RFR: 8263002: Remove CDS MiscCode region

[Coleen Phillimore]

On Sun, 7 Mar 2021 06:26:00 GMT, Ioi Lam <iklam at openjdk.org> wrote:

> The CDS MiscCode region is used for:
> (a) C++ vtables
> (b) Method trampolines
> 
> (a) can be moved to the ReadWrite region
> (b) were introduced in JDK-8145221 so we can delay writing into Methods. This was intended to improve copy-on-write sharing to reduce memory footprint. However, this hasn't been shown to have any significant effect (footprint of metadata usually is much smaller than the Java heap), and introduces a lot of complexity in the HotSpot code.
> 
> Removing (b) will make it easier to implement JDK-8026297 (Generating AdapterHandlerEntry during CDS dump), which will further improve start-up time.
> 
> ============
> Other benefits of removing the MiscCode region:
> 
> - We no longer have a read/write/executable region. This address the concern in JDK-8262922.
> - We can enable CDS on macOS/AArch64, which does not allow read/write/executable regions. (JDK-8253795)

wow.  Looks good to me.

src/hotspot/share/memory/archiveBuilder.cpp line 831:

> 829:       if (STATIC_DUMP) {
> 830:         if (!_builder->is_in_buffer_space(*p)) {
> 831:           tty->print_cr("ohashii %p %p", p, *p);

What is ohashii ?  Should this be an assert?  Or log_error instead of tty->print

src/hotspot/share/memory/cppVtables.cpp line 218:

> 216:   assert(DumpSharedSpaces, "must");
> 217:   size_t vtptrs_bytes = _num_cloned_vtable_kinds * sizeof(CppVtableInfo*);
> 218:   _index = (CppVtableInfo**)ArchiveBuilder::current()->rw_region()->allocate(vtptrs_bytes);

I thought these would be read-only?

src/hotspot/share/memory/dumpAllocStats.cpp line 106:

> 104: 
> 105:   assert(all_ro_bytes == ro_all, "everything should have been counted");
> 106: //assert(all_rw_bytes == rw_all, "everything should have been counted"); FIXME!

? Does this need to be fixed?

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Marked as reviewed by coleenp (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/2861