Re: On(c)e more: optimizer failure

[Stefan Kanthak]

Manuel López-Ibáñez <manuel.lopez-iba...@uma.es> wrote:

> FWIW: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=24021

Thanks.
So this bug may soon have a driver's license in some countries...

One more for the road:

$ cat wtf.c
double wtf(double x) {
    return sqrt(x * x); // can the square ever be negative?
}
$ gcc -m64 -o- -O3 -S
...
        mulsd   %xmm0, %xmm0
        pxor    %xmm1, %xmm1
        ucomisd %xmm0, %xmm1
        ja      .L10
        sqrtsd  %xmm0, %xmm0
        ret
.L10:
        jmp     sqrt
...

Is the special case 0.0 worth the almost always taken, thus wrong
predicted branch?

JFTR: the PXOR incurs two cycles penalty on many Intel processors!
      Better use XORPD there.

Stefan