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

[Daniel D. Daugherty]

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.
>