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

Wed Jan 13 12:00:58 UTC 2016

Thanks, Vladimir.

On 12.01.2016 20:24, Vladimir Kozlov wrote:
>> 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.

I changed the implementation to only capture the byte[] and char[] memory:
http://cr.openjdk.java.net/~thartmann/8144212/webrev.03/

The method GraphKit::capture_memory(src_type, dst_type) returns a new MergeMemNode if the src and dst types are different, merging the two.

Best,
Tobias

> 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.
>>>>>>>>>>