RFR 8057793 BigDecimal is no longer effectively immutable

[Brian Burkhalter]

Hello,

I created a formal webrev:

Issue:	https://bugs.openjdk.java.net/browse/JDK-8057793
Webrev:	http://cr.openjdk.java.net/~bpb/8057793/webrev.00/

Based on manual inspection of the revised code the patch looks good to me. The test submitted with the issue now succeeds as do all regression tests in jdk/test/java/math to which I also added the code from the test case in the issue report.

Note that this webrev is with respect to JDK 9.

Thanks,

Brian

On Sep 11, 2014, at 6:35 PM, Joe Darcy <joe.darcy at oracle.com> wrote:

> Hello,
> 
> Hmmm. I haven't dived into the details of the code, but setScale calls out to divide functionality so it is plausible a bug in divide could cause a problem in setScale.
> 
> Thanks for the bug report,
> 
> -Joe
> 
> On 9/9/2014 1:30 AM, Robert Gibson wrote:
>> 
>> 
>> Hi there,
>> 
>> I came across a case in BigDecimal division where the dividend ends up getting mutated, which is rather strange for a seemingly immutable class! (It's a subset of the cases where the Burnikel-Ziegler algorithm is used, I haven't done the analysis to find out under which exact conditions it's triggered.)
>> 
>> The attached patch - against the JDK8 version - should fix the problem, at the cost of an extra array copy.  Could somebody review and/or comment please?
>> 
>> Thanks,
>> Robert
>> 
>> --- MutableBigInteger.java      2014-09-04 09:42:23.426815000 +0200
>> +++ MutableBigInteger.java.patched      2014-09-04 09:46:21.344132000 +0200
>> @@ -1261,19 +1261,20 @@
>>              int sigma = (int) Math.max(0, n32 - b.bitLength());   // step 3: sigma = max{T | (2^T)*B < beta^n}
>>              MutableBigInteger bShifted = new MutableBigInteger(b);
>>              bShifted.safeLeftShift(sigma);   // step 4a: shift b so its length is a multiple of n
>> -            safeLeftShift(sigma);     // step 4b: shift this by the same amount
>> +            MutableBigInteger aShifted = new MutableBigInteger (this);
>> +            aShifted.safeLeftShift(sigma);     // step 4b: shift a by the same amount
>> -            // step 5: t is the number of blocks needed to accommodate this plus one additional bit
>> -            int t = (int) ((bitLength()+n32) / n32);
>> +            // step 5: t is the number of blocks needed to accommodate a plus one additional bit
>> +            int t = (int) ((aShifted.bitLength()+n32) / n32);
>>              if (t < 2) {
>>                  t = 2;
>>              }
>> -            // step 6: conceptually split this into blocks a[t-1], ..., a[0]
>> -            MutableBigInteger a1 = getBlock(t-1, t, n);   // the most significant block of this
>> +            // step 6: conceptually split a into blocks a[t-1], ..., a[0]
>> +            MutableBigInteger a1 = aShifted.getBlock(t-1, t, n);   // the most significant block of a
>>              // step 7: z[t-2] = [a[t-1], a[t-2]]
>> -            MutableBigInteger z = getBlock(t-2, t, n);    // the second to most significant block
>> +            MutableBigInteger z = aShifted.getBlock(t-2, t, n);    // the second to most significant block
>>              z.addDisjoint(a1, n);   // z[t-2]
>>              // do schoolbook division on blocks, dividing 2-block numbers by 1-block numbers
>> @@ -1284,7 +1285,7 @@
>>                  ri = z.divide2n1n(bShifted, qi);
>>                  // step 8b: z = [ri, a[i-1]]
>> -                z = getBlock(i-1, t, n);   // a[i-1]
>> +                z = aShifted.getBlock(i-1, t, n);   // a[i-1]
>>                  z.addDisjoint(ri, n);
>>                  quotient.addShifted(qi, i*n);   // update q (part of step 9)
>>              }
>> @@ -1292,7 +1293,7 @@
>>              ri = z.divide2n1n(bShifted, qi);
>>              quotient.add(qi);
>> -           ri.rightShift(sigma);   // step 9: this and b were shifted, so shift back
>> +           ri.rightShift(sigma);   // step 9: a and b were shifted, so shift back
>>              return ri;
>>          }
>>      }
>