Re: [PATCH] sysdeps: Simplify sin Taylor Series calculation

[Paul Zimmermann <Paul.Zimmermann@inria.fr>]

       Dear Akila,

thank you for the new version, this seems good to me.

Do you have commit access?

Best regards,
Paul Zimmermann

> From: Akila Welihinda <akilawelihinda@ucla.edu>
> Date: Sun, 12 Dec 2021 10:35:03 -0800
> Cc: Akila Welihinda <akilawelihinda@ucla.edu>
> 
> The macro TAYLOR_SIN adds the term `-0.5*da*a^2 + da` in hopes
> of regaining some precision as a function of da. However the
> comment says we add the term `-0.5*da*a^2 + 0.5*da` which is
> different. This fix updates the comment to reflect the
> code and also simplifies the calculation by replacing `a` with `x`
> because they always have the same value.
> 
> Signed-off-by: Akila Welihinda <akilawelihinda@ucla.edu>
> ---
>  sysdeps/ieee754/dbl-64/s_sin.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/sysdeps/ieee754/dbl-64/s_sin.c b/sysdeps/ieee754/dbl-64/s_sin.c
> index 7d89e3dfc2..a412c3642d 100644
> --- a/sysdeps/ieee754/dbl-64/s_sin.c
> +++ b/sysdeps/ieee754/dbl-64/s_sin.c
> @@ -51,16 +51,16 @@
>  #define POLYNOMIAL(xx) (POLYNOMIAL2 (xx) + s1)
>  
>  /* The computed polynomial is a variation of the Taylor series expansion for
> -   sin(a):
> +   sin(x):
>  
> -   a - a^3/3! + a^5/5! - a^7/7! + a^9/9! + (1 - a^2) * da / 2
> +   x - x^3/3! + x^5/5! - x^7/7! + x^9/9! - dx*x^2/2 + dx
>  
>     The constants s1, s2, s3, etc. are pre-computed values of 1/3!, 1/5! and so
>     on.  The result is returned to LHS.  */
> -#define TAYLOR_SIN(xx, a, da) \
> +#define TAYLOR_SIN(xx, x, dx) \
>  ({									      \
> -  double t = ((POLYNOMIAL (xx)  * (a) - 0.5 * (da))  * (xx) + (da));	      \
> -  double res = (a) + t;							      \
> +  double t = ((POLYNOMIAL (xx)  * (x) - 0.5 * (dx))  * (xx) + (dx));	      \
> +  double res = (x) + t;							      \
>    res;									      \
>  })
>  
> -- 
> 2.30.1 (Apple Git-130)

Reviewed-by: Paul Zimmermann <Paul.Zimmermann@inria.fr>