Re: [Qemu-devel] [PATCH v2] softfloat: enforce softfloat if the host's FMA is broken

[Philippe Mathieu-Daudé]

Hi Emilio,

On 12/25/18 8:03 AM, Emilio G. Cota wrote:
> The added branch to the FMA ops is marked as unlikely and therefore
> its impact on performance (measured with fp-bench) is within noise range
> when measured on an Intel(R) Xeon(R) Gold 6142 CPU @ 2.60GHz.

Did you forget to write here the result of your measurements?

> 
> Reported-by: Laurent Desnogues <address@hidden>

Suggested-by: Richard Henderson <address@hidden>

> Signed-off-by: Emilio G. Cota <address@hidden>

Reviewed-by: Philippe Mathieu-Daudé <address@hidden>

> ---
> v2: drop the cpuid bits, since that work (1) is only a performance
> optimization, not a correctness fix and (2) requires additional
> consolidation work that I do not have time to do right now.
> 
>  fpu/softfloat.c | 33 +++++++++++++++++++++++++++++++++
>  1 file changed, 33 insertions(+)
> 
> diff --git a/fpu/softfloat.c b/fpu/softfloat.c
> index 59eac97d10..9132d7a0b0 100644
> --- a/fpu/softfloat.c
> +++ b/fpu/softfloat.c
> @@ -1542,6 +1542,8 @@ soft_f64_muladd(float64 a, float64 b, float64 c, int 
> flags,
>      return float64_round_pack_canonical(pr, status);
>  }
>  
> +static bool force_soft_fma;
> +
>  float32 QEMU_FLATTEN
>  float32_muladd(float32 xa, float32 xb, float32 xc, int flags, float_status 
> *s)
>  {
> @@ -1562,6 +1564,11 @@ float32_muladd(float32 xa, float32 xb, float32 xc, int 
> flags, float_status *s)
>      if (unlikely(!f32_is_zon3(ua, ub, uc))) {
>          goto soft;
>      }
> +
> +    if (unlikely(force_soft_fma)) {
> +        goto soft;
> +    }
> +
>      /*
>       * When (a || b) == 0, there's no need to check for under/over flow,
>       * since we know the addend is (normal || 0) and the product is 0.
> @@ -1623,6 +1630,11 @@ float64_muladd(float64 xa, float64 xb, float64 xc, int 
> flags, float_status *s)
>      if (unlikely(!f64_is_zon3(ua, ub, uc))) {
>          goto soft;
>      }
> +
> +    if (unlikely(force_soft_fma)) {
> +        goto soft;
> +    }
> +
>      /*
>       * When (a || b) == 0, there's no need to check for under/over flow,
>       * since we know the addend is (normal || 0) and the product is 0.
> @@ -7974,3 +7986,24 @@ float128 float128_scalbn(float128 a, int n, 
> float_status *status)
>                                           , status);
>  
>  }
> +
> +static void __attribute__((constructor)) softfloat_init(void)
> +{
> +    union_float64 ua, ub, uc, ur;
> +
> +    if (QEMU_NO_HARDFLOAT) {
> +        return;
> +    }
> +    /*
> +     * Test that the host's FMA is not obviously broken. For example,
> +     * glibc < 2.23 can perform an incorrect FMA on certain hosts; see
> +     *   https://sourceware.org/bugzilla/show_bug.cgi?id=13304
> +     */
> +    ua.s = 0x0020000000000001ULL;
> +    ub.s = 0x3ca0000000000000ULL;
> +    uc.s = 0x0020000000000000ULL;
> +    ur.h = fma(ua.h, ub.h, uc.h);
> +    if (ur.s != 0x0020000000000001ULL) {
> +        force_soft_fma = true;
> +    }
> +}
>