Re: [PATCH] Avoid undefined behaviour in ibm128 implementation of llroundl

[Michael Hudson-Doyle <michael.hudson@canonical.com>]

Disregard this, I hadn't run all the tests I thought I had, sorry about
that.

On Sun, 21 Aug 2022 at 22:45, Michael Hudson-Doyle <
michael.hudson@canonical.com> wrote:

> Detecting an overflow edge case depended on signed overflow of a long
> long. Replace the signed long long with unsigned and cast it back to
> unsigned before comparisons (which is implementation defined behaviour,
> but I guess glibc does not support any one's complement
> architectures...).
>
> BZ #29488
> ---
>  sysdeps/ieee754/ldbl-128ibm/s_llroundl.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/sysdeps/ieee754/ldbl-128ibm/s_llroundl.c
> b/sysdeps/ieee754/ldbl-128ibm/s_llroundl.c
> index d85154e73a..e8117bfc2b 100644
> --- a/sysdeps/ieee754/ldbl-128ibm/s_llroundl.c
> +++ b/sysdeps/ieee754/ldbl-128ibm/s_llroundl.c
> @@ -28,7 +28,8 @@ long long
>  __llroundl (long double x)
>  {
>    double xh, xl;
> -  long long res, hi, lo;
> +  unsigned long long res;
> +  long long hi, lo;
>
>    ldbl_unpack (x, &xh, &xl);
>
> @@ -69,7 +70,7 @@ __llroundl (long double x)
>        res = hi + lo;
>
>        /* This is just sign(hi) == sign(lo) && sign(res) != sign(hi).  */
> -      if (__glibc_unlikely (((~(hi ^ lo) & (res ^ hi)) < 0)))
> +      if (__glibc_unlikely (((~(hi ^ lo) & (((long long)res) ^ hi)) < 0)))
>         goto overflow;
>
>        xh -= lo;
> @@ -95,7 +96,7 @@ __llroundl (long double x)
>             res -= 1;
>         }
>
> -      if (__glibc_unlikely (((~(hi ^ (res - hi)) & (res ^ hi)) < 0)))
> +      if (__glibc_unlikely (((~(hi ^ (((long long)res) - hi)) & (((long
> long)res) ^ hi)) < 0)))
>         goto overflow;
>
>        return res;
> --
> 2.34.1
>
>