2 * Written by J.T. Conklin <jtc@NetBSD.org>.
7 * Modified by Lex Wennmacher <wennmach@NetBSD.org>
11 #include <machine/asm.h>
18 RCSID("$DragonFly: src/lib/msun/i387/Attic/s_log1pf.S,v 1.1 2004/12/29 12:55:50 asmodai Exp $")
21 * The log1pf() function is provided to compute an accurate value of
22 * log(1 + x), even for tiny values of x. The i387 FPU provides the
23 * fyl2xp1 instruction for this purpose. However, the range of this
24 * instruction is limited to:
25 * -(1 - (sqrt(2) / 2)) <= x <= sqrt(2) - 1
26 * -0.292893 <= x <= 0.414214
27 * at least on older processor versions.
29 * log1pf() is implemented by testing the range of the argument.
30 * If it is appropriate for fyl2xp1, this instruction is used.
31 * Else, we compute log1pf(x) = ln(2)*ld(1 + x) the traditional way
34 * The range testing costs speed, but as the rationale for the very
35 * existence of this function is accuracy, we accept that.
37 * In order to reduce the cost for testing the range, we check if
38 * the argument is in the range
40 * which can be done with just one conditional branch. If x is
41 * inside this range, we use fyl2xp1. Outside of this range,
42 * the use of fyl2x is accurate enough.
49 XMM_ONE_ARG_FLOAT_PROLOGUE
53 fadd %st(0) /* ... x 2 */
54 fadd %st(0) /* ... x 4 */
56 fdivp /* ... x 0.25 */