reg_round.s

LINUX 1.0 内核c源代码

	.file "reg_round.S"
/*---------------------------------------------------------------------------+
 |  reg_round.S                                                              |
 |                                                                           |
 | Rounding/truncation/etc for FPU basic arithmetic functions.               |
 |                                                                           |
 | Copyright (C) 1993                                                        |
 |                       W. Metzenthen, 22 Parker St, Ormond, Vic 3163,      |
 |                       Australia.  E-mail   [e-mail]    |
 |                                                                           |
 | This code has four possible entry points.                                 |
 | The following must be entered by a jmp intruction:                        |
 |   fpu_reg_round, fpu_reg_round_sqrt, and fpu_Arith_exit.                  |
 |                                                                           |
 | The _round_reg entry point is intended to be used by C code.              |
 | From C, call as:                                                          |
 | void round_reg(FPU_REG *arg, unsigned int extent, unsigned int control_w) |
 |                                                                           |
 | For correct "up" and "down" rounding, the argument must have the correct  |
 | sign.                                                                     |
 |                                                                           |
 +---------------------------------------------------------------------------*/

/*---------------------------------------------------------------------------+
 | Four entry points.                                                        |
 |                                                                           |
 | Needed by both the fpu_reg_round and fpu_reg_round_sqrt entry points:     |
 |  %eax:%ebx  64 bit significand                                            |
 |  %edx       32 bit extension of the significand                           |
 |  %edi       pointer to an FPU_REG for the result to be stored             |
 |  stack      calling function must have set up a C stack frame and         |
 |             pushed %esi, %edi, and %ebx                                   |
 |                                                                           |
 | Needed just for the fpu_reg_round_sqrt entry point:                       |
 |  %cx  A control word in the same format as the FPU control word.          |
 | Otherwise, PARAM4 must give such a value.                                 |
 |                                                                           |
 |                                                                           |
 | The significand and its extension are assumed to be exact in the          |
 | following sense:                                                          |
 |   If the significand by itself is the exact result then the significand   |
 |   extension (%edx) must contain 0, otherwise the significand extension    |
 |   must be non-zero.                                                       |
 |   If the significand extension is non-zero then the significand is        |
 |   smaller than the magnitude of the correct exact result by an amount     |
 |   greater than zero and less than one ls bit of the significand.          |
 |   The significand extension is only required to have three possible       |
 |   non-zero values:                                                        |
 |       less than 0x80000000  <=> the significand is less than 1/2 an ls    |
 |                                 bit smaller than the magnitude of the     |
 |                                 true exact result.                        |
 |         exactly 0x80000000  <=> the significand is exactly 1/2 an ls bit  |
 |                                 smaller than the magnitude of the true    |
 |                                 exact result.                             |
 |    greater than 0x80000000  <=> the significand is more than 1/2 an ls    |
 |                                 bit smaller than the magnitude of the     |
 |                                 true exact result.                        |
 |                                                                           |
 +---------------------------------------------------------------------------*/

/*---------------------------------------------------------------------------+
 |  The code in this module has become quite complex, but it should handle   |
 |  all of the FPU flags which are set at this stage of the basic arithmetic |
 |  computations.                                                            |
 |  There are a few rare cases where the results are not set identically to  |
 |  a real FPU. These require a bit more thought because at this stage the   |
 |  results of the code here appear to be more consistent...                 |
 |  This may be changed in a future version.                                 |
 +---------------------------------------------------------------------------*/


#include "fpu_asm.h"
#include "exception.h"
#include "control_w.h"

/* Flags for FPU_bits_lost */
#define	LOST_DOWN	$1
#define	LOST_UP		$2

/* Flags for FPU_denormal */
#define	DENORMAL	$1
#define	UNMASKED_UNDERFLOW $2


#ifdef REENTRANT_FPU
/*	Make the code re-entrant by putting
	local storage on the stack: */
#define FPU_bits_lost	(%esp)
#define FPU_denormal	1(%esp)

#else
/*	Not re-entrant, so we can gain speed by putting
	local storage in a static area: */
.data
	.align 2,0
FPU_bits_lost:
	.byte	0
FPU_denormal:
	.byte	0
#endif REENTRANT_FPU


