[9] RFR(S): 8144212: JDK 9 b93 breaks Apache Lucene due to compact strings

Tue Jan 12 13:59:38 UTC 2016

On 11.01.2016 21:00, Vladimir Kozlov wrote:
> On 1/11/16 7:20 AM, Tobias Hartmann wrote:
>> On 08.01.2016 20:41, Vladimir Kozlov wrote:
>>> On 1/8/16 2:37 AM, Tobias Hartmann wrote:
>>>> On 07.01.2016 21:49, Vladimir Kozlov wrote:
>>>>> On 1/7/16 6:52 AM, Tobias Hartmann wrote:
>>>>>> Hi Vladimir,
>>>>>>
>>>>>> On 07.01.2016 00:58, Vladimir Kozlov wrote:
>>>>>>> Andrew is right.
>>>>>>
>>>>>> Yes, he's right that the membar is not needed in this case. I noticed that GraphKit::inflate_string() sets the output memory to TypeAryPtr::BYTES although inflate writes to a char[] array in this case. This caused the subsequent char load to be on a different slice allowing C2 to move the load to before the intrinsic.
>>>>>
>>>>> Right. It was the root of this bug, see below.
>>>>>
>>>>>>
>>>>>> I fixed this for the inflate and compress intrinsics.
>>>>>>
>>>>>>> GraphKit::inflate_string() should have SCMemProjNode as compress_string() does to prevent loads move up.
>>>>>>> StrInflatedCopyNode is not memory node.
>>>>>>
>>>>>> Okay, why are above changes not sufficient to prevent the load from moving up? Also, the comment for SCMemProjNode says:
>>>>>
>>>>> I did not get the question. Is it before your webrev.01 change? Or even with the change?
>>>>
>>>> I meant with webrev.01 but you answered my question below.
>>>>
>>>>>>     // This class defines a projection of the memory  state of a store conditional node.
>>>>>>     // These nodes return a value, but also update memory.
>>>>>>
>>>>>> But inflate does not return any value.
>>>>>
>>>>> Hmm, according to bottom type inflate produce memory:
>>>>>
>>>>> StrInflatedCopyNode::bottom_type() const { return Type::MEMORY; }
>>>>>
>>>>> So it really does not need SCMemProjNode. Sorry about that.
>>>>> So load was LoadUS which is char load and originally memory slice of inflate was incorrect BYTES.
>>>>
>>>> Exactly.
>>>>
>>>>> Instead of SCMemProjNode we should have to change the idx of your dst_type:
>>>>>
>>>>> set_memory(str, dst_type);
>>>>
>>>> Yes, that's what I do now in webrev.01 by passing the dst_type as an argument to inflate_string.
>>>>
>>>>> And you should rollback part of changes in escape.cpp and macro.cpp.
>>>>
>>>> Okay, I'll to that.
>>>>
>>>>>> Here is the new webrev, including the SCMemProjNode and adapting escape analysis and macro expansion accordingly:
>>>>>> http://cr.openjdk.java.net/~thartmann/8144212/webrev.01/
>>>>>
>>>>> In general when src & dst arrays have different type we may need to use TypeOopPtr::BOTTOM to prevent related store & loads bypass these copy nodes.
>>>>
>>>> Okay, should we then use BOTTOM for both the input and output type?
>>>
>>> Only input. Output type corresponds to dst array type which you set correctly now.
>>
>> It seems like that this is not sufficient. As Roland pointed out (off-thread), there may still be a problem in the following case:
>>   StoreC
>>   inflate_string
>>   LoadC
>>
>> The memory graph (def->use) now looks like this:
>>   LoadC -> inflate_string -> ByteMem
>>                 ... StoreC-> CharMem
> 
> I did not get this. If StoreC node is created before inflate_string - inflate_string should point to it be barrier for LoadC.

Note that the StoreC and inflate_string are *not* writing to the same char[] array. The test looks like this:

 char c1[] = new char[1];
 char c2[] = new char[1];

 c2[0] = 42;
 // Inflate String from byte[] to char[]
 s.getChars(0, 1, c1, 0);
 // Read char[] memory written before inflation
 return c2[0];

The result should be 42. The problem is that inflate_string does not point to StoreC because inflate_string uses a byte[] as input and in this case also writes to a different char[]. Even if we set the input to BOTTOM, inflate_string points to 7 Parm (BOTTOM) but not to the char[] memory produced by 96 StoreC:
http://cr.openjdk.java.net/~thartmann/8144212/inflate_bottom.png

349 LoadUS then reads from the output char[] memory of inflate_string which does not include the result of StoreC. The test fails because the return value is != 42.

My solution is to capture both the byte[] and char[] memory by using a MergeMem node as input to inflate_string.

>  If StoreC followed inflate_string and LoadC followed StoreC - LoadC should point to StoreC. If LoadC does not follow StoreC then result is relaxed.

Yes, these cases work fine.

