dmd simple loop disassembly - redundant instruction?

[bearophile]

Ivan Kazmenko:

> I am studying the difference between x86 generated code of DMD 
> and C/C++ compilers on Windows (simply put: why exactly, and by 
> what margin, DMD-compiled D code is often slower than 
> GCC-compiled C/C++ equivalent).
>
> Now, I have this simple D program:
>
> -----
> immutable int MAX_N = 1_000_000;
> void main () {
>     int [MAX_N] a;
>     foreach (i; 0..MAX_N)
>         a[i] = i;
> }
> -----
>
> (I know there's iota in std.range, and it turns out to be even 
> slower - but that's a high level function, and I'm trying to 
> understand the lower-level details now.)
>
> The assembly (dmd -O -release -inline -noboundscheck, then 
> obj2asm) has the following piece corresponding to the cycle:
>
> -----
> L2C:		mov	-03D0900h[EDX*4][EBP],EDX
> 		mov	ECX,EDX
> 		inc	EDX
> 		cmp	EDX,0F4240h
> 		jb	L2C
> -----

ldc2 optimizes the useless loop away:


__Dmain:
	xorl	%eax, %eax
	ret


If I modify the code returning some value from the int main:
     return a[7];


ldc2 gives the loop code:

LBB0_1:
	movl	%eax, 12(%esp,%eax,4)
	incl	%eax
	cmpl	$1000000, %eax
	jne	LBB0_1


If I use iota ldc2 copiles the loop to exactly the same asm:

foreach (i; MAX_N.iota)

Bye,
bearophile