Re: Performance

[bearophile via Digitalmars-d]

This C++ code:

double plus(const unsigned int nSteps) {
    const double p0 = 0.0045;
    const double p1 = 1.00045452-p0;
    double tot = 1.346346;
    bool b = true;

    for (unsigned int i = 0; i < nSteps; i++) {
        switch (b) {
            case true:
                tot += p0;
                break;
            case false:
                tot += p1;
                break;
        }

        b = !b;
    }

    return tot;
}


G++ 4.8.0 gives the asm (using -Ofast, that implies unsafe FP 
optimizations):

__Z4plusj:
        movl    4(%esp), %ecx
        testl   %ecx, %ecx
        je      L7
        fldl    LC0
        xorl    %edx, %edx
        movl    $1, %eax
        fldl    LC2
        jmp     L6
        .p2align 4,,7
L11:
        fxch    %st(1)
        addl    $1, %edx
        xorl    $1, %eax
        cmpl    %ecx, %edx
        faddl   LC1
        je      L12
        fxch    %st(1)
L6:
        cmpb    $1, %al
        je      L11
        addl    $1, %edx
        xorl    $1, %eax
        cmpl    %ecx, %edx
        fadd    %st, %st(1)
        jne     L6
        fstp    %st(0)
        jmp     L10
        .p2align 4,,7
L12:
        fstp    %st(1)
L10:
        rep ret
L7:
        fldl    LC0
        ret

Bye,
bearophile