RFR: 7143664: Clean up OrderAccess implementations and usage
david.holmes at oracle.com
Mon Feb 9 07:14:23 UTC 2015
On 29/01/2015 8:48 AM, David Holmes wrote:
> Updated webrev:
First let me say that Erik and I had a lot of offline discussions about
this since he first proposed his reworking. My initial concern was
ensuring the model changes were appropriate as this is a complex area
and there can be different views of how things can/should be expressed.
As per the bug report I was aware of a number of issues with the
existing expression of semantics, but Erik showed me things were even
worse than I had thought :)
Basically the definitions of stand-alone release() and acquire() as they
currently are in orderAccess.hpp are somewhat broken. "release"
semantics must establish a barrier between accesses prior to the
release() and a specific store after the release - hence release_store()
and similarly for acquire() and a preceding load, hence load_acquire().
The general definitions of acquire() and release() as one-way movement
barriers don't provide useful semantics - in part because regardless of
which way you move things once everything is on the same side of the
barrier you can't tell in which direction they moved. :)
Further, those general definitions would imply that release_store() and
release();store are semantically very different - but they are currently
implemented (as are things like release_store_fence) as if semantically
And finally, while orderAccess.hpp suggests that release() can be
implemented by a storeStore|loadStore barrier, there is code throughout
the VM that expects it is exactly a storeStore|storeload barrier. (And
similarly for acquire). C1 for example explicitly defines:
membar_acquire := LoadLoad + LoadStore
membar_release := LoadStore + StoreStore
membar := StoreLoad (aka fence)
and if I then look at where "release" is used in the codebase they all
seem incorrect based on "release" semantics, but correct if you expect
release() to mean LoadStore|StoreStore (and in most cases you would then
actually use a release_store).
So the changes Erik proposes to ensure that release() and acquire() are
defined with respect to that subsequent-store or preceding-load, bring
expectations in the code back into alignment. It means release(); store
is the same as release_store() semantically, and the latter is a means
for optimizing when you place the barrier and the store directly
together - where often you can't because the actual store/load may be
buried within other accessor methods eg:
We can fix that by adding set_with_release/load_with_acquire, or passing
in barrier enums C++11 style, but that is a lot of rework.
On the implementation side I'm a template novice and was originally
skeptical about the technique, but having implemented it for a couple of
platforms I'm now a fan - even if I couldn't walk you through the exact
behaviour of the template specialization code :)
So on to the actual review comments ...
Minor nit: I would prefer the term "bound" rather than "joined" when
referring to load_acquire and store_release.
In the implementation table I think "sparc" should be "sparc TSO".
I still think we need to say something about C++ volatile and about
compiler barriers in general.
No comment (this just removes stale/unused terminology)
No comments - ppc64 folk need to review these.
In OrderAccess::fence doesn't the existing asm already act as a compiler
barrier; and on uniprocessor we need neither compiler nor hardware
barriers; so isn't the compiler_barrier() redundant?
No comments - The Zero folk will need to approve this change in terminology.
You've removed the GNU_SOURCE ifdefs which means the new code has to be
tested using gcc on Solaris. We don't officially support that platform
so I have no way to verify these particular changes.
I think it makes sense to use the same code here as the linux x86
version. But as above can't verify this for gcc on Solaris; also query
about need for compiler_barrier() in fence().
Same comment about fence() and compiler_barrier() :)
Do you have a reference to the MSVC volatile semantics?
Not clear about the 32-bit versus 64-bit changes - mainly because I
don't know why they were different in the first place.
Why are windows release_store_fence specializations different to linux
Not clear about your change to the float version of release_store_fence:
a) do we need to change it?
b) given what we changed it to isn't that the default ?
General comment: copyright dates need updating in most, if not all, files.
More information about the hotspot-dev