RFR: 8264268: Don't use oop types for derived pointers

[Stefan Karlsson]

The JIT compiler embeds pointers to addresses within an object. These are called derived pointers. When the GC moves objects, these pointers need to be updated explicitly, because the GC only deals with the real oops of the objects (base pointer).

The code that deals with this uses oop* for the address containing the base pointer. This is fine, the address contains an oop. However, it also uses oop* for the interior pointer, even though the contents is not a valid oop.

This creates temporary oops that does not conform to the normal requirements for oops. For example, the lower three bits could be set. This makes it problematic to write stricter verification code.

I propose that we use intptr_t* instead of oop*, and only use oop* when the location is known to contain a valid oop.

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Commit messages:
 - 8264268: Don't use oop types for derived pointers

Changes: https://git.openjdk.java.net/jdk/pull/3214/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3214&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264268
  Stats: 51 lines in 5 files changed: 7 ins; 6 del; 38 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3214.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3214/head:pull/3214

PR: https://git.openjdk.java.net/jdk/pull/3214