On Wed, 17 Jul 2024 13:48:59 GMT, fabioromano1 <d...@openjdk.org> wrote:
>> src/java.base/share/classes/java/math/MutableBigInteger.java line 1978:
>>
>>> 1976: * is either correct, or rounded up by one if the value
>>> is too high
>>> 1977: * and too close to the next perfect square.
>>> 1978: */
>>
>> Contrary to my previous believe and own experiments, I now think this code
>> is incorrect.
>>
>> Let `long t = 3037000503L` and `long x = t * t`. The code computes `long s
>> == 3037000502L`, an underestimate of the correct square root `t` by 1.
>> Underestimates are neither detected nor corrected.
>> Of course, the corresponding remainder `long r = x - s * s`, namely `r =
>> 6074001005L`, is just barely too large as it does _not_ meet `r <= 2 * s`.
>
> In fact, if you run this code:
> `long limit = 1L << 32;
> for (long n = 0; n < limit; n++) {
> long x = n * n;
> if (n != (long) Math.sqrt(x >= 0 ? x : x + 0x1p64)) {
> System.out.println(n);
> }
> }`
>
> now you find a lot of counterexamples. The question is: why, until recently,
> if I did run the same code I could not find a counterexample?
I hope these errors are not due to an implementation change in the virtual
machine instructions...
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PR Review Comment: https://git.openjdk.org/jdk/pull/19710#discussion_r1681105179