.text
	.align 2,144
.globl fpu_reg_round
.globl fpu_reg_round_sqrt
.globl fpu_Arith_exit
.globl _round_reg

/* Entry point when called from C */
_round_reg:
	pushl	%ebp
	movl	%esp,%ebp
	pushl	%esi
	pushl	%edi
	pushl	%ebx

	movl	PARAM1,%edi
	movl	SIGH(%edi),%eax
	movl	SIGL(%edi),%ebx
	movl	PARAM2,%edx
	movl	PARAM3,%ecx
	jmp	fpu_reg_round_sqrt

fpu_reg_round:			/* Normal entry point */
	movl	PARAM4,%ecx

fpu_reg_round_sqrt:		/* Entry point from wm_sqrt.S */

#ifdef REENTRANT_FPU
	pushl	%ebx		/* adjust the stack pointer */
#endif REENTRANT_FPU

#ifdef PARANOID
/* Cannot use this here yet */
/*	orl	%eax,%eax */
/*	jns	L_entry_bugged */
#endif PARANOID

	cmpl	EXP_UNDER,EXP(%edi)
	jle	xMake_denorm			/* The number is a de-normal */

	movb	$0,FPU_denormal			/* 0 -> not a de-normal */

xDenorm_done:
	movb	$0,FPU_bits_lost		/* No bits yet lost in rounding */

	movl	%ecx,%esi
	andl	CW_PC,%ecx
	cmpl	PR_64_BITS,%ecx
	je	LRound_To_64

	cmpl	PR_53_BITS,%ecx
	je	LRound_To_53

	cmpl	PR_24_BITS,%ecx
	je	LRound_To_24

#ifdef PECULIAR_486
/* With the precision control bits set to 01 "(reserved)", a real 80486
   behaves as if the precision control bits were set to 11 "64 bits" */
	cmpl	PR_RESERVED_BITS,%ecx
	je	LRound_To_64
#ifdef PARANOID
	jmp	L_bugged_denorm_486
#endif PARANOID
#else
#ifdef PARANOID
	jmp	L_bugged_denorm	/* There is no bug, just a bad control word */
#endif PARANOID
#endif PECULIAR_486


/* Round etc to 24 bit precision */
LRound_To_24:
	movl	%esi,%ecx
	andl	CW_RC,%ecx
	cmpl	RC_RND,%ecx
	je	LRound_nearest_24

	cmpl	RC_CHOP,%ecx
	je	LCheck_truncate_24

	cmpl	RC_UP,%ecx		/* Towards +infinity */
	je	LUp_24

	cmpl	RC_DOWN,%ecx		/* Towards -infinity */
	je	LDown_24

#ifdef PARANOID
	jmp	L_bugged_round24
#endif PARANOID

LUp_24:
	cmpb	SIGN_POS,SIGN(%edi)
	jne	LCheck_truncate_24	/* If negative then  up==truncate */

	jmp	LCheck_24_round_up

LDown_24:
	cmpb	SIGN_POS,SIGN(%edi)
	je	LCheck_truncate_24	/* If positive then  down==truncate */

LCheck_24_round_up:
	movl	%eax,%ecx
	andl	$0x000000ff,%ecx
	orl	%ebx,%ecx
	orl	%edx,%ecx
	jnz	LDo_24_round_up
	jmp	LRe_normalise

LRound_nearest_24:
	/* Do rounding of the 24th bit if needed (nearest or even) */
	movl	%eax,%ecx
	andl	$0x000000ff,%ecx
	cmpl	$0x00000080,%ecx
	jc	LCheck_truncate_24	/* less than half, no increment needed */

	jne	LGreater_Half_24	/* greater than half, increment needed */

	/* Possibly half, we need to check the ls bits */
	orl	%ebx,%ebx
	jnz	LGreater_Half_24	/* greater than half, increment needed */

	orl	%edx,%edx
	jnz	LGreater_Half_24	/* greater than half, increment needed */

	/* Exactly half, increment only if 24th bit is 1 (round to even) */
	testl	$0x00000100,%eax
	jz	LDo_truncate_24

