RFR: JDK-8061964: Insufficient compiler barriers for GCC in OrderAccess functions

[Mikael Gerdin]

Hi all,

Please review this attempt at fixing the OrderAccess functions on Linux 
x86 with GCC.

While working on another bug I recently discovered that g++ was 
reordering stores across a call to OrderAccess::storestore on Linux x86.

The G1 code attempts to do an ordered publishing of two values:
_saved_mark_word = _top;
OrderAccess::storestore();
_gc_time_stamp = curr_gc_time_stamp;

The types involved are
HeapWord* _top, _saved_mark_word;
volatile unsigned _gc_time_stamp;

The incorrect behavior seems to have started when JDK-6973570 was fixed 
in JDK 7.
Below, _top is at offset 0x58, _saved_mark_word at 0x18 and 
_gc_time_stamp at 0x138, %rbx is "this".

/net/jre/onestop/jdk/1.7.0/promoted/all//b108/binaries/linux-x64/jre/lib/amd64/server/libjvm.so:
   3d9f4d:       39 d0                   cmp    %edx,%eax
   3d9f4f:       73 1c                   jae    3d9f6d 
<G1OffsetTableContigSpace::set_saved_mark()+0x3d>
   3d9f51:       48 8b 43 58             mov    0x58(%rbx),%rax
   3d9f55:       48 89 43 18             mov    %rax,0x18(%rbx)
   3d9f59:       48 8b 05 40 f9 70 00    mov    0x70f940(%rip),%rax    # 
ae98a0 <_DYNAMIC+0x12f8>
   3d9f60:       48 c7 00 00 00 00 00    movq   $0x0,(%rax)
   3d9f67:       89 93 38 01 00 00       mov    %edx,0x138(%rbx)

/net/jre/onestop/jdk/1.7.0/promoted/all//b109/binaries/linux-x64/jre/lib/amd64/server/libjvm.so
   3da05d:       39 d0                   cmp    %edx,%eax
   3da05f:       73 15                   jae    3da076 
<G1OffsetTableContigSpace::set_saved_mark()+0x36>
   3da061:       48 8b 43 58             mov    0x58(%rbx),%rax
   3da065:       c7 45 f4 00 00 00 00    movl   $0x0,-0xc(%rbp)
   3da06c:       89 93 38 01 00 00       mov    %edx,0x138(%rbx)
   3da072:       48 89 43 18             mov    %rax,0x18(%rbx)

In b109 the store of %rax to 0x18(%rbx) has been ordered after the store 
of %edx to 0x138(%rbx) in the same build as JDK-6973570 was integrated.

My suggestion to fix this is to extend all the OrderAccess::release* 
variants on x86 with a:
__asm__ volatile ("" : : : "memory");
to attempt to prevent GCC from reordering any memory accesses across 
those function calls.

I've verified that this solves the issue in the assembly with our 
current JDK 9 build platform compilers.
I've also verified that this particular piece of code is compiled 
correctly on our other x86 platforms: Solaris, Windows and OS X.

Webrev:
http://cr.openjdk.java.net/~mgerdin/8061964/webrev.0/
Bug:
https://bugs.openjdk.java.net/browse/JDK-8061964
Testing:
JPRT, inspecting generated assembly for the function 
G1OffsetTableContigSpace::record_top_and_timestamp (as the method is 
currently named).
Suggestions of further testing is greatly appreciated.

Thanks
Mikael