RFR: 8177346: hotspot change for 8176513 breaks jdk9 build on Ubuntu 16.04

[Phil Race]

I am not blaming that change as being "bad" in any way.
I don't see how anyone could have foreseen this side effect.

-phil.

On 3/21/2017 12:56 PM, Daniel D. Daugherty wrote:
> Adding folks involved with the fix for 8176513.
>
> Dan
>
>
>
> On 3/21/17 1:40 PM, Phil Race wrote:
>> Please review a small JDK 9 build fix for a build failure due to
>>
>> hotspot/src/share/vm/opto/library_call.cpp:2578:3: error: 
>> ‘need_mem_bar’ may be used uninitialized in this function 
>> [-Werror=maybe-uninitialized]
>>    if (need_mem_bar) insert_mem_bar(Op_MemBarCPUOrder);
>>
>> https://bugs.openjdk.java.net/browse/JDK-8177346
>>
>> I can't explain why GCC 5.4 woke up and decided to start reporting 
>> this warning when
>> nothing logically changed that would trigger it. However I think it 
>> should already have
>> been triggered -  it is clear to me that the compiler has a point.
>>
>> The fix is simple and provided in-line below :-
>> ---
>>
>> hg diff src/share/vm/opto/library_call.cpp
>> diff --git a/src/share/vm/opto/library_call.cpp 
>> b/src/share/vm/opto/library_call.cpp
>> --- a/src/share/vm/opto/library_call.cpp
>> +++ b/src/share/vm/opto/library_call.cpp
>> @@ -2372,7 +2372,7 @@
>>    // the barriers get omitted and the unsafe reference begins to 
>> "pollute"
>>    // the alias analysis of the rest of the graph, either 
>> Compile::can_alias
>>    // or Compile::must_alias will throw a diagnostic assert.)
>> -  bool need_mem_bar;
>> +  bool need_mem_bar = false;
>>    switch (kind) {
>>        case Relaxed:
>>            need_mem_bar = mismatched && !adr_type->isa_aryptr();
>>
>> ---
>>
>> In fact this initialisation pattern is already used in other nearly 
>> identical cases in
>> the same function.
>> eg this one about 20 lines before my update
>>   bool mismatched = false;
>>
>> and this one about 20 lines after ..
>>   bool requires_atomic_access = false;
>>
>> So there is ample precedent.
>>
>> I will get the appropriate RDP2 approvals to push once code review is 
>> complete.
>> I also anticipate that by the time this happens I will need to push 
>> it to
>> http://hg.openjdk.java.net/jdk9/dev/hotspot/
>> as per the email here :
>> http://mail.openjdk.java.net/pipermail/hotspot-dev/2017-March/026155.html 
>>
>>
>> Let me know if you think otherwise.
>>
>> JPRT is being used to build/test the fix.
>>
>> -phil.
>>
>