LGreater_Half_24:			/* Rounding: increment at the 24th bit */
LDo_24_round_up:
	andl	$0xffffff00,%eax	/* Truncate to 24 bits */
	xorl	%ebx,%ebx
	movb	LOST_UP,FPU_bits_lost
	addl	$0x00000100,%eax
	jmp	LCheck_Round_Overflow

LCheck_truncate_24:
	movl	%eax,%ecx
	andl	$0x000000ff,%ecx
	orl	%ebx,%ecx
	orl	%edx,%ecx
	jz	LRe_normalise		/* No truncation needed */

LDo_truncate_24:
	andl	$0xffffff00,%eax	/* Truncate to 24 bits */
	xorl	%ebx,%ebx
	movb	LOST_DOWN,FPU_bits_lost
	jmp	LRe_normalise


/* Round etc to 53 bit precision */
LRound_To_53:
	movl	%esi,%ecx
	andl	CW_RC,%ecx
	cmpl	RC_RND,%ecx
	je	LRound_nearest_53

	cmpl	RC_CHOP,%ecx
	je	LCheck_truncate_53

	cmpl	RC_UP,%ecx		/* Towards +infinity */
	je	LUp_53

	cmpl	RC_DOWN,%ecx		/* Towards -infinity */
	je	LDown_53

#ifdef PARANOID
	jmp	L_bugged_round53
#endif PARANOID

LUp_53:
	cmpb	SIGN_POS,SIGN(%edi)
	jne	LCheck_truncate_53	/* If negative then  up==truncate */

	jmp	LCheck_53_round_up

LDown_53:
	cmpb	SIGN_POS,SIGN(%edi)
	je	LCheck_truncate_53	/* If positive then  down==truncate */

LCheck_53_round_up:
	movl	%ebx,%ecx
	andl	$0x000007ff,%ecx
	orl	%edx,%ecx
	jnz	LDo_53_round_up
	jmp	LRe_normalise

LRound_nearest_53:
	/* Do rounding of the 53rd bit if needed (nearest or even) */
	movl	%ebx,%ecx
	andl	$0x000007ff,%ecx
	cmpl	$0x00000400,%ecx
	jc	LCheck_truncate_53	/* less than half, no increment needed */

	jnz	LGreater_Half_53	/* greater than half, increment needed */

	/* Possibly half, we need to check the ls bits */
	orl	%edx,%edx
	jnz	LGreater_Half_53	/* greater than half, increment needed */

	/* Exactly half, increment only if 53rd bit is 1 (round to even) */
	testl	$0x00000800,%ebx
	jz	LTruncate_53

LGreater_Half_53:			/* Rounding: increment at the 53rd bit */
LDo_53_round_up:
	movb	LOST_UP,FPU_bits_lost
	andl	$0xfffff800,%ebx	/* Truncate to 53 bits */
	addl	$0x00000800,%ebx
	adcl	$0,%eax
	jmp	LCheck_Round_Overflow

LCheck_truncate_53:
	movl	%ebx,%ecx
	andl	$0x000007ff,%ecx
	orl	%edx,%ecx
	jz	LRe_normalise

LTruncate_53:
	movb	LOST_DOWN,FPU_bits_lost
	andl	$0xfffff800,%ebx	/* Truncate to 53 bits */
	jmp	LRe_normalise


/* Round etc to 64 bit precision */
LRound_To_64:
	movl	%esi,%ecx
	andl	CW_RC,%ecx
	cmpl	RC_RND,%ecx
	je	LRound_nearest_64

	cmpl	RC_CHOP,%ecx
	je	LCheck_truncate_64

	cmpl	RC_UP,%ecx		/* Towards +infinity */
	je	LUp_64

	cmpl	RC_DOWN,%ecx		/* Towards -infinity */
	je	LDown_64

#ifdef PARANOID
	jmp	L_bugged_round64
#endif PARANOID

LUp_64:
	cmpb	SIGN_POS,SIGN(%edi)
	jne	LCheck_truncate_64	/* If negative then  up==truncate */