RFR: 8281213: Unsafe uses of long and size_t in MemReporterBase::diff_in_current_scale [v4]

[Evgeny Astigeevich]

On Tue, 10 Jan 2023 11:27:43 GMT, Afshin Zafari <duke at openjdk.org> wrote:

>> ### Description
>> MemReporterBase::diff_in_current_scale is defined as follows:
>> 
>>   inline long diff_in_current_scale(size_t s1, size_t s2) const {
>>     long amount = (long)(s1 - s2);
>>     long scale = (long)_scale;
>>     amount = (amount > 0) ? (amount + scale / 2) : (amount - scale / 2);
>>     return amount / scale;
>>   }
>> 
>> Long and size_t can have different sizes: 4 bytes and 8 bytes (LLP64). The result of 's1 - s2' might not fit into long. It might not fit into int64_t. For example: s1 is SIZE_MAX and s2 is SIZE_MAX-MAX_INT64-1.
>> 
>> Size_t should be used instead of long. Assertions must be added to check:
>> s1 >= s2 and (amount - scale/2) >= 0 and (amount + scale/2) <= SIZE_MAX.
>> 
>> ### Patch
>> `long` is replaced by `size_t`. Comparison to 0 is implemented accordingly since size_t is always >= 0.
>> Since s1 can be less than s2 in some invocations of this method, no assert is written for `(s1 >= s2)` case.
>> 
>> ### Test
>> local: runtime/NMT/Jcmd*
>> mach5: tier1
>
> Afshin Zafari has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains seven additional commits since the last revision:
> 
>  - 8281213: Unsafe uses of long and size_t in MemReporterBase::diff_in_current_scale
>  - Merge branch 'master' into _8281213
>  - 8281213: Unsafe uses of long and size_t in MemReporterBase::diff_in_current_scale
>  - 8281213: Unsafe uses of long and size_t in MemReporterBase::diff_in_current_scale
>  - 8281213: Unsafe uses of long and size_t in MemReporterBase::diff_in_current_scale
>  - 8281213: Unsafe uses of long and size_t in MemReporterBase::diff_in_current_scale
>  - 8281213: Unsafe uses of long and size_t in MemReporterBase::diff_in_current_scale

src/hotspot/share/services/memReporter.hpp line 94:

> 92:       return 0;
> 93:     }
> 94:     assert(scaled <= INT64_MAX, "overflow");

It looks like you have decided to support values in range [-INT64_MAX, INT64_MAX] not in [INT64_MIN, INT64_MAX]. I am OK with this.
If I am correct, could you please comment your decision in the code?

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