Array reverse

[bearophile]

I've seen a difference in the performance of 
std.algorithm.reverse applied on an array compared to home-made 
code, so I have written a little test.

Below you see the code, and the asm of the functions, compiled 
with DMD 2.060, 32 bit (-O -release -inline).

Feel free to recompile the code yourself to see if I have done 
some mistake :-)

------------------------------

import core.stdc.stdio: printf;
import std.algorithm: reverse;

void reverseArr(T)(T[] arr) {
     for (auto x = arr.ptr, y = &arr[$ - 1]; x < y; ) {
         auto aux = *x;
         *x++ = *y;
         *y-- = aux;
     }
}

void main() {
     auto a = [10, 20, 30, 40];
     foreach(x; a) printf("%d ", x); printf("\n");
     a.reverseArr();
     foreach(x; a) printf("%d ", x); printf("\n");
     a.reverse();
     foreach(x; a) printf("%d ", x); printf("\n");
}

------------------------------

reverseArr:
		push	EBX
		mov	ECX,0Ch[ESP]
		mov	EBX,0Ch[ESP]
		push	ESI
		mov	ESI,0Ch[ESP]
		lea	ESI,-4[ESI*4][ECX]
		cmp	ECX,ESI
		jae	L2B
L19:	mov	EAX,[EBX]
		mov	EDX,[ESI]
		mov	[EBX],EDX
		add	EBX,4
		mov	[ESI],EAX
		add	ESI,0FFFFFFFCh
		cmp	EBX,ESI
		jb	L19
L2B:	pop	ESI
		pop	EBX
		ret	8

-------------------

std.algorithm.reverse:
L0:		sub	ESP,020h
		push	EBX
		push	ESI
		push	EDI
		cmp	dword ptr 030h[ESP],0
		je	LE1
L11:	cmp	dword ptr 030h[ESP],0
		mov	EAX,030h[ESP]
		mov	EDX,034h[ESP]
		mov	018h[ESP],EAX
		mov	01Ch[ESP],EDX
		jne	L32
		mov	EAX,022Dh
		call	near ptr _D3std9algorithm7__arrayZ
L32:	mov	EAX,030h[ESP]
		mov	EDX,034h[ESP]
		mov	024h[ESP],EDX
		mov	ECX,030h[ESP]
		lea	EDX,-1[ECX]
		mov	020h[ESP],EAX
		cmp	EDX,ECX
		mov	010h[ESP],EDX
		jb	L5E
		mov	EAX,0258h
		call	near ptr _D3std9algorithm7__arrayZ
L5E:	mov	ECX,01Ch[ESP]
		mov	EBX,030h[ESP]
		mov	EDX,024h[ESP]
		lea	EBX,-4[EBX*4][EDX]
		mov	ESI,[ECX]
		mov	EDX,[EBX]
		mov	[ECX],EDX
		mov	ECX,030h[ESP]
		cmp	ECX,ECX
		mov	[EBX],ESI
		ja	L86
		cmp	ECX,1
		jae	L90
L86:	mov	EAX,0173h
		call	near ptr _D3std9algorithm7__arrayZ
L90:	mov	EDX,034h[ESP]
		mov	EDI,010h[ESP]
		mov	030h[ESP],EDI
		add	EDX,4
		mov	034h[ESP],EDX
		cmp	dword ptr 030h[ESP],0
		je	LE1
		mov	EBX,030h[ESP]
		lea	ECX,-1[EBX]
		cmp	ECX,EBX
		mov	014h[ESP],ECX
		jbe	LC6
		mov	EAX,01E8h
		call	near ptr _D3std9algorithm7__arrayZ
LC6:	mov	ESI,014h[ESP]
		mov	EDX,034h[ESP]
		mov	030h[ESP],ESI
		mov	034h[ESP],EDX
		cmp	dword ptr 030h[ESP],0
		jne	L11
LE1:	pop	EDI
		pop	ESI
		pop	EBX
		add	ESP,020h
		ret	8

------------------------------

Bye,
bearophile