ppc/gen/abs.s

   1 /*
   2  * Copyright (c) 1999 Apple Computer, Inc. All rights reserved.
   3  *
   4  * @APPLE_LICENSE_HEADER_START@
   5  *
   6  * Copyright (c) 1999-2003 Apple Computer, Inc.  All Rights Reserved.
   7  *
   8  * This file contains Original Code and/or Modifications of Original Code
   9  * as defined in and that are subject to the Apple Public Source License
  10  * Version 2.0 (the 'License'). You may not use this file except in
  11  * compliance with the License. Please obtain a copy of the License at
  12  * http://www.opensource.apple.com/apsl/ and read it before using this
  13  * file.
  14  *
  15  * The Original Code and all software distributed under the License are
  16  * distributed on an 'AS IS' basis, WITHOUT WARRANTY OF ANY KIND, EITHER
  17  * EXPRESS OR IMPLIED, AND APPLE HEREBY DISCLAIMS ALL SUCH WARRANTIES,
  18  * INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY,
  19  * FITNESS FOR A PARTICULAR PURPOSE, QUIET ENJOYMENT OR NON-INFRINGEMENT.
  20  * Please see the License for the specific language governing rights and
  21  * limitations under the License.
  22  *
  23  * @APPLE_LICENSE_HEADER_END@
  24  */
  25 /* Copyright (c) 1992,1997 NeXT Software, Inc.  All rights reserved.
  26  *
  27  *      File:   libc/gen/ppc/abs.s
  28  *      Author: Derek B Clegg, NeXT Software, Inc.
  29  *
  30  * HISTORY
  31  *  24-Jan-1997 Umesh Vaishampayan (umeshv@NeXT.com)
  32  *      Ported to PPC.
  33  *  10-Nov-92  Derek B Clegg (dclegg@next.com)
  34  *      Created.
  35  *  13-Jan-93  Derek B Clegg (dclegg@next.com)
  36  *      Optimized.
  37  *
  38  * ANSI X3.159-1989:
  39  *   int abs(int j);
  40  *
  41  * Description:
  42  *   The `abs' function computes the absolute value of an integer `j'.
  43  *   If the result cannot be represented, the behavior is undefined.
  44  * Returns:
  45  *   The `abs' function returns the absolute value.
  46  */
  47 #include <architecture/ppc/asm_help.h>
  48 #include <architecture/ppc/pseudo_inst.h>
  49
  50 /* We calculate abs(x) as
  51  *   s = x >> 31;
  52  *   y = x + s;
  53  *   return y ^ s;
  54  *
  55  * If x >= 0, then s = 0, so clearly we return x.  On the other hand, if
  56  * x < 0, then we may write x as ~z + 1, where z = -x.  In this case,
  57  * s = -1, so y = x - 1 = ~z, and hence we return -1 ^ (x - 1) = -1 ^ ~z
  58  * = z = -x.
  59  */
  60 LEAF(_abs)
  61         srawi   a1,a0,31
  62         add     a2,a1,a0
  63         xor     a0,a2,a1
  64         blr
  65 END(_abs)