On Fri, 31 Mar 2023 07:25:48 GMT, Thomas Stuefe <stu...@openjdk.org> wrote:
>> Roman Kennke has updated the pull request incrementally with one additional >> commit since the last revision: >> >> Use int instead of size_t for cached offsets, to match the uncached offset >> type and avoid build failures > > src/hotspot/cpu/aarch64/macroAssembler_aarch64.cpp line 6234: > >> 6232: orr(hdr, hdr, markWord::unlocked_value); >> 6233: // Clear lock-bits, into t2 >> 6234: eor(t2, hdr, markWord::unlocked_value); > > In arm, I use a combination of bic and orr instead. That gives me, with just > two instructions, added safety against someone handing in a "11" marked MW. I > know, should never happen, but better safe. > > > ldr(new_hdr, Address(obj, oopDesc::mark_offset_in_bytes())); > bic(new_hdr, new_hdr, markWord::lock_mask_in_place); // new header (00) > orr(old_hdr, new_hdr, markWord::unlocked_value); // old header (01) > > (note that I moved MW loading down into MA::fast_lock for unrelated reasons). > > Unfortunately, on aarch64 there seem to be no bic variants that accept > immediates. So it would take one more instruction to get the same result: > > > - // Load (object->mark() | 1) into hdr > - orr(hdr, hdr, markWord::unlocked_value); > - // Clear lock-bits, into t2 > - eor(t2, hdr, markWord::unlocked_value); > + // Prepare new and old header > + mov(t2, markWord::lock_mask_in_place); > + bic(t2, hdr, t2); > + orr(hdr, t2, markWord::unlocked_value); > > > But maybe there is a better way that does not need three instructions. There is a BFC (Bitfield Clear) pseudo-instruction that uses the BFM instruction. ------------- PR Review Comment: https://git.openjdk.org/jdk/pull/10907#discussion_r1154955795
