RFR JDK-8141491: Unaligned memory access in Bits.c

John Rose john.r.rose at oracle.com
Tue Jan 26 19:04:01 UTC 2016

On Jan 26, 2016, at 10:48 AM, Andrew Haley <aph at redhat.com> wrote:
> On 01/26/2016 06:32 PM, John Rose wrote:
>> On Jan 26, 2016, at 1:04 AM, Andrew Haley <aph at redhat.com> wrote:
>>> I agree that memcpy is the right thing to use.  It's portable and is
>>> inlined well on production-quality C compilers.
>> But it is not strong enough to uphold the Java memory model,
>> because it is allows to copy byte-wise, which can tear shorts,
>> ints, or longs, creating illegal race states.
>> So we try to avoid memcpy when we can.
> Yes, I see.  I guess the best we can do is something like the fun and
> games in Unsafe.{get, put}LongUnaligned(), which always do the best
> they can to align everything.

Unsafe.copyMemory bottoms out to Copy::conjoint_memory_atomic.
IMO that's a better starting point than memcpy.  Perhaps it can be
given an additional parameter (or overloading) to specify a swap size.

— John

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