Re: [Qemu-devel] [RFC PATCH] fpu: add compile time check for old glibc/libm and fma

[Laurent Desnogues]

Hi,

On Thu, Dec 20, 2018 at 12:10 PM Alex Bennée <address@hidden> wrote:
>
> Some versions of glibc have been reported to have problems with
> fused-multiply-accumulate operations. If the underlying fma
> implementation does a two step operation it will instroduce subtle
> rounding errors. Newer versions of the library seem to deal with this
> better and modern hardware has fused operations which the library can
> use.

Thanks for taking care of this issue.  The fix was tested on our
machines and it works as expected.

So:

Tested-by: Laurent Desnogues <address@hidden>

> Reported-by: Laurent Desnogues <address@hidden>
> Signed-off-by: Alex Bennée <address@hidden>
> Cc: Emilio G. Cota <address@hidden>
> ---
>  fpu/softfloat.c | 25 +++++++++++++++++++++++++
>  1 file changed, 25 insertions(+)
>
> diff --git a/fpu/softfloat.c b/fpu/softfloat.c
> index 59eac97d10..9c2dbd04b5 100644
> --- a/fpu/softfloat.c
> +++ b/fpu/softfloat.c
> @@ -203,6 +203,25 @@ GEN_INPUT_FLUSH3(float64_input_flush3, float64)
>  # define QEMU_HARDFLOAT_3F64_USE_FP 0
>  #endif
>
> +/*
> + * Choose whether to accelerate fused multiply-accumulate operations
> + * with hard float functions. Some versions of glibc's maths library
> + * have been reported to be broken on x86 without FMA instructions.
> + */
> +#if defined(__x86_64__)
> +/* this was actually reported as glibc-2.12-1.149.el6_6.5.x86_64 was
> + * broken but glibc 2.12-1.209 works but out of caution lets disable
> + * for all older glibcs.
> + */
> +#if defined(__GLIBC__) && (__GLIBC__ == 2 && __GLIBC_MINOR__ <= 12)
> +#define QEMU_HARDFLOAT_USE_FMA 0
> +#else
> +#define QEMU_HARDFLOAT_USE_FMA 1
> +#endif
> +#else
> +#define QEMU_HARDFLOAT_USE_FMA 1
> +#endif
> +
>  /*
>   * QEMU_HARDFLOAT_USE_ISINF chooses whether to use isinf() over
>   * float{32,64}_is_infinity when !USE_FP.
> @@ -1551,6 +1570,9 @@ float32_muladd(float32 xa, float32 xb, float32 xc, int 
> flags, float_status *s)
>      ub.s = xb;
>      uc.s = xc;
>
> +    if (!QEMU_HARDFLOAT_USE_FMA) {
> +        goto soft;
> +    }
>      if (unlikely(!can_use_fpu(s))) {
>          goto soft;
>      }
> @@ -1612,6 +1634,9 @@ float64_muladd(float64 xa, float64 xb, float64 xc, int 
> flags, float_status *s)
>      ub.s = xb;
>      uc.s = xc;
>
> +    if (!QEMU_HARDFLOAT_USE_FMA) {
> +        goto soft;
> +    }
>      if (unlikely(!can_use_fpu(s))) {
>          goto soft;
>      }
> --
> 2.17.1
>