On Sat, 22 Mar 2003, Till Riedel wrote:
> > > res=pow((float)base,(float)dim);
> this is actually not a smart thing. it was cut and paste from libvorbis.
> pow is a function for doubles. if you i use powf everything works fine.
This should work OK. The casts to float should have no effect except to
lose precision is the operands don't fit in about 24 bits.
OTOH, powf() is known to be broken. I haven't committed a fix like the
following for too long:
%%%
Index: e_powf.c
===================================================================
RCS file: /home/ncvs/src/lib/msun/src/e_powf.c,v
retrieving revision 1.9
diff -u -2 -r1.9 e_powf.c
--- e_powf.c 17 Jun 2002 15:28:59 -0000 1.9
+++ e_powf.c 17 Jun 2002 15:41:06 -0000
@@ -45,5 +45,9 @@
lg2 = 6.9314718246e-01, /* 0x3f317218 */
lg2_h = 6.93145752e-01, /* 0x3f317200 */
+#if 0
+lg2_l = 1.42860677e-06, /* 0x35bfbe8e */
+#else
lg2_l = 1.42860654e-06, /* 0x35bfbe8c */
+#endif
ovt = 4.2995665694e-08, /* -(128-log2(ovfl+.5ulp)) */
cp = 9.6179670095e-01, /* 0x3f76384f =2/(3ln2) */
@@ -160,7 +164,7 @@
s_h = s;
GET_FLOAT_WORD(is,s_h);
- SET_FLOAT_WORD(s_h,is&0xfffff000);
+ SET_FLOAT_WORD(s_h,is&0xfffc0000);
/* t_h=ax+bp[k] High */
- SET_FLOAT_WORD(t_h,((ix>>1)|0x20000000)+0x0040000+(k<<21));
+ SET_FLOAT_WORD(t_h,((ix>>1)|0x20000000)+0x00400000+(k<<21));
t_l = ax - (t_h-bp[k]);
s_l = v*((u-s_h*t_h)-s_h*t_l);
@@ -229,5 +233,5 @@
t = p_l+p_h;
GET_FLOAT_WORD(is,t);
- SET_FLOAT_WORD(t,is&0xfffff000);
+ SET_FLOAT_WORD(t,is&0xfffffff8);
u = t*lg2_h;
v = (p_l-(t-p_h))*lg2+t*lg2_l;
%%%
The change to lg2_l just fixes a tiny error. We gain precision by
representing log(2) as approximately (lg2_h + lg2_l) in infinite precision.
The bits in lg2_l are relatively uncritical since most of them are to
give more than float precision. But they sometimes matter. The Cygnus
extensions to support float precision have a number of off by 1 or 2 bit
errors converting the low values. I fixed a couple of these, but IIRC
there was only one that caused an error that was reported by ucbtest (not
this one).
The changes in the magic numbers are too fix larger errors. I don't
rememebr anything else about them except that the errors were reported
by ucbtest and the changes made ucbtest happy.
Bruce
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