[aarch64-port-dev ] RFR(M): 8233743: AArch64: Make r27 conditionally allocatable

[Nick Gasson]

Hi Andrew,

>>
>> CompressedKlassPointers::base() => 0xffff0b4b5000
>> CompressedKlassPointers::shift() => 3
> 
> This is bad. Can you have a look at the allocation code to see why the search
> for an appropriate address range fails?
> 

We have a loop in Metaspace::allocate_metaspace_compressed_klass_ptrs 
that searches for a 4G aligned location for the compressed class space 
on AArch64, but this search is not done if CDS is in use and the archive 
was loaded successfully, because in that case the class space has 
already been mapped (i.e. `metaspace_rs.is_reserved()' is true).

Previously it was only possible to map the CDS archive at 0x800000000. 
The compressed class base is set to the start of this region which 
happens to be 4G aligned so our MacroAssembler::load_klass optimisation 
applies and we emit the short code sequence.

With the recent change in 8231610, if the CDS archive cannot be mapped 
at that address (e.g. because of ASLR or because the heap is mapped 
there) then the CDS archive will be relocated to an arbitrary address 
decided by mmap. That's where the oddly-aligned compressed klass base 
above comes from. This causes MacroAssembler::load_klass to emit the 
inefficient sequence which then overflows the buffer for the itable stub 
(the worst-case size estimate there is wrong, which needs to be fixed 
separately).

A minimal way to reproduce this is:

$ java -XX:HeapBaseMinAddress=33G -Xshare:on -Xlog:cds=debug -version
...
[0.050s][info ][cds] CDS archive was created with max heap size = 128M, 
and the following configuration:
[0.050s][info ][cds]     narrow_klass_base = 0x0000fffec7507000, 
narrow_klass_shift = 3
...
#  guarantee(masm->pc() <= s->code_end()) failed: itable #2: overflowed 
buffer, estimated len: 180, actual len: 184, overrun: 4


I suggest we move the 4G-aligned search from 
allocate_metaspace_compressed_klass_ptrs into its own function that can 
then be called from MetaspaceShared::reserve_shared_space when 
requested_address==NULL (i.e. the fallback path when mmap at 0x800000000 
fails). If you're happy with this I'll make a patch for review?


Thanks,
Nick