ppc/gen/abs.s

   1 /*
   2  * Copyright (c) 1999 Apple Computer, Inc. All rights reserved.
   3  *
   4  * @APPLE_LICENSE_HEADER_START@
   5  *
   6  * The contents of this file constitute Original Code as defined in and
   7  * are subject to the Apple Public Source License Version 1.1 (the
   8  * "License").  You may not use this file except in compliance with the
   9  * License.  Please obtain a copy of the License at
  10  * http://www.apple.com/publicsource and read it before using this file.
  11  *
  12  * This Original Code and all software distributed under the License are
  13  * distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY KIND, EITHER
  14  * EXPRESS OR IMPLIED, AND APPLE HEREBY DISCLAIMS ALL SUCH WARRANTIES,
  15  * INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY,
  16  * FITNESS FOR A PARTICULAR PURPOSE OR NON-INFRINGEMENT.  Please see the
  17  * License for the specific language governing rights and limitations
  18  * under the License.
  19  *
  20  * @APPLE_LICENSE_HEADER_END@
  21  */
  22 /* Copyright (c) 1992,1997 NeXT Software, Inc.  All rights reserved.
  23  *
  24  *      File:   libc/gen/ppc/abs.s
  25  *      Author: Derek B Clegg, NeXT Software, Inc.
  26  *
  27  * HISTORY
  28  *  24-Jan-1997 Umesh Vaishampayan (umeshv@NeXT.com)
  29  *      Ported to PPC.
  30  *  10-Nov-92  Derek B Clegg (dclegg@next.com)
  31  *      Created.
  32  *  13-Jan-93  Derek B Clegg (dclegg@next.com)
  33  *      Optimized.
  34  *
  35  * ANSI X3.159-1989:
  36  *   int abs(int j);
  37  *
  38  * Description:
  39  *   The `abs' function computes the absolute value of an integer `j'.
  40  *   If the result cannot be represented, the behavior is undefined.
  41  * Returns:
  42  *   The `abs' function returns the absolute value.
  43  */
  44 #include <architecture/ppc/asm_help.h>
  45 #include <architecture/ppc/pseudo_inst.h>
  46
  47 /* We calculate abs(x) as
  48  *   s = x >> 31;
  49  *   y = x + s;
  50  *   return y ^ s;
  51  *
  52  * If x >= 0, then s = 0, so clearly we return x.  On the other hand, if
  53  * x < 0, then we may write x as ~z + 1, where z = -x.  In this case,
  54  * s = -1, so y = x - 1 = ~z, and hence we return -1 ^ (x - 1) = -1 ^ ~z
  55  * = z = -x.
  56  */
  57 LEAF(_abs)
  58         srawi   a1,a0,31
  59         add     a2,a1,a0
  60         xor     a0,a2,a1
  61         blr
  62 END(_abs)