| | 1 | /* |
| | 2 | * Written by J.T. Conklin <jtc@NetBSD.org>. |
| | 3 | * Public domain. |
| | 4 | * |
| | 5 | * $NetBSD: s_log1pf.S,v 1.10 2003/09/16 18:17:11 wennmach Exp $ |
| | 6 | */ |
| | 7 | |
| | 8 | /* |
| | 9 | * Modified by Lex Wennmacher <wennmach@NetBSD.org> |
| | 10 | * Still public domain. |
| | 11 | */ |
| | 12 | |
| | 13 | #include <machine/asm.h> |
| | 14 | |
| | 15 | #include "abi.h" |
| | 16 | |
| | 17 | /* |
| | 18 | * The log1pf() function is provided to compute an accurate value of |
| | 19 | * log(1 + x), even for tiny values of x. The i387 FPU provides the |
| | 20 | * fyl2xp1 instruction for this purpose. However, the range of this |
| | 21 | * instruction is limited to: |
| | 22 | * -(1 - (sqrt(2) / 2)) <= x <= sqrt(2) - 1 |
| | 23 | * -0.292893 <= x <= 0.414214 |
| | 24 | * at least on older processor versions. |
| | 25 | * |
| | 26 | * log1pf() is implemented by testing the range of the argument. |
| | 27 | * If it is appropriate for fyl2xp1, this instruction is used. |
| | 28 | * Else, we compute log1pf(x) = ln(2)*ld(1 + x) the traditional way |
| | 29 | * (using fyl2x). |
| | 30 | * |
| | 31 | * The range testing costs speed, but as the rationale for the very |
| | 32 | * existence of this function is accuracy, we accept that. |
| | 33 | * |
| | 34 | * In order to reduce the cost for testing the range, we check if |
| | 35 | * the argument is in the range |
| | 36 | * -0.25 <= x <= 0.25 |
| | 37 | * which can be done with just one conditional branch. If x is |
| | 38 | * inside this range, we use fyl2xp1. Outside of this range, |
| | 39 | * the use of fyl2x is accurate enough. |
| | 40 | * |
| | 41 | */ |
| | 42 | |
| | 43 | .text |
| | 44 | .align 4 |
| | 45 | ENTRY(log1pf) |
| | 46 | XMM_ONE_ARG_FLOAT_PROLOGUE |
| | 47 | flds ARG_FLOAT_ONE |
| | 48 | fabs |
| | 49 | fld1 /* ... x 1 */ |
| | 50 | fadd %st(0) /* ... x 2 */ |
| | 51 | fadd %st(0) /* ... x 4 */ |
| | 52 | fld1 /* ... 4 1 */ |
| | 53 | fdivp /* ... x 0.25 */ |
| | 54 | fcompp |
| | 55 | fnstsw %ax |
| | 56 | andb $69,%ah |
| | 57 | jne use_fyl2x |
| | 58 | jmp use_fyl2xp1 |
| | 59 | |
| | 60 | .align 4 |
| | 61 | use_fyl2x: |
| | 62 | fldln2 |
| | 63 | flds ARG_FLOAT_ONE |
| | 64 | fld1 |
| | 65 | faddp |
| | 66 | fyl2x |
| | 67 | XMM_FLOAT_EPILOGUE |
| | 68 | ret |
| | 69 | |
| | 70 | .align 4 |
| | 71 | use_fyl2xp1: |
| | 72 | fldln2 |
| | 73 | flds ARG_FLOAT_ONE |
| | 74 | fyl2xp1 |
| | 75 | XMM_FLOAT_EPILOGUE |
| | 76 | ret |
| | 77 | END(log1pf) |
| | 78 | |
| | 79 | .section .note.GNU-stack,"",%progbits |
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