[PATCH] amdgcn, libm: fix infinite loop

[Jeff Johnston]

Patch applied.  Thanks.

-- Jeff J.

On Fri, Aug 8, 2025 at 4:15 AM Thomas Schwinge <tschwinge@baylibre.com>
wrote:

> Hi Andrew!
>
> On 2025-08-07T13:09:41+0000, Andrew Stubbs <ams@baylibre.com> wrote:
> > The end condition on this loop, unlike all the other similar loops, is
> > "i >= 0", which is a problem because "i <<= 1" can go negative and then
> zero if
> > you continue shifting, and so back to true, again.  This isn't a problem
> for
> > the loop in the scalar implementation, but it means we need to mask the
> shift
> > in the vector implementation.
> >
> > This fixes GCC PR#121392.
>
> Thanks, I confirm this fixes the issue.
>
> I suggest you 'git push', or I can do it for you if you don't have access
> to newlib Git -- or, I'll be happy to sponsor you to get access, as
> maintainer of the GCN parts in newlib.
> (<https://sourceware.org/cgi-bin/pdw/ps_form.cgi> has the instructions
> for the latter.)
>
>
> Grüße
>  Thomas
>
>
> >  newlib/libm/machine/amdgcn/v64sf_fmod.c | 7 +++++--
> >  1 file changed, 5 insertions(+), 2 deletions(-)
> >
> > diff --git a/newlib/libm/machine/amdgcn/v64sf_fmod.c
> b/newlib/libm/machine/amdgcn/v64sf_fmod.c
> > index 7302420ad..b62b81929 100644
> > --- a/newlib/libm/machine/amdgcn/v64sf_fmod.c
> > +++ b/newlib/libm/machine/amdgcn/v64sf_fmod.c
> > @@ -70,8 +70,11 @@ DEF_VS_MATH_FUNC (v64sf, fmodf, v64sf x, v64sf y)
> >    v64si iy;
> >    VECTOR_IF (hy < 0x00800000, cond)  // subnormal y
> >      iy = VECTOR_INIT (-126);
> > -    for (v64si i = (hy << 8); !ALL_ZEROES_P (cond & (i >= 0)); i <<= 1)
> > -      VECTOR_COND_MOVE (iy, iy - 1, cond & (i >= 0));
> > +    for (v64si i = (hy << 8); !ALL_ZEROES_P (cond & (i >= 0)); /* i <<=
> 1 */)
> > +      {
> > +     VECTOR_COND_MOVE (iy, iy - 1, cond & (i >= 0));
> > +     VECTOR_COND_MOVE (i, i << 1, cond & (i >= 0));
> > +      }
> >    VECTOR_ELSE (cond)
> >      VECTOR_COND_MOVE (iy, (hy >> 23) - 127, cond);
> >    VECTOR_ENDIF
> > --
> > 2.50.0
>
>
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