Thanks,
Tobias

>> The intrinsic hides the dependency between LoadC and StoreC, causing the load to read from memory not containing the result of the StoreC. I was able to write a regression test for this (see 'TestStringIntrinsicMemoryFlow::testInflate2').
>>
>> Setting the input to BOTTOM, generates the following graph:
>> http://cr.openjdk.java.net/~thartmann/8144212/inflate_bottom.png
>> The 349 LoadUS does not read the result of the 96 StoreC because the StrInflateCopyNode does not capture it's memory. The test fails.
>>
>> I adapted the fix to emit a MergeMemoryNode to capture the entire memory state as input to the intrinsic. The graph then looks like this:
>>   LoadC -> inflate_string -> MergeMem(ByteMem, StoreC(CharMem))
>> http://cr.openjdk.java.net/~thartmann/8144212/inflate_merge.png
>>
>> Here is the new webrev:
>> http://cr.openjdk.java.net/~thartmann/8144212/webrev.02/
>> Probably, we could also only capture the byte and char slices instead of merging everything. What do you think?
>>
>> Best,
>> Tobias
>>
>>>>>> Related question:
>>>>>> In library_call.cpp, I now use TypeAryPtr::get_array_body_type(dst_elem) to get the correct TypeAryPtr for the destination (we support both BYTES and CHARS). For a char[] destination, it returns:
>>>>>>     char[int:>=0]:exact+any *
>>>>>>
>>>>>> which is equal to the type of the char load.
>>>>>
>>>>> Please, explain this. I thought string's array will always be byte[] when compressed strings are enabled. Is it used for getChars() which returns char array?
>>>>
>>>> Yes, both the compress and inflate intrinsics are used for different types of src and dst arrays. See comment in library_call.cpp:
>>>>
>>>> // compressIt == true --> generate a compressed copy operation (compress char[]/byte[] to byte[])
>>>> //   int StringUTF16.compress(char[] src, int srcOff, byte[] dst, int dstOff, int len)
>>>> //   int StringUTF16.compress(byte[] src, int srcOff, byte[] dst, int dstOff, int len)
>>>> // compressIt == false --> generate an inflated copy operation (inflate byte[] to char[]/byte[])
>>>> //   void StringLatin1.inflate(byte[] src, int srcOff, char[] dst, int dstOff, int len)
>>>> //   void StringLatin1.inflate(byte[] src, int srcOff, byte[] dst, int dstOff, int len)
>>>>
>>>> I.e., the inflate intrinsic is used for inflation from byte[] to byte[]/char[].
>>>>
>>>>> Should we also be more careful in inflate_string_slow()? Is it used?
>>>>
>>>> No, inflate_string_slow() is only called from PhaseStringOpts::copy_latin1_string() where it is used to inflate from byte[] to byte[].
>>>>
>>>>>> I also tried to derive the type from the array by using dst_type->isa_aryptr(). However, this returns a more specific type:
>>>>>>     char[int:1]:NotNull:exact *
>>>>>>
>>>>>> Using this results in C2 assuming that the subsequent char load is independent and again moving it to before the intrinsic. I don't understand why that is. Shouldn't the second type be a "subtype" of the first type?
>>>>>
>>>>> It is indeed strange. What memory type of LoadUS? It could be bug.
>>>>
>>>> LoadUS has memory type "char[int:>=0]:exact+any *" which has alias index 4. dst_type->isa_aryptr() returns memory type "char[int:1]:NotNull:exact *" which has alias index 8.
>>>>
>>>> I will look into this again and try to understand what happens.
>>>
>>> It could that aryptr is pointer to array and load type is pointer to array's element.
>>>
>>> Thanks,
>>> Vladimir
>>>
>>>>
>>>> Thanks,
>>>> Tobias
>>>>
>>>>>>> On 1/6/16 5:34 AM, Andrew Haley wrote:
>>>>>>>> On 01/06/2016 01:06 PM, Tobias Hartmann wrote:
>>>>>>>>
>>>>>>>>> The problem here is that C2 reorders memory instructions and moves
>>>>>>>>> an array load before an array store. The MemBarCPUOrder is now used
>>>>>>>>> (compiler internally) to prevent this. We do the same for normal
>>>>>>>>> array copys in PhaseMacroExpand::expand_arraycopy_node(). No actual
>>>>>>>>> code is emitted. See also the comment in memnode.hpp:
>>>>>>>>>
>>>>>>>>>      // Ordering within the same CPU.  Used to order unsafe memory references
>>>>>>>>>      // inside the compiler when we lack alias info.  Not needed "outside" the
>>>>>>>>>      // compiler because the CPU does all the ordering for us.
>>>>>>>>>
>>>>>>>>> "CPU does all the ordering for us" means that even with a relaxed
>>>>>>>>> memory ordering, loads are never moved before dependent stores.
>>>>>>>>>
>>>>>>>>> Or did I misunderstand your question?
>>>>>>>>
>>>>>>>> No, I don't think so.  I was just checking: I am very aware that
>>>>>>>> HotSpot has presented those of use with relaxed memory order machines
>>>>>>>> with some interesting gotchas over the years, that's all.  I'm a bit
>>>>>>>> surprised that C2 needs this barrier, given that there is a
>>>>>>>> read-after-write dependency, but never mind.
>>>>>>>>
>>>>>>>> Thanks,
>>>>>>>>
>>>>>>>> Andrew.
>>>>>>>>