[PATCH] expf: Fixed the hi+lo approximation to log(2). The normal 17+24 bit decomposition

[Jeff Johnston]

Hi Fabian,

Looking at the FreeBSD code, I see there are other changes that differ from
newlib's version starting from:

else if(hx < 0x39000000)  { /* when |x|<2**-14 */     <=== in newlib this
is 2** - 23
   if(huge+x>one) return one+x;/* trigger inexact */
}
else k = 0;

    /* x is now in primary range */
t  = x*x;
if(k >= -125)
   SET_FLOAT_WORD(twopk,((u_int32_t)(0x7f+k))<<23);
else
   SET_FLOAT_WORD(twopk,((u_int32_t)(0x7f+(k+100)))<<23);
c  = x - t*(P1+t*P2);
if(k==0) return one-((x*c)/(c-(float)2.0)-x);
else y = one-((lo-(x*c)/((float)2.0-c))-hi);
if(k >= -125) {
   if(k==128) return y*2.0F*0x1p127F;
   return y*twopk;
} else {
   return y*twopk*twom100;
}

Would you like to port over those changes as well?  Some of them are from
18 years ago and there are a few from 6 years ago (ones with 0x7f).

-- Jeff J.

On Tue, Jan 28, 2025 at 5:52 AM Fabian Schriever <
fabian.schriever@gtd-gmbh.de> wrote:

> that was used doesn't work normally here, since we want to be able to
> multiply `hi' by the exponent of x _exactly_, and the exponent of x has
> more than 7 significant bits for most denormal x's, so the multiplication
> was not always exact despite a cloned comment claiming that it was.  (The
> comment is correct in the double precision case -- with the normal 33+53
> bit decomposition the exponent can have 20 significant bits and the extra
> bit for denormals is only the 11th.)
>
> Fixing this had little or no effect for denormals (I think because
> more precision is inherently lost for denormals than is lost by roundoff
> errors in the multiplication).
>
> The fix is to reduce the precision of the decomposition to 16+24 bits.
> Due to 2 bugs in the old deomposition and numerical accidents, reducing
> the precision actually increased the precision of hi+lo.  The old hi+lo
> had about 39 bits instead of at least 41 like it should have had.
> There were off-by-1-bit errors in each of hi and lo, apparently due
> to mistranslation from the double precision hi and lo.  The correct
> 16 bit hi happens to give about 19 bits of precision, so the correct
> hi+lo gives about 43 bits instead of at least 40.  The end result is
> that expf() is now perfectly rounded (to nearest) except in 52561 cases
> instead of except in 67027 cases, and the maximum error is 0.5013 ulps
> instead of 0.5023 ulps.
>
> Commit and message are from FreeBSD.
> Reference:
> https://github.com/freebsd/freebsd-src/commit/06d803185574a0ba37434b1e98b322e54cc32a4e
> Original Author: Bruce Evans
>
> We have taken the liberty to also add the f suffix to all float
> constants to ensure they are not doubles.
> ---
>  newlib/libm/math/ef_exp.c | 34 +++++++++++++++++-----------------
>  1 file changed, 17 insertions(+), 17 deletions(-)
>
> diff --git a/newlib/libm/math/ef_exp.c b/newlib/libm/math/ef_exp.c
> index d6e25bfcd..85e1cf9d3 100644
> --- a/newlib/libm/math/ef_exp.c
> +++ b/newlib/libm/math/ef_exp.c
> @@ -26,20 +26,20 @@ static const float
>  #else
>  static float
>  #endif
> -one    = 1.0,
> -halF[2]        = {0.5,-0.5,},
> -huge   = 1.0e+30,
> -twom100 = 7.8886090522e-31,      /* 2**-100=0x0d800000 */
> -ln2HI[2]   ={ 6.9313812256e-01,                /* 0x3f317180 */
> -            -6.9313812256e-01,},       /* 0xbf317180 */
> -ln2LO[2]   ={ 9.0580006145e-06,        /* 0x3717f7d1 */
> -            -9.0580006145e-06,},       /* 0xb717f7d1 */
> -invln2 =  1.4426950216e+00,            /* 0x3fb8aa3b */
> -P1   =  1.6666667163e-01, /* 0x3e2aaaab */
> -P2   = -2.7777778450e-03, /* 0xbb360b61 */
> -P3   =  6.6137559770e-05, /* 0x388ab355 */
> -P4   = -1.6533901999e-06, /* 0xb5ddea0e */
> -P5   =  4.1381369442e-08; /* 0x3331bb4c */
> +one    = 1.0f,
> +halF[2]        = {0.5f,-0.5f,},
> +huge   = 1.0e+30f,
> +twom100 = 7.8886090522e-31f,      /* 2**-100=0x0d800000 */
> +ln2HI[2]   ={ 6.9314575195e-01f,               /* 0x3f317200 */
> +            -6.9314575195e-01f,},      /* 0xbf317200 */
> +ln2LO[2]   ={ 1.4286067653e-06f,       /* 0x35bfbe8e */
> +            -1.4286067653e-06f,},      /* 0xb5bfbe8e */
> +invln2 =  1.4426950216e+00f,           /* 0x3fb8aa3b */
> +P1   =  1.6666667163e-01f, /* 0x3e2aaaab */
> +P2   = -2.7777778450e-03f, /* 0xbb360b61 */
> +P3   =  6.6137559770e-05f, /* 0x388ab355 */
> +P4   = -1.6533901999e-06f, /* 0xb5ddea0e */
> +P5   =  4.1381369442e-08f; /* 0x3331bb4c */
>
>  #ifdef __STDC__
>         float __ieee754_expf(float x)   /* default IEEE double exp */
> @@ -60,7 +60,7 @@ P5   =  4.1381369442e-08; /* 0x3331bb4c */
>          if(FLT_UWORD_IS_NAN(hx))
>              return x+x;                /* NaN */
>          if(FLT_UWORD_IS_INFINITE(hx))
> -           return (xsb==0)? x:0.0;             /* exp(+-inf)={inf,0} */
> +           return (xsb==0)? x:0.0f;            /* exp(+-inf)={inf,0} */
>         if(sx > FLT_UWORD_LOG_MAX)
>             return __math_oflowf(0); /* overflow */
>         if(sx < 0 && hx > FLT_UWORD_LOG_MIN)
> @@ -85,8 +85,8 @@ P5   =  4.1381369442e-08; /* 0x3331bb4c */
>      /* x is now in primary range */
>         t  = x*x;
>         c  = x - t*(P1+t*(P2+t*(P3+t*(P4+t*P5))));
> -       if(k==0)        return one-((x*c)/(c-(float)2.0)-x);
> -       else            y = one-((lo-(x*c)/((float)2.0-c))-hi);
> +       if(k==0)        return one-((x*c)/(c-2.0f)-x);
> +       else            y = one-((lo-(x*c)/(2.0f-c))-hi);
>         if(k >= -125) {
>             __uint32_t hy;
>             GET_FLOAT_WORD(hy,y);
> --
> 2.33.0.windows.1
>
>
>
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