RFR: 8281213: Unsafe uses of long and size_t in MemReporterBase::diff_in_current_scale [v3]

[Kim Barrett]

On Sat, 24 Dec 2022 06:33:19 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

>> Afshin Zafari has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   8281213: Unsafe uses of long and size_t in MemReporterBase::diff_in_current_scale
>
> src/hotspot/share/services/memReporter.hpp line 80:
> 
>> 78: 
>> 79:     size_t amount = s1 - s2;
>> 80:     assert(amount <= SIZE_MAX - _scale / 2, "size_t overflow");
> 
> This assert can fail for sufficiently large `amount`, even though the desired
> value is computable.  The value we want is
> 
> size_t scaled = (amount + _scale/2) / _scale;
> 
> rounded to nearest, without risk of overflow. We can split `amount` into `p + q`,
> where `q = amount % _scale` and `p = amount - q` (which is also `(amount / _scale) * _scale`).
> Then use
> 
> size_t scaled = (p + q + _scale/2) / _scale;
> 
> =>
> 
> size_t scaled = (p / _scale) + ((q + _scale/2) / _scale);
> 
> The lefthand side of the addition is exact.
> The righthand side is 0 if `q <= (_scale - 1)/2`, else 1. (The -1 is to
> account for odd _scale values.)  So
> 
> size_t scaled = (amount / _scale);
> if ((amount % _scale) <= (_scale - 1)/2) {
>   scaled += 1;
> }

And after all that I wrote the conditional backward.  That should be `(amount % _scale) > (_scale - 1)/2`.

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PR: https://git.openjdk.org/jdk/pull/11514