RFR 8057793 BigDecimal is no longer effectively immutable
Brian Burkhalter
brian.burkhalter at oracle.com
Fri Sep 12 23:54:05 UTC 2014
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;
>> }
>> }
>
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