arch/s390/lib/div64.c

   1 /*
   2  *  arch/s390/lib/div64.c
   3  *
   4  *  __div64_32 implementation for 31 bit.
   5  *
   6  *    Copyright (C) IBM Corp. 2006
   7  *    Author(s): Martin Schwidefsky (schwidefsky@de.ibm.com),
   8  */
   9
  10 #include <linux/types.h>
  11 #include <linux/module.h>
  12
  13 #ifdef CONFIG_MARCH_G5
  14
  15 /*
  16  * Function to divide an unsigned 64 bit integer by an unsigned
  17  * 31 bit integer using signed 64/32 bit division.
  18  */
  19 static uint32_t __div64_31(uint64_t *n, uint32_t base)
  20 {
  21         register uint32_t reg2 asm("2");
  22         register uint32_t reg3 asm("3");
  23         uint32_t *words = (uint32_t *) n;
  24         uint32_t tmp;
  25
  26         /* Special case base==1, remainder = 0, quotient = n */
  27         if (base == 1)
  28                 return 0;
  29         /*
  30          * Special case base==0 will cause a fixed point divide exception
  31          * on the dr instruction and may not happen anyway. For the
  32          * following calculation we can assume base > 1. The first
  33          * signed 64 / 32 bit division with an upper half of 0 will
  34          * give the correct upper half of the 64 bit quotient.
  35          */
  36         reg2 = 0UL;
  37         reg3 = words[0];
  38         asm volatile(
  39                 "       dr      %0,%2\n"
  40                 : "+d" (reg2), "+d" (reg3) : "d" (base) : "cc" );
  41         words[0] = reg3;
  42         reg3 = words[1];
  43         /*
  44          * To get the lower half of the 64 bit quotient and the 32 bit
  45          * remainder we have to use a little trick. Since we only have
  46          * a signed division the quotient can get too big. To avoid this
  47          * the 64 bit dividend is halved, then the signed division will
  48          * work. Afterwards the quotient and the remainder are doubled.
  49          * If the last bit of the dividend has been one the remainder
  50          * is increased by one then checked against the base. If the
  51          * remainder has overflown subtract base and increase the
  52          * quotient. Simple, no ?
  53          */
  54         asm volatile(
  55                 "       nr      %2,%1\n"
  56                 "       srdl    %0,1\n"
  57                 "       dr      %0,%3\n"
  58                 "       alr     %0,%0\n"
  59                 "       alr     %1,%1\n"
  60                 "       alr     %0,%2\n"
  61                 "       clr     %0,%3\n"
  62                 "       jl      0f\n"
  63                 "       slr     %0,%3\n"
  64                 "       alr     %1,%2\n"
  65                 "0:\n"
  66                 : "+d" (reg2), "+d" (reg3), "=d" (tmp)
  67                 : "d" (base), "2" (1UL) : "cc" );
  68         words[1] = reg3;
  69         return reg2;
  70 }
  71
  72 /*
  73  * Function to divide an unsigned 64 bit integer by an unsigned
  74  * 32 bit integer using the unsigned 64/31 bit division.
  75  */
  76 uint32_t __div64_32(uint64_t *n, uint32_t base)
  77 {
  78         uint32_t r;
  79
  80         /*
  81          * If the most significant bit of base is set, divide n by
  82          * (base/2). That allows to use 64/31 bit division and gives a
  83          * good approximation of the result: n = (base/2)*q + r. The
  84          * result needs to be corrected with two simple transformations.
  85          * If base is already < 2^31-1 __div64_31 can be used directly.
  86          */
  87         r = __div64_31(n, ((signed) base < 0) ? (base/2) : base);
  88         if ((signed) base < 0) {
  89                 uint64_t q = *n;
  90                 /*
  91                  * First transformation:
  92                  * n = (base/2)*q + r
  93                  *   = ((base/2)*2)*(q/2) + ((q&1) ? (base/2) : 0) + r
  94                  * Since r < (base/2), r + (base/2) < base.
  95                  * With q1 = (q/2) and r1 = r + ((q&1) ? (base/2) : 0)
  96                  * n = ((base/2)*2)*q1 + r1 with r1 < base.
  97                  */
  98                 if (q & 1)
  99                         r += base/2;
 100                 q >>= 1;
 101                 /*
 102                  * Second transformation. ((base/2)*2) could have lost the
 103                  * last bit.
 104                  * n = ((base/2)*2)*q1 + r1
 105                  *   = base*q1 - ((base&1) ? q1 : 0) + r1
 106                  */
 107                 if (base & 1) {
 108                         int64_t rx = r - q;
 109                         /*
 110                          * base is >= 2^31. The worst case for the while
 111                          * loop is n=2^64-1 base=2^31+1. That gives a
 112                          * maximum for q=(2^64-1)/2^31 = 0x1ffffffff. Since
 113                          * base >= 2^31 the loop is finished after a maximum
 114                          * of three iterations.
 115                          */
 116                         while (rx < 0) {
 117                                 rx += base;
 118                                 q--;
 119                         }
 120                         r = rx;
 121                 }
 122                 *n = q;
 123         }
 124         return r;
 125 }
 126
 127 #else /* MARCH_G5 */
 128
 129 uint32_t __div64_32(uint64_t *n, uint32_t base)
 130 {
 131         register uint32_t reg2 asm("2");
 132         register uint32_t reg3 asm("3");
 133         uint32_t *words = (uint32_t *) n;
 134
 135         reg2 = 0UL;
 136         reg3 = words[0];
 137         asm volatile(
 138                 "       dlr     %0,%2\n"
 139                 : "+d" (reg2), "+d" (reg3) : "d" (base) : "cc" );
 140         words[0] = reg3;
 141         reg3 = words[1];
 142         asm volatile(
 143                 "       dlr     %0,%2\n"
 144                 : "+d" (reg2), "+d" (reg3) : "d" (base) : "cc" );
 145         words[1] = reg3;
 146         return reg2;
 147 }
 148
 149 #endif /* MARCH_G5 */
 150
 151 EXPORT_SYMBOL(__div64_32);