RFR: 8291555: Implement alternative fast-locking scheme [v47]

[Dean Long]

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

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PR Review Comment: https://git.openjdk.org/jdk/pull/10907#discussion_r1154955795