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fix the arithmetic of multi-word division. 

The Multiword division was not handling some degenerate high
guesses for the intermediate division result guess. The end result
was an assertion. Recover from this case.

(Does the addinb back of bp need to be optimized better?)
(cherry picked from commit 6715426833)
This commit is contained in:
Stephen Williams 2009-03-13 21:59:44 -07:00
parent ef12137931
commit c47ea0721c
1 changed files with 15 additions and 9 deletions
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24
vvp/vthread.cc
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@ -1974,7 +1974,8 @@ static unsigned long divide2words(unsigned long a, unsigned long b,
static unsigned long* divide_bits(unsigned long*ap, unsigned long*bp, unsigned wid)
{
// Do all our work a cpu-word at a time. The "words" variable
// is the number of words of the wid.
unsigned words = (wid+CPU_WORD_BITS-1) / CPU_WORD_BITS;
unsigned btop = words-1;
@ -1985,6 +1986,8 @@ static unsigned long* divide_bits(unsigned long*ap, unsigned long*bp, unsigned w
if (btop==0 && bp[0]==0)
return 0;
// The result array will eventually accumulate the result. The
// diff array is a difference that we use in the intermediate.
unsigned long*diff = new unsigned long[words];
unsigned long*result= new unsigned long[words];
for (unsigned idx = 0 ; idx < words ; idx += 1)
@ -2012,10 +2015,11 @@ static unsigned long* divide_bits(unsigned long*ap, unsigned long*bp, unsigned w
// cur_res is a guestimate of the result this far. It
// may be 1 too big. (But it will also be >0) Try it,
// and if the difference comes out negative, then adjust
// then.
// and if the difference comes out negative, then adjust.
// diff = (bp * cur_res) << cur_ptr;
multiply_array_imm(diff+cur_ptr, bp, words-cur_ptr, cur_res);
// ap -= diff
unsigned long carry = 1;
for (unsigned idx = cur_ptr ; idx < words ; idx += 1)
ap[idx] = add_with_carry(ap[idx], ~diff[idx], carry);
@ -2027,12 +2031,14 @@ static unsigned long* divide_bits(unsigned long*ap, unsigned long*bp, unsigned w
// case, we know that cur_res was too large by 1. Correct by
// adding 1b back in and reducing cur_res.
if ((carry&1) == 0) {
cur_res -= 1;
carry = 0;
for (unsigned idx = cur_ptr ; idx < words ; idx += 1)
ap[idx] = add_with_carry(ap[idx], bp[idx-cur_ptr], carry);
// The sign *must* have changed again.
assert(carry == 1);
// Keep adding b back in until the remainder
// becomes positive again.
do {
cur_res -= 1;
carry = 0;
for (unsigned idx = cur_ptr ; idx < words ; idx += 1)
ap[idx] = add_with_carry(ap[idx], bp[idx-cur_ptr], carry);
} while (carry == 0);
}
result[cur_ptr] = cur_res;