On Tue, Sep 26, 2017 at 06:21:16PM +0200, Jan Stary wrote:
> These (diff below) seem to be obvious typos in k_sin.c,
> but only in comments. The comment that says
>
> if x < 2^-27 (hx<0x3e400000), return x with inexact if x!=0
>
> also puzzles me a bit: what the code does is
>
> GET_HIGH_WORD(ix,x);
> ix &= 0x7fffffff; /* high word of x */
> if(ix<0x3e400000) /* |x| < 2**-27 */
> {if((int)x==0) return x;} /* generate inexact */
>
> If the high word of a double x is less than 0x3e400000,
> how could (int)x be anything else than zero?
The result might be zero, but you want the inexact flag to be set in
those cases. I think case 2 describes it correctly: only return 0 with
the inexact flag not set if x equals zero.
-Otto
>
> The "if x!= 0" also puzzles me: the sine of zero _is_ zero, exactly.
> What would change with this:
>
> if (ix < 0x3e400000)
> return x;
>
> What is the role of the iy indicator?
> Is it easier/faster to test the integer indicator for iy == 0
> than it would be to test y == 0 itself? Or is there another reason?
>
> Jan
>
>
> Index: k_sin.c
> ===================================================================
> RCS file: /cvs/src/lib/libm/src/k_sin.c,v
> retrieving revision 1.3
> diff -u -p -r1.3 k_sin.c
> --- k_sin.c 27 Oct 2009 23:59:30 -0000 1.3
> +++ k_sin.c 26 Sep 2017 16:07:45 -0000
> @@ -10,15 +10,15 @@
> * ====================================================
> */
>
> -/* __kernel_sin( x, y, iy)
> +/* __kernel_sin(x, y, iy)
> * kernel sin function on [-pi/4, pi/4], pi/4 ~ 0.7854
> * Input x is assumed to be bounded by ~pi/4 in magnitude.
> * Input y is the tail of x.
> - * Input iy indicates whether y is 0. (if iy=0, y assume to be 0).
> + * Input iy indicates whether y is 0. (if iy=0, assume y to be 0).
> *
> * Algorithm
> * 1. Since sin(-x) = -sin(x), we need only to consider positive x.
> - * 2. if x < 2^-27 (hx<0x3e400000 0), return x with inexact if x!=0.
> + * 2. if x < 2^-27 (hx<0x3e400000), return x with inexact if x!=0.
> * 3. sin(x) is approximated by a polynomial of degree 13 on
> * [0,pi/4]
> * 3 13
