[musl] Re: Potentially incorrect musl scalbn results on AArch64?

[Szabolcs Nagy <nsz@port70.net>]

* Alexander Richardson <Alexander.Richardson@cl.cam.ac.uk> [2021-02-25 10:27:11 +0000]:
> Hello,
> 
> I've recently been tracking down testsuite failures on FreeBSD aarch64
> and as part of this updated the FreeBSD scalbn* implementations to use
> the musl versions. However, two of the scalbn tests are failing on
> non-x86 architectures (https://godbolt.org/z/rax7f6)
> For example, scalbn(1, -1023) returns
> "1.1125369292536006915451e-308"/0x0.8p-1022 on x86, but if I run the
> tests on aarch64 I get 0 instead.

i added musl list on cc

i cannot reproduce your issue (i.e. the c code works for me on
all targets as is)

one issue can be that if freebsd incorrectly sets the fpu on
aarch64 into flush-subnormals-to-zero mode.

or a clang compiler bug (which we have seen before wrt floating
point optimizations, although not wrong results, only wrong fenv)

> I'm not particularly familiar with floating-point calculations, but it
> appears to me that this could be caused by x86's extended precision
> during calculations?
> If I cast the result to (long double) on aarch64 prior to the
> multiplication, I get the expected result on AArch64 (but that's
> obviously slow and won't work on architectures where long double ==
> double).
> I've attached the current workaround, but I'm sure there is a better
> solution to this. Or possibly the test is incorrect and 0 is a
> perfectly valid result?
> 
> Kind regards,
> Alex
> 
> 
> diff --git a/lib/msun/src/s_scalbn.c b/lib/msun/src/s_scalbn.c
> index 219cd8f0c989..0d344840862f 100644
> --- a/lib/msun/src/s_scalbn.c
> +++ b/lib/msun/src/s_scalbn.c
> @@ -29,6 +29,19 @@ double scalbn(double x, int n)
>         }
>         u.i = (uint64_t)(0x3ff+n)<<52;
>         x = y * u.f;
> +#if !defined(__amd64__) && !defined(__i386__)
> +       /*
> +        * x86 performs the multiplication with higher precision, but on
> +        * non-x86 architectures we might get 0 instead of a tiny value. To work
> +        * around this problem perform the multiplication with float128 (slow).
> +        * TODO: This doesn't work on e.g. MIPS where long double == double.
> +        */
> +       if (x == 0.) {
> +               x = (long double)y * u.f;
> +               /* fprintf(stderr, "\ttrying again: %a/%a\n", x,
> (double)((long double)y * u.f)); */
> +               return x;
> +       }
> +#endif
>         return x;
>  }