Activate -mrecip with -ffast-math?

[Richard Guenther]

On 6/18/07, Uros Bizjak <ubizjak@gmail.com> wrote:
> On 6/18/07, tbp <tbptbp@gmail.com> wrote:
>
> > Until now, the contract was: you have to deal with (and contain) NaN
> > and infinities. Fair enough, even if tricky that remained manageable.
> > But if i can't expect a mere division by 0, or sqrt of 0 (quite common
> > with FTZ/DAZ on) to give me respectively an infinite and 0 and instead
> > get a NaN (which i can't filter, you remember?) because of the NR
> > round, that's pure madness.
>
> Attached patch to should fix these troubles for the cost of 2 extra
> clocks. The trick is to limit the result just below infinity for
> rsqrt, and this keeps 0.0*(inf-) -> 0.0.

I guess I'm still confused how this will fix sqrt(x) -> rsqrt for x == 0, so,
can we have a testcase enumerating the now bogus cases?

Thx,
Richard.

> Uros.
>
> Index: i386.c
> ===================================================================
> --- i386.c      (revision 125790)
> +++ i386.c      (working copy)
> @@ -22590,7 +22590,7 @@ void ix86_emit_swdivsf (rtx res, rtx a,
>  void ix86_emit_swsqrtsf (rtx res, rtx a, enum machine_mode mode,
>                          bool recip)
>  {
> -  rtx x0, e0, e1, e2, e3, three, half;
> +  rtx x0, e0, e1, e2, e3, three, half, bignum;
>
>    x0 = gen_reg_rtx (mode);
>    e0 = gen_reg_rtx (mode);
> @@ -22600,15 +22600,18 @@ void ix86_emit_swsqrtsf (rtx res, rtx a,
>
>    three = CONST_DOUBLE_FROM_REAL_VALUE (dconst3, SFmode);
>    half = CONST_DOUBLE_FROM_REAL_VALUE (dconsthalf, SFmode);
> +  bignum = gen_lowpart (SFmode, GEN_INT (0x7f7fffff));
>
>    if (VECTOR_MODE_P (mode))
>      {
>        three = ix86_build_const_vector (SFmode, true, three);
>        half = ix86_build_const_vector (SFmode, true, half);
> +      bignum = ix86_build_const_vector (SFmode, true, bignum);
>      }
>
>    three = force_reg (mode, three);
>    half = force_reg (mode, half);
> +  bignum = force_reg (mode, bignum);
>
>    /* sqrt(a) = 0.5 * a * rsqrtss(a) * (3.0 - a * rsqrtss(a) * rsqrtss(a))
>       1.0 / sqrt(a) = 0.5 * rsqrtss(a) * (3.0 - a * rsqrtss(a) * rsqrtss(a)) */
> @@ -22617,6 +22620,9 @@ void ix86_emit_swsqrtsf (rtx res, rtx a,
>    emit_insn (gen_rtx_SET (VOIDmode, x0,
>                           gen_rtx_UNSPEC (mode, gen_rtvec (1, a),
>                                           UNSPEC_RSQRT)));
> +  emit_insn (gen_rtx_SET (VOIDmode, x0,
> +                         gen_rtx_SMIN (mode, x0, bignum)));
> +
>    /* e0 = x0 * a */
>    emit_insn (gen_rtx_SET (VOIDmode, e0,
>                           gen_rtx_MULT (mode, x0, a)));
>