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

[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.

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Commit messages:
 - 8241613: Suspicious calls to MacroAssembler::null_check(Register, offset)

Changes: https://git.openjdk.org/jdk/pull/13026/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13026&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8241613
  Stats: 3 lines in 1 file changed: 0 ins; 3 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/13026.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/13026/head:pull/13026

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