A bug in div64.s

Liu Fred-a18596 fred.liu at motorola.com
Mon Sep 12 17:21:42 EST 2005

Hello, Greetings,


I found there was a bug in arch/ppc/lib/div64.s.

The original div64_32() function would produce a zero divide exception in case of div64_32(0x100000000, 0xFFFFFFFF).


Here is the bug-fixed new div64_32()function.




 * Divide a 64-bit unsigned number by a 32-bit unsigned number.

 * This routine assumes that the top 32 bits of the dividend are

 * non-zero to start with.

 * On entry, r3 points to the dividend, which get overwritten with

 * the 64-bit quotient, and r4 contains the divisor.

 * On exit, r3 contains the remainder.


 * Copyright (C) 2002 Paul Mackerras, IBM Corp.


 * This program is free software; you can redistribute it and/or

 * modify it under the terms of the GNU General Public License

 * as published by the Free Software Foundation; either version

 * 2 of the License, or (at your option) any later version.


#include <asm/ppc_asm.h>

#include <asm/processor.h>



               lwz          r5,0(r3)   # get the dividend into r5/r6

               lwz          r6,4(r3)

               cmplw    r5,r4

               li              r7,0

               li              r8,0

               blt           1f

               divwu      r7,r5,r4   # if dividend.hi >= divisor,

               mullw      r0,r7,r4   # quotient.hi = dividend.hi / divisor

               subf.       r5,r0,r5   # dividend.hi %= divisor

               beq         3f

1:            mr           r11,r5                     # here dividend.hi != 0

               andis.      r0,r5,0xc000

               bne         2f

               cntlzw     r0,r5                       # we are shifting the dividend right

               li              r10,-1                     # to make it < 2^32, and shifting

               srw         r10,r10,r0              # the divisor right the same amount,

               addc.      r9,r4,r10 # rounding up (so the estimate cannot

               andc       r11,r6,r10              # ever be too large, only too small)

               andc       r9,r9,r10

               mfxer   r12                    #

               rlwinm. r12, r12, 3, 31, 31    # XER[CA]

               cmpwi   r12, 1                 # carry ?

               bne     5f

               ori     r9, r9, 0x1            # carry !


               or            r11,r5,r11

               rotlw       r9,r9,r0

               rotlw       r11,r11,r0

               divwu      r11,r11,r9              # then we divide the shifted quantities

2:            mullw      r10,r11,r4              # to get an estimate of the quotient,

               mulhwu  r9,r11,r4 # multiply the estimate by the divisor,

               subfc      r6,r10,r6 # take the product from the divisor,

               add         r8,r8,r11 # and add the estimate to the accumulated

               subfe.     r5,r9,r5   # quotient

               bne         1b

3:            cmplw    r6,r4

               blt           4f

               divwu      r0,r6,r4   # perform the remaining 32-bit division

               mullw      r10,r0,r4 # and get the remainder

               add         r8,r8,r0

               subf        r6,r10,r6

4:            stw          r7,0(r3)   # return the quotient in *r3

               stw          r8,4(r3)

               mr           r3,r6                       # return the remainder in r3







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