glibc/sysdeps/libm-i387/e_powf.S

/* ix87 specific implementation of pow function.
   Copyright (C) 1996 Free Software Foundation, Inc.
   This file is part of the GNU C Library.
   Contributed by Ulrich Drepper <drepper@cygnus.com>, 1996.

   The GNU C Library is free software; you can redistribute it and/or
   modify it under the terms of the GNU Library General Public License as
   published by the Free Software Foundation; either version 2 of the
   License, or (at your option) any later version.

   The GNU C Library is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
   Library General Public License for more details.

   You should have received a copy of the GNU Library General Public
   License along with the GNU C Library; see the file COPYING.LIB.  If not,
   write to the Free Software Foundation, Inc., 59 Temple Place - Suite 330,
   Boston, MA 02111-1307, USA.  */

#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)
	ASM_TYPE_DIRECTIVE(limit,@object)
limit:	.double 0.29
	ASM_SIZE_DIRECTIVE(limit)

#ifdef PIC
#define MO(op) op##@GOTOFF(%ecx)
#else
#define MO(op) op
#endif

	.text
ENTRY(__ieee754_powf)
	flds	4(%esp)		// x
	flds	8(%esp)		// y : x

#ifdef	PIC
	call	1f
1:	popl	%ecx
	addl	$_GLOBAL_OFFSET_TABLE_+[.-1b], %ecx
#endif
	subl	$8,%esp

	/* First see whether `y' is a natural number.  In this case we
	   can use a more precise algorithm.  */
	fld	%st		// y : y : x
	fistpll	(%esp)		// y : x
	fildll	(%esp)		// int(y) : y : x
	fucomp	%st(1)		// y : x
	fnstsw
	sahf
	jne	2f

	/* OK, we have an integer value for y.  */
	ftst			// y : x
	fstp	%st(0)		// x
	fnstsw
	sahf
	popl	%eax
	popl	%edx
	jnc	4f		// y >= 0, jump
	fdivrl	MO(one)		// 1/x		(now referred to as x)
	negl	%eax
	adcl	$0, %edx
	negl	%edx
4:	fldl	MO(one)		// 1 : x
	fxch

6:	shrdl	$1, %edx, %eax
	jnc	5f
	fxch
	fmul	%st(1)		// x : ST*x
	fxch
5:	fmul	%st(0), %st	// x*x : ST*x
	movl	%eax, %ecx
	orl	%edx, %ecx
	jnz	6b
	fstp	%st(0)		// ST*x
	ret

	.align ALIGNARG(4)
2:	/* y is a real number.  */
	fxch			// x : y
	fldl	MO(one)		// 1.0 : x : y
	fld	%st(1)		// x : 1.0 : x : y
	fsub	%st(1)		// x-1 : 1.0 : x : y
	fabs			// |x-1| : 1.0 : x : y
	fcompl	MO(limit)	// 1.0 : x : y
	fnstsw
	fxch			// x : 1.0 : y
	sahf
	ja	7f
	fsub	%st(1)		// x-1 : 1.0 : y
	fyl2xp1			// log2(x) : y
	jmp	8f

7:	fyl2x			// log2(x) : y
8:	fmul	%st(1)		// y*log2(x) : y
	fst	%st(1)		// y*log2(x) : y*log2(x)
	frndint			// int(y*log2(x)) : y*log2(x)
	fsubr	%st, %st(1)	// int(y*log2(x)) : fract(y*log2(x))
	fxch			// fract(y*log2(x)) : int(y*log2(x))
	f2xm1			// 2^fract(y*log2(x))-1 : int(y*log2(x))
	faddl	MO(one)		// 2^fract(y*log2(x)) : int(y*log2(x))
	fscale			// 2^fract(y*log2(x))*2^int(y*log2(x)) : int(y*log2(x))
	addl	$8, %esp
	fstp	%st(1)		// 2^fract(y*log2(x))*2^int(y*log2(x))
	ret
END(__ieee754_powf)