Date: Thu, 5 Dec 2013 11:52:25 -0800 From: Steve Kargl <sgk@troutmask.apl.washington.edu> To: Filipe Maia <filipe.c.maia@gmail.com> Cc: freebsd-numerics@freebsd.org Subject: Re: dead code in lgamma_r[f]? Message-ID: <20131205195225.GA65732@troutmask.apl.washington.edu> In-Reply-To: <CAN5hRiV6N1arc4RDv=9JbRiXp-J9o3WzAbeZSOpAxks2ZeG%2B_w@mail.gmail.com> References: <20131205173700.GA64575@troutmask.apl.washington.edu> <20131205182324.GA65135@troutmask.apl.washington.edu> <CAN5hRiV6N1arc4RDv=9JbRiXp-J9o3WzAbeZSOpAxks2ZeG%2B_w@mail.gmail.com>
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On Thu, Dec 05, 2013 at 08:06:22PM +0100, Filipe Maia wrote:
> What if ix == 0x433xxxxx?
See below.
> Wouldn't the if clause be false?
See below.
> > Index: src/e_lgamma_r.c
> > ===================================================================
> > --- src/e_lgamma_r.c (revision 1427)
> > +++ src/e_lgamma_r.c (working copy)
> > @@ -173,19 +173,15 @@
> > */
> > z = floor(y);
> > if(z!=y) { /* inexact anyway */
> > - y *= 0.5;
> > - y = 2.0*(y - floor(y)); /* y = |x| mod 2.0 */
> > - n = (int) (y*4.0);
> > + y /= 2;
> > + y = 2*(y - floor(y)); /* y = |x| mod 2.0 */
> > + n = (int)(y*4);
If you have a y value that corresponds to ix=0x433xxxxx where
one of the x is nonzero, I believe that you end up going through
this portion of code.
> > } else {
You end up here if and only if z==y, which means y is integral.
As noted in the original email, the 'if(ix>=0x43400000)' below
is dead code because this code in __ieee754_lgamma_r():
if(ix>=0x43300000) /* |x|>=2**52, must be -integer */
return one/zero;
t = sin_pi(x);
prevents the call to sin_pi(x).
> > - if(ix>=0x43400000) {
> > - y = zero; n = 0; /* y must be even */
> > - } else {
> > - if(ix<0x43300000) z = y+two52; /* exact */
If we again look at the code from __ieee754_lgamma_r(), we see
that sin_pi() is called if ix < 0x43300000, so by the time we
arrive at the 'if(ix<0x43300000)' statement we already know that
the condition is true.
> > - GET_LOW_WORD(n,z);
> > - n &= 1;
> > - y = n;
> > - n<<= 2;
> > - }
> > + z = y+two52; /* exact */
> > + GET_LOW_WORD(n,z);
> > + n &= 1;
> > + y = n;
> > + n<<= 2;
> > }
> > switch (n) {
> > case 0: y = __kernel_sin(pi*y,zero,0); break;
--
Steve
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