[riscv-port] RFR: 8278994: riscv: RVC support [v10]

[Xiaolin Zheng]

On Mon, 10 Jan 2022 06:46:59 GMT, Fei Yang <fyang at openjdk.org> wrote:

>> Yes - it seems a little obscure though.
>> 
>> 1. The background is that:
>> 
>> about the `nmethod_entry_barrier`:
>> 
>> nmethod_entry_barrier (begin) {   <-  when we reach here, we may get a 2-byte alignment because of RVC's existence.
>>    ... 
>>    amoswap                            
>>    <the swapped address in the code segment>    <-  though the size of the whole stub is a multiple of 4 currently, 
>>                                                     here we may also get a 2-byte alignment because of the above issue.
>> } nmethod_entry_barrier (end)
>> 
>> 
>> and about the trampoline:
>> 
>> trampoline (begin) {  <-  when we reach here, we may get a 2-byte alignment because of RVC's existence.
>>    auipc
>>    ld
>>    jalr
>>    <the real jump target in the code segment>  <-  same as the above explanation.
>> } trampline (end)
>> 
>> 
>> 2. The reason to pass another argument is that: 
>> 
>> we might consider just keeping the start address of the code slices (nmethod_entry_barrier and trampoline) aligned to 4-byte to solve this issue, but this assumption is based on 'the size of the code slices (nmethod_entry_barrier and trampoline themselves) are a multiple of 4', and we don't know if they will change in the future by RVC. So I explicitly pass the real target address to the `align` function as a new argument to make sure the code is always right in the future.
>> 
>> I am pleased with other suggestions for this part.
>
> For the nmethod_entry_barrier case, I would omit the extra_offset parameter (and also the nmethod_barrier_guard_offset funtion). I think your newly-added assertion at line 265 is enough.
> 
> But looks like it's different in the trampoline case where I see we need a wordSize (8bytes) alignment. So we may keep the change for this case.

Yes, I agree with your point. That trampoline part is special and requires to be scrupulous to make changes.

So maybe I'd better remove the `nmethod_barrier_guard_offset` part right?

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PR: https://git.openjdk.java.net/riscv-port/pull/34