Compressed oops are copied bytewise on windows amd64.

[David Holmes]

I filed:

7100935: win32: memmove is not atomic but is used for 
pd_conjoint_*_atomic operations

It's odd that the windows versions continued to use memmove when its use 
on other platforms was removed for similar reasons quite some time ago.

I was quite startled to find that your test program actually crashed the 
VM on my 64-bit Windows system, so I bumped the priority to a P2.

David

On 14/10/2011 7:20 PM, David Holmes wrote:
> On 14/10/2011 12:05 AM, Siebenborn, Axel wrote:
>>> On 13/10/2011 5:24 PM, Siebenborn, Axel wrote:
>>>> on windows-amd64 the interpreter copies compressed oops using
>>> memmove.
>>>>
>>>> However, memmove is not thread safe and might copy bytewise.
>>>>
>>>> Another thread can see a partly copied compressed oop.
>>>
>>> I'm not seeing the connection with compressed oops here.
>>
>> Compressed oops are copied using pd_conjoint_jints_atomic. This
>> function copies using memmove on Windows. Uncompressed oops are copied
>> using a copy loop, similar to my suggested fix.
>
> Ok - but the problem is the basic non-atomicity of memmove as it relates
> to conjoint_jints_atomic - right? Given an oop is a multi-word entity
> the overall copy will not be atomic anyway.
>
>> The problem of memmove is that it copies byte per byte until it
>> reaches an
>> alignment of 8 bytes. After that it copies 64 bits. In the context of
>> these functions, atomic means, that a 32 bit value is copied using 32
>> bit load and store instructions. To use a copy loop written in c++
>> implies, that the c++ compiler doesn't generate code that stores
>> (aligned) values not atomically.
>
> AFAICS memmove is defined to do byte-copies so anything larger than a
> byte is just an optimization. Seems memmove should never have been used
> for an atomic copy.
>
> I'll file a bug for this.
>
>>> That said I think win-x64 should do the same as linux-x64 and presently
>>> they differ (perhaps a tool issue, I'm not sure)
>>>
>>
>> On linux the copy functions are implemented using assembler. I'm not
>> an expert of the windows tool chain. I don't know if it's easy to do
>> the assembler implementation here. However, the c++ loop is less error
>> prone and portable.
>
> Yes lots of assembler on all other x986 platforms but not on Windows.
>
> Thanks,
> David
> -----
>
>> Axel