RFR: 8365147: AArch64: Replace DMB + LD + DMB with LDAR for C1 volatile field loads [v3]

[Ruben]

On Wed, 12 Nov 2025 11:27:08 GMT, Andrew Haley <aph at openjdk.org> wrote:

>> Samuel Chee has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains five commits:
>> 
>>  - Address review comments. Refine.
>>    
>>    Change-Id: I9cc0308300548c1892d39791e00b41ef13c95e63
>>  - Merge from the main branch
>>  - Address review comments
>>    
>>    Change-Id: Ica13be8094ac0f057066042ef0a5ec5927b98dfd
>>  - Refine code generation for mem2reg_volatile
>>    
>>    The patch is contributed by @theRealAph.
>>    
>>    Change-Id: I7ab1854dd238cdce72a4ab218b5b4ee84ad39586
>>  - 8365147: AArch64: Replace DMB + LD + DMB with LDAR for C1 volatile loads
>>    
>>    Replaces the DMB ISH + LD + DMB ISHLD sequence with LDAR
>>    for volatile field loads - for example, AtomicLong::get.
>>    
>>    This is valid, as originally the DMBs were necessary due to
>>    the case described here - https://bugs.openjdk.org/browse/JDK-8179954.
>>    As in the rare case where the LD can be reordered with an LDAR
>>    or STLR from the C2 implementation for stores and loads, these
>>    DMBs are required.
>>    However, acquire/release operations use a sequentially consistent model
>>    which does not allow reordering between them. Hence, the LD can be
>>    replaced with an LDAR to disallow reordering with a STLR/LDAR
>>    and the first DMB can be removed.
>>    
>>    The LDAR has acquire semantics, so it's impossible for
>>    memory accesses after to be reordered before; the DMB ISHLD is
>>    not required. Therefore, a singular LDAR is sufficient.
>>    
>>    This excludes floats and doubles, as they do not have
>>    equivalent load-acquire instructions.
>>    
>>    Change-Id: Ia93607f8bb20c2d974fe6b2e586dd3239bb2728c
>
> src/hotspot/cpu/aarch64/c1_LIRAssembler_aarch64.cpp line 948:
> 
>> 946: }
>> 947: 
>> 948: void LIR_Assembler::load_generic(LIR_Address *from_addr, LIR_Opr dest,
> 
> Suggestion:
> 
> void LIR_Assembler::load_relaxed(LIR_Address *from_addr, LIR_Opr dest,
> 
> Standard terminology.

Thanks @theRealAph. Updated.

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/26748#discussion_r2541853251