RFR: 8308076: X86_64: make rheapbase register allocatable in zero based compressedOops mode [v4]

[Quan Anh Mai]

On Tue, 16 May 2023 05:57:12 GMT, kuaiwei <duke at openjdk.org> wrote:

>> In x86 64 mode, decode heap oop could use SIB without base if heap base is zero. like
>> 
>> 0d1     movl    R11, [,R9 << 3 + #72] (zero base compressed oop addressing) # compressed ptr ! Field: java/lang/ClassLoader.classAssertionStatus 
>> 
>> So rheapbase( r12 )  can be allocated as general register.
>> 
>> Tier 1/2 tests are passed without new failure.
>
> kuaiwei has updated the pull request incrementally with one additional commit since the last revision:
> 
>   fix reinit_heabase and add cpu specific compressedOops.hpp

src/hotspot/cpu/x86/x86_64.ad line 382:

> 380: 
> 381: static bool need_r12_heapbase() {
> 382:   return CompressedOops::need_heapbase_reg();

Why not just use `PreserveHeapbaseReg`, this is a machine-dependent flag so I don't find it appealing to introduce a machine-independent check.

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PR Review Comment: https://git.openjdk.org/jdk/pull/13976#discussion_r1206114536