[prev in list] [next in list] [prev in thread] [next in thread] 

List:       openjdk-serviceability-dev
Subject:    Re: RFR: 8291555: Implement alternative fast-locking scheme [v47]
From:       Dean Long <dlong () openjdk ! org>
Date:       2023-03-31 22:41:56
Message-ID: eFKylrRhxzxD-uJVDa7spgeImzrULap7LvWpAkHCahI=.b7d92c9e-86e1-4dec-9e86-cedbc5057384 () github ! com
[Download RAW message or body]

On Fri, 31 Mar 2023 07:25:48 GMT, Thomas Stuefe <stuefe@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


[prev in list] [next in list] [prev in thread] [next in thread]