RFR: JDK-8301224: Replace NULL with nullptr in share/gc/shared/ [v3]

[Johan Sjölen]

> Hi, this PR changes all occurrences of NULL to nullptr for the subdirectory share/gc/shared/. Unfortunately the script that does the change isn't perfect, and so we
> need to comb through these manually to make sure nothing has gone wrong. I also review these changes but things slip past my eyes sometimes.
> 
> Here are some typical things to look out for:
> 
> 1. No changes but copyright header changed (probably because I reverted some changes but forgot the copyright).
> 2. Macros having their NULL changed to nullptr, these are added to the script when I find them. They should be NULL.
> 3. nullptr in comments and logs. We try to use lower case "null" in these cases as it reads better. An exception is made when code expressions are in a comment.
> 
> An example of this:
> 
> ```c++
> // This function returns null
> void* ret_null();
> // This function returns true if *x == nullptr
> bool is_nullptr(void** x);
> 
> 
> Note how `nullptr` participates in a code expression here, we really are talking about the specific value `nullptr`.
> 
> Thanks!

Johan Sjölen has updated the pull request incrementally with one additional commit since the last revision:

  Another fix

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/12249/files
  - new: https://git.openjdk.org/jdk/pull/12249/files/64b6ba01..8feb47e2

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12249&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12249&range=01-02

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/12249.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12249/head:pull/12249

PR: https://git.openjdk.org/jdk/pull/12249