On Tue, Oct 10, 2017 at 12:08:06PM +0000, Visa Hankala wrote:
> On Mon, Oct 09, 2017 at 10:39:47PM +0200, Juan Francisco Cantero Hurtado
> wrote:
> > Marc Feeley (Gambit Scheme) has been helping me with a bug on Gambit on
> > Loongson. Apparently the bug is on our side.
> >
> > I've created this little test based on his code:
> >
> > #include <stdio.h>
> > #include <math.h>
> >
> > int main()
> > {
> > double x = 0.5;
> > double y = 1074.0;
> > printf("x=%.20g y=%.20g pow(x,y)=%.20g\n",x,y,pow(x,y));
> > return 0;
> > }
> >
> > On OpenBSD/amd64, Linux/amd64, Linux/aarch64 and Linux/mips64:
> > x=0.5 y=1074 pow(x,y)=4.9406564584124654418e-324
> >
> > On OpenBSD/loongson:
> > x=0.5 y=1074 pow(x,y)=-4.9406564584124654418e-324
> >
> > Is the negative result expected?
>
> No. On mips64, ldexp(3) seems to use a garbage value when figuring out
> the sign of a denormal value. This applies to infinity values as well.
>
> The patch below should fix the bug. The t3 register contains the binary
> representation of the floating-point `x' parameter. I can also make
> a regression test for the patch.
>
> OK?
Works for me. Thanks for the quick fix.
>
> Index: arch/mips64/gen/ldexp.S
> ===================================================================
> RCS file: src/lib/libc/arch/mips64/gen/ldexp.S,v
> retrieving revision 1.7
> diff -u -p -r1.7 ldexp.S
> --- arch/mips64/gen/ldexp.S 27 Oct 2015 05:54:49 -0000 1.7
> +++ arch/mips64/gen/ldexp.S 10 Oct 2017 11:52:56 -0000
> @@ -143,20 +143,20 @@ LEAF(ldexp, 0)
> xori t2, 1
> 1:
> dmtc1 t2, $f0 # save denormalized result (LSW)
> - bge v1, zero, 1f # should result be negative?
> + bge t3, zero, 1f # should result be negative?
> neg.d $f0, $f0 # negate result
> 1:
> j ra
> 7:
> dmtc1 zero, $f0 # result is zero
> - beq t0, zero, 1f # is result positive?
> + bge t3, zero, 1f # is result positive?
> neg.d $f0, $f0 # negate result
> 1:
> j ra
> 8:
> dli t1, 0x7ff0000000000000 # result is infinity (MSW)
> dmtc1 t1, $f0
> - bge v1, zero, 1f # should result be negative infinity?
> + bge t3, zero, 1f # should result be negative infinity?
> neg.d $f0, $f0 # result is negative infinity
> 1:
> add.d $f0, $f0 # cause overflow faults if enabled
>
--
Juan Francisco Cantero Hurtado http://juanfra.info
