Re: sparc64 fmulq emulation glitch?

[Rene Rebe <rene@exactcode.de>]

On Friday 28 July 2006 01:51, David Miller wrote:
> From: Rene Rebe <rene@exactcode.de>
> Date: Mon, 26 Jun 2006 09:02:12 +0200
> 
> > int main ()
> > {
> >         long double a = 2;
> >         long double b = 1.0;
> >         a *= b;
> >         printf ("%Lf\n", a);
> > }
> > 
> > built with: gcc -m64 -mhard-quad-float
> > 
> > yields: 4
> 
> Something is wrong with _FP_MUL_MEAT_2_wide_3mul, which tries to
> optimize the 2-limb multiply into 3 multiplies instead of 4.  None of
> the other platforms utilizing the soft-fp layer in the kernel try to
> use the _3mul version either.
> 
> Correctness trumps performance, so we can just use the full 4 multiply
> version to fix this bug.
> 
> Thanks for the report.
> 
> diff-tree 92f282988b4ce3967ee8399f7d1184ebfa04e48b (from 64821324ca49f24be1a66f2f432108f96a24e596)
> Author: David S. Miller <davem@sunset.davemloft.net>
> Date:   Thu Jul 27 16:49:21 2006 -0700
> 
>     [SPARC64]: Fix quad-float multiply emulation.
>     
>     Something is wrong with the 3-multiply (vs. 4-multiply) optimized
>     version of _FP_MUL_MEAT_2_*(), so just use the slower version
>     which actually computes correct values.
>     
>     Noticed by Rene Rebe
>     
>     Signed-off-by: David S. Miller <davem@davemloft.net>
> 
> diff --git a/include/asm-sparc64/sfp-machine.h b/include/asm-sparc64/sfp-machine.h
> index 5015bb8..89d4243 100644
> --- a/include/asm-sparc64/sfp-machine.h
> +++ b/include/asm-sparc64/sfp-machine.h
> @@ -34,7 +34,7 @@
>  #define _FP_MUL_MEAT_D(R,X,Y)					\
>    _FP_MUL_MEAT_1_wide(_FP_WFRACBITS_D,R,X,Y,umul_ppmm)
>  #define _FP_MUL_MEAT_Q(R,X,Y)					\
> -  _FP_MUL_MEAT_2_wide_3mul(_FP_WFRACBITS_Q,R,X,Y,umul_ppmm)
> +  _FP_MUL_MEAT_2_wide(_FP_WFRACBITS_Q,R,X,Y,umul_ppmm)
>  
>  #define _FP_DIV_MEAT_S(R,X,Y)	_FP_DIV_MEAT_1_imm(S,R,X,Y,_FP_DIV_HELP_imm)
>  #define _FP_DIV_MEAT_D(R,X,Y)	_FP_DIV_MEAT_1_udiv_norm(D,R,X,Y)
> 

I can confirm this patch corrects the emulation.

Yours thankfully,

-- 
  René Rebe - ExactCODE - Berlin (Europe / Germany)
  http://exactcode.de | http://t2-project.org | http://rene.rebe.name
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