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Re: [PATCH] Speedup tanf range reduction
On 22/08/2018 09:29, Wilco Dijkstra wrote:
> Speedup tanf range reduction by using the new sincosf range
> reduction algorithm. Overall code quality is improved due to
> inlining, so there is a speedup even if no range reduction is
> required.
>
> Passes GLIBC testsuite on AArch64. Some files are no longer
> required - they are removed in the next patch.
>
> tanf througput gains on Cortex-A72:
> * |x| < M_PI_4 : 1.1x
> * |x| < M_PI_2 : 1.2x
> * |x| < 2 * M_PI: 1.5x
> * |x| < 120.0 : 1.6x
> * |x| < Inf : 12.1x
>
> ChangeLog:
> 2018-08-22 Wilco Dijkstra <wdijkstr@arm.com>
>
> * sysdeps/ieee754/flt-32/s_tanf.c (__tanf): Use fast range reduction.
tanf seems to be the only place where __ieee754_rem_pio2f is used, so we
can remove sysdeps/ieee754/flt-32/e_rem_pio2f.c and
sysdeps/powerpc/fpu/e_rem_pio2f.c. Also, I would like to certify that
it is faster than powerpc version (I would expect so, but at least it
would be good to actually get some numbers).
>
> --
> diff --git a/sysdeps/ieee754/flt-32/s_tanf.c b/sysdeps/ieee754/flt-32/s_tanf.c
> index ba3af54913669e4abdfd864307856ec44138f9b9..a397665c4bab7785049935ef526472afedf82e34 100644
> --- a/sysdeps/ieee754/flt-32/s_tanf.c
> +++ b/sysdeps/ieee754/flt-32/s_tanf.c
> @@ -21,6 +21,30 @@ static char rcsid[] = "$NetBSD: s_tanf.c,v 1.4 1995/05/10 20:48:20 jtc Exp $";
> #include <math.h>
> #include <math_private.h>
> #include <libm-alias-float.h>
> +#include "s_sincosf.h"
> +
> +static inline int32_t
> +rem_pio2f (float x, float *y)
> +{
> + double dx = x;
> + int n;
> + const sincos_t *p = &__sincosf_table[0];
> +
> + if (__glibc_likely (abstop12 (x) < abstop12 (120.0f)))
> + dx = reduce_fast (dx, p, &n);
> + else
> + {
> + uint32_t xi = asuint (x);
> + int sign = xi >> 31;
> +
> + dx = reduce_large (xi, &n);
> + dx = sign ? -dx : dx;
> + }
> +
> + y[0] = dx;
> + y[1] = dx - y[0];
> + return n;
> +}
>
> float __tanf(float x)
> {
> @@ -42,7 +66,7 @@ float __tanf(float x)
>
> /* argument reduction needed */
> else {
> - n = __ieee754_rem_pio2f(x,y);
> + n = rem_pio2f(x,y);
> return __kernel_tanf(y[0],y[1],1-((n&1)<<1)); /* 1 -- n even
> -1 -- n odd */
> }
>