Re: Superfluous branches due to insufficient flow analysis

["Stefan Kanthak" <stefan.kanthak@nexgo.de>]

"Gabriel Ravier" <gabravier@gmail.com> wrote:

Please don't FULL QUOTE!

> On 8/13/21 8:58 PM, Stefan Kanthak wrote:
>> Hi,
>>
>> compile the following naive implementation of nextafter() for AMD64:
>>
>> JFTR: ignore the aliasing casts, they don't matter here!
>>
>> $ cat repro.c

[...]

> Shouldn't this kind of stuff go to the Bugzilla ?

I don't mind if you or someone else take it there.
If you do so, here's one for the road:

$ cat alternate.c
double nextafter(double from, double to)
{
    if (from == to)
        return to;

    if ((from != from) || (to != to))
        return from + to;

    if (from == 0.0)
        return to < 0.0 ? -0x1.0p-1074 : 0x1.0p-1074;

    unsigned long long ull = *(unsigned long long *) &from;

    if ((from > to) == (from > 0.0))
        ull--;
    else
        ull++;

    return *(double *) &ull;
}
$ gcc -m64 -o- -O3 -S alternate.c
nextafter:
        ucomisd %xmm0, %xmm1 # sets PF if comparands are unordered,
                             #  i.e. at least one is a NaN
        jnp     .L20         # OUCH: this branch is unlikely taken!
.L13:                        # reached only if at least one
                             #  argument is a NaN!
        ucomisd %xmm0, %xmm0 # OUCH: SUPERFLUOUS!
        jnp     .L21         # OUCH: SUPERFLUOUS!
.L14:
        addsd   %xmm1, %xmm0
        ret
.L21:                        # reached only if at least one
                             #  argument is a NaN!
        ucomisd %xmm1, %xmm1 # OUCH: SUPERFLUOUS!
        jp      .L14         # OUCH: SUPERFLUOUS!
        pxor    %xmm2, %xmm2
        ucomisd %xmm2, %xmm0
        jnp     .L22         # OUCH: SUPERFLUOUS!
                             #       xmm0 can't be NaN here!
                             # OUCH: and if it were, this branch
                             #        would unlikely be taken!
.L8:
        comisd  %xmm0, %xmm1
        movq    %xmm0, %rdx
        leaq    -1(%rdx), %rax
        seta    %r8b
        comisd  %xmm0, %xmm2
        seta    %cl
        addq    $1, %rdx
        cmpb    %cl, %r8b
        cmovne  %rdx, %rax
        movq    %rax, %xmm0
        ret
.L20:
        jne     .L13
        movapd  %xmm1, %xmm0
        ret
.L22:
        jne     .L8
        movabsq $-9223372036854775808, %rdx
        movq    %xmm1, %rax
        andq    %rdx, %rax
        orq     $1, %rax
        movq    %rax, %xmm0
        ret

Stefan