wm_sqrt.s

freebsd v4.4内核源码

	.file	"wm_sqrt.S"
/*
 *  wm_sqrt.S
 *
 * Fixed point arithmetic square root evaluation.
 *
 * Call from C as:
 *   void wm_sqrt(FPU_REG *n, unsigned int control_word)
 *
 *
 * Copyright (C) 1992,1993,1994
 *                       W. Metzenthen, 22 Parker St, Ormond, Vic 3163,
 *                       Australia.  E-mail   [e-mail]
 * All rights reserved.
 *
 * This copyright notice covers the redistribution and use of the
 * FPU emulator developed by W. Metzenthen. It covers only its use
 * in the 386BSD, FreeBSD and NetBSD operating systems. Any other
 * use is not permitted under this copyright.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must include information specifying
 *    that source code for the emulator is freely available and include
 *    either:
 *      a) an offer to provide the source code for a nominal distribution
 *         fee, or
 *      b) list at least two alternative methods whereby the source
 *         can be obtained, e.g. a publically accessible bulletin board
 *         and an anonymous ftp site from which the software can be
 *         downloaded.
 * 3. All advertising materials specifically mentioning features or use of
 *    this emulator must acknowledge that it was developed by W. Metzenthen.
 * 4. The name of W. Metzenthen may not be used to endorse or promote
 *    products derived from this software without specific prior written
 *    permission.
 *
 * THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES,
 * INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY
 * AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL
 * W. METZENTHEN BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
 * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
 * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
 * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
 * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
 * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
 * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 *
 *
 * The purpose of this copyright, based upon the Berkeley copyright, is to
 * ensure that the covered software remains freely available to everyone.
 *
 * The software (with necessary differences) is also available, but under
 * the terms of the GNU copyleft, for the Linux operating system and for
 * the djgpp ms-dos extender.
 *
 * W. Metzenthen   June 1994.
 *
 *
 * $FreeBSD: src/sys/gnu/i386/fpemul/wm_sqrt.s,v 1.4.2.1 1999/09/05 08:10:00 peter Exp $
 *
 */


/*---------------------------------------------------------------------------+
 |  wm_sqrt(FPU_REG *n, unsigned int control_word)                           |
 |    returns the square root of n in n.                                     |
 |                                                                           |
 |  Use Newton's method to compute the square root of a number, which must   |
 |  be in the range  [1.0 .. 4.0),  to 64 bits accuracy.                     |
 |  Does not check the sign or tag of the argument.                          |
 |  Sets the exponent, but not the sign or tag of the result.                |
 |                                                                           |
 |  The guess is kept in %esi:%edi                                           |
 +---------------------------------------------------------------------------*/

#include <gnu/i386/fpemul/exception.h>
#include <gnu/i386/fpemul/fpu_asm.h>


.data
/*
	Local storage:
 */
	.align 4,0
accum_3:
	.long	0		/* ms word */
accum_2:
	.long	0
accum_1:
	.long	0
accum_0:
	.long	0

/* The de-normalised argument:
//                  sq_2                  sq_1              sq_0
//        b b b b b b b ... b b b   b b b .... b b b   b 0 0 0 ... 0
//           ^ binary point here */
fsqrt_arg_2:
	.long	0		/* ms word */
fsqrt_arg_1:
	.long	0
fsqrt_arg_0:
	.long	0		/* ls word, at most the ms bit is set */

.text
	.align 2,144

.globl _wm_sqrt

_wm_sqrt:
	pushl	%ebp
	movl	%esp,%ebp
	pushl	%esi
	pushl	%edi
	pushl	%ebx

	movl	PARAM1,%esi

	movl	SIGH(%esi),%eax
	movl	SIGL(%esi),%ecx
	xorl	%edx,%edx

/* We use a rough linear estimate for the first guess.. */

	cmpl	EXP_BIAS,EXP(%esi)
	jnz	sqrt_arg_ge_2

	shrl	$1,%eax			/* arg is in the range  [1.0 .. 2.0) */
	rcrl	$1,%ecx
	rcrl	$1,%edx

sqrt_arg_ge_2:
/* From here on, n is never accessed directly again until it is
// replaced by the answer. */

	movl	%eax,fsqrt_arg_2		/* ms word of n */
	movl	%ecx,fsqrt_arg_1
	movl	%edx,fsqrt_arg_0

/* Make a linear first estimate */
	shrl	$1,%eax
	addl	$0x40000000,%eax
	movl	$0xaaaaaaaa,%ecx
	mull	%ecx
	shll	%edx			/* max result was 7fff... */
	testl	$0x80000000,%edx	/* but min was 3fff... */
	jnz	sqrt_prelim_no_adjust

	movl	$0x80000000,%edx	/* round up */

sqrt_prelim_no_adjust:
	movl	%edx,%esi	/* Our first guess */

/* We have now computed (approx)   (2 + x) / 3, which forms the basis
   for a few iterations of Newton's method */

	movl	fsqrt_arg_2,%ecx	/* ms word */

/* From our initial estimate, three iterations are enough to get us
// to 30 bits or so. This will then allow two iterations at better
// precision to complete the process.

