On Mon, 10 Jan 2022 06:46:59 GMT, Fei Yang <fyang@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? ------------- PR: https://git.openjdk.java.net/riscv-port/pull/34