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

[Afshin Zafari]

> ### 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 incrementally with one additional commit since the last revision:

  8281213: Unsafe uses of long and size_t in MemReporterBase::diff_in_current_scale

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/11514/files
  - new: https://git.openjdk.org/jdk/pull/11514/files/e147bee4..d9528485

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11514&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11514&range=01-02

  Stats: 22 lines in 1 file changed: 0 ins; 12 del; 10 mod
  Patch: https://git.openjdk.org/jdk/pull/11514.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11514/head:pull/11514

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