While solving some Project Euler problems with D and C, I have found about five fold performance difference. I have reduced the code to a small benchmark (that shows a smaller difference).

Probably the problem comes from %10 and /10 done on uint. DMD uses div, while GCC uses a known trick for small integer division/modulus. On the web there are pages that explain how to implement this small integer division optimization (example: http://libdivide.com/ ).

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Timings, n = 100_000_000:
  GCC:  3.13 s
  DMD: 11.69 s


dmd -O -release -inline testd.d
gcc -O3 -s testc.c -o testc.exe

32 bit
GCC 4.5.2
DMD 2.052beta

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// D2 code
import std.c.stdio: printf;

int main() {
    uint total = 0;
    uint i;
    for (i = 0; i < 100000000; i++) {
        uint n = i;
        uint res = n % 10;
        n /= 10;
        while (n != 0) {
            res = (n % 10) + (10 * res);
            n /= 10;
        }
        total += res;
    }
    printf("%u\n", total); // 461784529
    return 0;
}

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// C code
#include "stdio.h"
typedef unsigned long uint;

int main() {
    uint total = 0;
    uint i;
    for (i = 0; i < 100000000; i++) {
        uint n = i;
        uint res = n % 10;
        n /= 10;
        while (n != 0) {
            res = (n % 10) + (10 * res);
            n /= 10;
        }
        total += res;
    }
    printf("%u\n", total); // 461784529
    return 0;
}

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DMD inner loop:

L1C:	lea	EBX,[EBX*4][EBX]
		mov	EAX,ESI
		mov	ECX,0Ah
		xor	EDX,EDX
		add	EBX,EBX
		div	ECX
		add	EBX,EDX
		test EAX,EAX
		mov	ESI,EAX
		jne	L1C

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GCC inner loop:

L7:
	movl	%ecx, %eax
	mull	%esi
	leal	(%ebx,%ebx,4), %ebx
	shrl	$3, %edx
	leal	(%edx,%edx,4), %eax
	addl	%eax, %eax
	subl	%eax, %ecx
	testl	%edx, %edx
	leal	(%ecx,%ebx,2), %ebx
	movl	%edx, %ecx
	jne	L7