RFR: 8241613: Suspicious calls to MacroAssembler::null_check(Register, offset) [v6]

[Matias Saavedra Silva]

> In several places in HotSpot, the method MacroAssembler::null_check(Register, offset) is called in a way that never produces any null check in the assembly code. The method null_check(Register, offset) calls needs_explicit_null_check(offset) to determine if it must emit a null check in the assembly code or not.
> 
> needs_explicit_null_check(offset) returns true only if the offset is negative or bigger than the os page size. 
> the offset being passed is the offset of a field in the header of Java object or a Java array. In both cases, the offset is always positive and smaller than an os page size. A null_check() call with a single parameter will always produce a null check in assembly.
> 
> The cases suggested in the issue have been addressed by either removing or preserving the null_check. Verified with tier 1-3 tests.

Matias Saavedra Silva has updated the pull request incrementally with one additional commit since the last revision:

  Added global assert

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Changes:
  - all: https://git.openjdk.org/jdk/pull/13026/files
  - new: https://git.openjdk.org/jdk/pull/13026/files/c6d7947e..5e48eb99

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13026&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13026&range=04-05

  Stats: 32 lines in 13 files changed: 6 ins; 26 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/13026.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13026/head:pull/13026

PR: https://git.openjdk.org/jdk/pull/13026