// Compute  (g + n/g)/2  at each iteration (g is the guess). */
	shrl	%ecx		/* Doing this first will prevent a divide */
				/* overflow later. */

	movl	%ecx,%edx	/* msw of the arg / 2 */
	divl	%esi		/* current estimate */
	shrl	%esi		/* divide by 2 */
	addl	%eax,%esi	/* the new estimate */

	movl	%ecx,%edx
	divl	%esi
	shrl	%esi
	addl	%eax,%esi

	movl	%ecx,%edx
	divl	%esi
	shrl	%esi
	addl	%eax,%esi

/* Now that an estimate accurate to about 30 bits has been obtained (in %esi),
// we improve it to 60 bits or so.

// The strategy from now on is to compute new estimates from
//      guess := guess + (n - guess^2) / (2 * guess) */

/* First, find the square of the guess */
	movl	%esi,%eax
	mull	%esi
/* guess^2 now in %edx:%eax */

	movl	fsqrt_arg_1,%ecx
	subl	%ecx,%eax
	movl	fsqrt_arg_2,%ecx	/* ms word of normalized n */
	sbbl	%ecx,%edx
	jnc	sqrt_stage_2_positive
/* subtraction gives a negative result
// negate the result before division */
	notl	%edx
	notl	%eax
	addl	$1,%eax
	adcl	$0,%edx

	divl	%esi
	movl	%eax,%ecx

	movl	%edx,%eax
	divl	%esi
	jmp	sqrt_stage_2_finish

sqrt_stage_2_positive:
	divl	%esi
	movl	%eax,%ecx

	movl	%edx,%eax
	divl	%esi

	notl	%ecx
	notl	%eax
	addl	$1,%eax
	adcl	$0,%ecx

sqrt_stage_2_finish:
	sarl	$1,%ecx		/* divide by 2 */
	rcrl	$1,%eax

	/* Form the new estimate in %esi:%edi */
	movl	%eax,%edi
	addl	%ecx,%esi

	jnz	sqrt_stage_2_done	/* result should be [1..2) */

#ifdef PARANOID
/* It should be possible to get here only if the arg is ffff....ffff*/
	cmp	$0xffffffff,fsqrt_arg_1
	jnz	sqrt_stage_2_error
#endif PARANOID

/* The best rounded result.*/
	xorl	%eax,%eax
	decl	%eax
	movl	%eax,%edi
	movl	%eax,%esi
	movl	$0x7fffffff,%eax
	jmp	sqrt_round_result

#ifdef PARANOID
sqrt_stage_2_error:
	pushl	EX_INTERNAL|0x213
	call	EXCEPTION
#endif PARANOID

sqrt_stage_2_done:

/* Now the square root has been computed to better than 60 bits */

/* Find the square of the guess*/
	movl	%edi,%eax		/* ls word of guess*/
	mull	%edi
	movl	%edx,accum_1

	movl	%esi,%eax
	mull	%esi
	movl	%edx,accum_3
	movl	%eax,accum_2

	movl	%edi,%eax
	mull	%esi
	addl	%eax,accum_1
	adcl	%edx,accum_2
	adcl	$0,accum_3

/*	movl	%esi,%eax*/
/*	mull	%edi*/
	addl	%eax,accum_1
	adcl	%edx,accum_2
	adcl	$0,accum_3

/* guess^2 now in accum_3:accum_2:accum_1*/

	movl	fsqrt_arg_0,%eax		/* get normalized n*/
	subl	%eax,accum_1
	movl	fsqrt_arg_1,%eax
	sbbl	%eax,accum_2
	movl	fsqrt_arg_2,%eax		/* ms word of normalized n*/
	sbbl	%eax,accum_3
	jnc	sqrt_stage_3_positive

/* subtraction gives a negative result*/
/* negate the result before division */
	notl	accum_1
	notl	accum_2
	notl	accum_3
	addl	$1,accum_1
	adcl	$0,accum_2

#ifdef PARANOID
	adcl	$0,accum_3	/* This must be zero */
	jz	sqrt_stage_3_no_error

sqrt_stage_3_error:
	pushl	EX_INTERNAL|0x207
	call	EXCEPTION

sqrt_stage_3_no_error:
#endif PARANOID

	movl	accum_2,%edx
	movl	accum_1,%eax
	divl	%esi
	movl	%eax,%ecx

	movl	%edx,%eax
	divl	%esi

	sarl	$1,%ecx		/ divide by 2*/
	rcrl	$1,%eax

	/* prepare to round the result*/

	addl	%ecx,%edi
	adcl	$0,%esi

	jmp	sqrt_stage_3_finished

sqrt_stage_3_positive:
	movl	accum_2,%edx
	movl	accum_1,%eax
	divl	%esi
	movl	%eax,%ecx

	movl	%edx,%eax
	divl	%esi

	sarl	$1,%ecx		/* divide by 2*/
	rcrl	$1,%eax

	/* prepare to round the result*/

	notl	%eax		/* Negate the correction term*/
	notl	%ecx
	addl	$1,%eax
	adcl	$0,%ecx		/* carry here ==> correction == 0*/
	adcl	$0xffffffff,%esi

	addl	%ecx,%edi
	adcl	$0,%esi

sqrt_stage_3_finished:

