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

Tue Jan 12 19:24:30 UTC 2016

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

Yes, that is right solution here.

Thanks,
Vladimir

On 1/12/16 5:59 AM, Tobias Hartmann wrote:
> 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.
>>>>>>>>>