| /* |
| * Written by J.T. Conklin <jtc@netbsd.org>. |
| * Adapted for use as log2 by Ulrich Drepper <drepper@cygnus.com>. |
| * Public domain. |
| * |
| * Changed to use fyl2xp1 for values near 1, <drepper@cygnus.com>. |
| * Adapted for x86-64 by Andreas Jaeger <aj@suse.de>. |
| */ |
| |
| #include <machine/asm.h> |
| |
| #ifdef __ELF__ |
| .section .rodata |
| #else |
| .text |
| #endif |
| .align ALIGNARG(4) |
| ASM_TYPE_DIRECTIVE(one,@object) |
| one: .double 1.0 |
| ASM_SIZE_DIRECTIVE(one) |
| /* It is not important that this constant is precise. It is only |
| a value which is known to be on the safe side for using the |
| fyl2xp1 instruction. */ |
| ASM_TYPE_DIRECTIVE(limit,@object) |
| limit: .double 0.29 |
| ASM_SIZE_DIRECTIVE(limit) |
| |
| |
| #ifdef PIC |
| #define MO(op) op##(%rip) |
| #else |
| #define MO(op) op |
| #endif |
| |
| .text |
| ENTRY(__ieee754_log2l) |
| fldl MO(one) |
| fldt 8(%rsp) // x : 1 |
| fxam |
| fnstsw |
| fld %st // x : x : 1 |
| testb $1, %ah |
| jnz 3f // in case x is NaN or ±Inf |
| 4: fsub %st(2), %st // x-1 : x : 1 |
| fld %st // x-1 : x-1 : x : 1 |
| fabs // |x-1| : x-1 : x : 1 |
| fcompl MO(limit) // x-1 : x : 1 |
| fnstsw // x-1 : x : 1 |
| andb $0x45, %ah |
| jz 2f |
| fstp %st(1) // x-1 : 1 |
| fyl2xp1 // log(x) |
| ret |
| |
| 2: fstp %st(0) // x : 1 |
| fyl2x // log(x) |
| ret |
| |
| 3: testb $4, %ah |
| jnz 4b // in case x is ±Inf |
| fstp %st(1) |
| fstp %st(1) |
| ret |
| END (__ieee754_log2l) |