/* The result in %esi:%edi:%esi should be good to about 90 bits here,
// and the rounding information here does not have sufficient accuracy
// in a few rare cases. */
	cmpl	$0xffffffe0,%eax
	ja	sqrt_near_exact_x

	cmpl	$0x00000020,%eax
	jb	sqrt_near_exact

	cmpl	$0x7fffffe0,%eax
	jb	sqrt_round_result

	cmpl	$0x80000020,%eax
	jb	sqrt_get_more_precision

sqrt_round_result:
/* Set up for rounding operations*/
	movl	%eax,%edx
	movl	%esi,%eax
	movl	%edi,%ebx
	movl	PARAM1,%edi
	movl	EXP_BIAS,EXP(%edi)	/* Result is in  [1.0 .. 2.0)*/
	movl	PARAM2,%ecx
	jmp	FPU_round_sqrt


sqrt_near_exact_x:
/* First, the estimate must be rounded up.*/
	addl	$1,%edi
	adcl	$0,%esi

sqrt_near_exact:
/* This is an easy case because x^1/2 is monotonic.
// We need just find the square of our estimate, compare it
// with the argument, and deduce whether our estimate is
// above, below, or exact. We use the fact that the estimate
// is known to be accurate to about 90 bits. */
	movl	%edi,%eax		/* ls word of guess*/
	mull	%edi
	movl	%edx,%ebx		/* 2nd ls word of square*/
	movl	%eax,%ecx		/* ls word of square*/

	movl	%edi,%eax
	mull	%esi
	addl	%eax,%ebx
	addl	%eax,%ebx

#ifdef PARANOID
	cmp	$0xffffffb0,%ebx
	jb	sqrt_near_exact_ok

	cmp	$0x00000050,%ebx
	ja	sqrt_near_exact_ok

	pushl	EX_INTERNAL|0x214
	call	EXCEPTION

sqrt_near_exact_ok:
#endif PARANOID

	or	%ebx,%ebx
	js	sqrt_near_exact_small

	jnz	sqrt_near_exact_large

	or	%ebx,%edx
	jnz	sqrt_near_exact_large

/* Our estimate is exactly the right answer*/
	xorl	%eax,%eax
	jmp	sqrt_round_result

sqrt_near_exact_small:
/* Our estimate is too small*/
	movl	$0x000000ff,%eax
	jmp	sqrt_round_result
	
sqrt_near_exact_large:
/* Our estimate is too large, we need to decrement it*/
	subl	$1,%edi
	sbbl	$0,%esi
	movl	$0xffffff00,%eax
	jmp	sqrt_round_result


sqrt_get_more_precision:
/* This case is almost the same as the above, except we start*/
/* with an extra bit of precision in the estimate.*/
	stc			/* The extra bit.*/
	rcll	$1,%edi		/* Shift the estimate left one bit*/
	rcll	$1,%esi

	movl	%edi,%eax		/* ls word of guess*/
	mull	%edi
	movl	%edx,%ebx		/* 2nd ls word of square*/
	movl	%eax,%ecx		/* ls word of square*/

	movl	%edi,%eax
	mull	%esi
	addl	%eax,%ebx
	addl	%eax,%ebx

/* Put our estimate back to its original value*/
	stc			/* The ms bit.*/
	rcrl	$1,%esi		/* Shift the estimate left one bit*/
	rcrl	$1,%edi

#ifdef PARANOID
	cmp	$0xffffff60,%ebx
	jb	sqrt_more_prec_ok

	cmp	$0x000000a0,%ebx
	ja	sqrt_more_prec_ok

	pushl	EX_INTERNAL|0x215
	call	EXCEPTION

sqrt_more_prec_ok:
#endif PARANOID

	or	%ebx,%ebx
	js	sqrt_more_prec_small

	jnz	sqrt_more_prec_large

	or	%ebx,%ecx
	jnz	sqrt_more_prec_large

/* Our estimate is exactly the right answer*/
	movl	$0x80000000,%eax
	jmp	sqrt_round_result

sqrt_more_prec_small:
/* Our estimate is too small*/
	movl	$0x800000ff,%eax
	jmp	sqrt_round_result
	
sqrt_more_prec_large:
/* Our estimate is too large*/
	movl	$0x7fffff00,%eax
	jmp	sqrt_round_result