ppc/gen/abs.s

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