RFR(XS): 8055530: assert(_exits.control()->is_top() || !_gvn.type(ret_phi)->empty()) failed: return value must be well defined

[Roland Westrelin]

Thanks for looking at this, Vladimir.

> Why not bailout compilation when you detect such case? You changed only the assert.

I thought about it. But it seems to me that similar inconsistencies might occur elsewhere during compilation and that we wouldn't necessarily detect them. So why a special case here?

Roland.

> 
> Thanks,
> Vladimir
> 
> On 12/19/14 5:50 AM, Roland Westrelin wrote:
>> http://cr.openjdk.java.net/~roland/8055530/webrev.00/
>> 
>> From a core file:
>> 
>> method being compiled is sun.nio.cs.ISO_8859_1::newDecoder
>> 
>> public java.nio.charset.CharsetDecoder newDecoder();
>>   Code:
>>    0: new #7; //class sun/nio/cs/ISO_8859_1$Decoder
>>    3: dup
>>    4: aload_0
>>    5: aconst_null
>>    6: invokespecial #8; //Method sun/nio/cs/ISO_8859_1$Decoder."<init>":(Ljava/nio/charset/Charset;Lsun/nio/cs/ISO_8859_1$1;)V
>>    9: areturn
>> 
>> _exits.control() is a Region node.
>> _gvn.type(ret_phi) is top
>> 
>> ret_phi has a single input a CheckCastPP from the initialization node. Type of the CheckCastPP is TypeInstPtr sun/nio/cs/ISO_8859_1$Decoder. The type of the Phi when it's created in Parse::build_exits() is set to tf()->range()->field_at(TypeFunc::Parms). Here it's TypeInstPtr sun/nio/cs/UTF_8$Decoder, so not the type from the signature (java.nio.charset.CharsetDecoder) but a subtype. That would be the case because in TypeOopPtr::make_from_klass_common(), we define the type with unique_concrete_subklass() and CharsetDecoder is abstract. So UTF_8$Decoder is the only loaded subclass when the compilation starts and ISO_8859_1$Decoder is loaded during compilation. So this compilation will have to be retried because the assumption in TypeOopPtr::make_from_klass_common() that there's a single subclass will be invalid. When the type of the Phi is computed in do_exits(), UTF_8$Decoder and ISO_8859_1$Decoder are joined which given one is not a subclass of the other gives a result ab!
> ove the ce
> nter line, which filter turns into top.
>> 
>> I could reproduce the assert failure with a test case I wrote and stop the compiler thread, load a class concurrently and restart the compiler thread.
>> 
>> Roland.
>>