Re: FE Exception triggered by comparison

[Alexander Monakov <amonakov@ispras.ru>]

On Wed, 27 Feb 2019, Rich Felker wrote:

> Are there reasons we should perhaps use the __builtin versions of
> these when __GNUC__ indicates they're available? I like our bit test
> versions we have now, and I think they're sufficiently efficient, but
> I'm open to changes if there's a good reason.

Well, it really depends on what one considers 'sufficiently efficient'.
Instead of comparing a register with itself and testing flags (2 instructions)
you get (for 'int f(double x){return isnan(x);}'):

f:
        movabsq $9223372036854775807, %rdx
        movq    %xmm0, %rax
        andq    %rdx, %rax
        movabsq $9218868437227405312, %rdx
        cmpq    %rdx, %rax
        seta    %al
        movzbl  %al, %eax
        ret

(note that movq %xmm0, %rax is going to be more costly than a normal
move as it crosses from fp to integer domain in the cpu)

I think musl bit test can be implemented more efficiently via right-shifting
the representation in %rax first, avoiding 64-bit immediates, but even then
I'd say the "native" version is preferable.

Alexander