Re: [Qemu-devel] [PATCH] powerpc: use float64 for frsqrte

[Alexander Graf <agraf@suse.de>]

On 06/03/2014 11:14 AM, Tristan Gingold wrote:
> Remove the code that reduce the result to float32 as the frsqrte
> instruction is defined to return a double-precision estimate of
> the reciprocal square root.
>
> Although reducing the fractional part is harmless (as the estimation
> must have at least 12 bits of precision according to the old PEM),
> reducing the exponent range is not correct.
>
> Signed-off-by: Tristan Gingold <gingold@adacore.com>

I couldn't find a reference to doubles in ISA 2.07. Is frsqrte supposed 
to return doubles on all cores? Or is this implementation specific? 
Also, is frsqrte the only instruction affected?


Alex

> ---
>   target-ppc/fpu_helper.c |    3 ---
>   1 file changed, 3 deletions(-)
>
> diff --git a/target-ppc/fpu_helper.c b/target-ppc/fpu_helper.c
> index cd8f015..da93d12 100644
> --- a/target-ppc/fpu_helper.c
> +++ b/target-ppc/fpu_helper.c
> @@ -977,7 +977,6 @@ uint64_t helper_fres(CPUPPCState *env, uint64_t arg)
>   uint64_t helper_frsqrte(CPUPPCState *env, uint64_t arg)
>   {
>       CPU_DoubleU farg;
> -    float32 f32;
>   
>       farg.ll = arg;
>   
> @@ -991,8 +990,6 @@ uint64_t helper_frsqrte(CPUPPCState *env, uint64_t arg)
>           }
>           farg.d = float64_sqrt(farg.d, &env->fp_status);
>           farg.d = float64_div(float64_one, farg.d, &env->fp_status);
> -        f32 = float64_to_float32(farg.d, &env->fp_status);
> -        farg.d = float32_to_float64(f32, &env->fp_status);
>       }
>       return farg.ll;
>   }