Re: Performance

[bearophile via Digitalmars-d]

double plus(in uint nSteps) pure nothrow @safe /*@nogc*/ {
    enum double p0 = 0.0045;
    enum double p1 = 1.00045452-p0;
    double tot = 1.346346;
    auto b = true;

    foreach (immutable i; 0 .. nSteps) {
        final switch (b) {
            case true:
                tot += p0;
                break;
            case false:
                tot += p1;
                break;
        }

        b = !b;
    }

    return tot;
}

And this is the 32 bit X86 asm generated by ldc2 for the plus 
function:

__D4test4plusFNaNbNfxkZd:
        pushl   %ebp
        movl    %esp, %ebp
        pushl   %esi
        andl    $-8, %esp
        subl    $24, %esp
        movsd   LCPI0_0, %xmm0
        testl   %eax, %eax
        je      LBB0_8
        xorl    %ecx, %ecx
        movb    $1, %dl
        movsd   LCPI0_1, %xmm1
        movsd   LCPI0_2, %xmm2
        .align  16, 0x90
LBB0_2:
        testb   $1, %dl
        jne     LBB0_3
        addsd   %xmm1, %xmm0
        jmp     LBB0_7
        .align  16, 0x90
LBB0_3:
        movzbl  %dl, %esi
        andl    $1, %esi
        je      LBB0_5
        addsd   %xmm2, %xmm0
LBB0_7:
        xorb    $1, %dl
        incl    %ecx
        cmpl    %eax, %ecx
        jb      LBB0_2
LBB0_8:
        movsd   %xmm0, 8(%esp)
        fldl    8(%esp)
        leal    -4(%ebp), %esp
        popl    %esi
        popl    %ebp
        ret
LBB0_5:
        movl    $11, 4(%esp)
        movl    $__D4test12__ModuleInfoZ, (%esp)
        calll   __d_switch_error

Bye,
bearophile