi
 	call	_d_newarrayiT
 	movl	%esi, %ecx
 	
 (I haven't taken the running time, so I don't know if this idiom is more
efficient with LDC).


For arrays that escape, it's quite probably more efficient since it only gets 
initialized once this way.
For arrays that can get stack-promoted it may not be, because (non 
zero-)initialized arrays aren't eligible for that yet (as you can tell by the 
_d_newarray* call, which GC-allocates a new array and initializes it). So which 
is more efficient in that case depends on whether a GC allocation is more 
expensive than re-initializing 4 KB of data.