On 11/14/12 7:11 AM, Alex Rønne Petersen wrote:
On 14-11-2012 15:32, Andrei Alexandrescu wrote:
On 11/14/12 4:23 AM, David Nadlinger wrote:
On Wednesday, 14 November 2012 at 00:04:56 UTC, deadalnix wrote:
That is what java's volatile do. It have several uses cases, including
valid double check locking (It has to be noted that this idiom is used
incorrectly in druntime ATM, which proves both its usefullness and
that it require language support) and disruptor which I wanted to
implement for message passing in D but couldn't because of lack of
support at the time.
What stops you from using core.atomic.{atomicLoad, atomicStore}? I don't
know whether there might be a weird spec loophole which could
theoretically lead to them being undefined behavior, but I'm sure that
they are guaranteed to produce the right code on all relevant compilers.
You can even specify the memory order semantics if you know what you are
doing (although this used to trigger a template resolution bug in the
frontend, no idea if it works now).
David
This is a simplification of what should be going on. The
core.atomic.{atomicLoad, atomicStore} functions must be intrinsics so
the compiler generate sequentially consistent code with them (i.e. not
perform certain reorderings). Then there are loads and stores with
weaker consistency semantics (acquire, release, acquire/release, and
consume).
Andrei
They already work as they should:
* DMD: They use inline asm, so they're guaranteed to not be reordered.
Calls aren't reordered with DMD either, so even if the former wasn't the
case, it'd still work.
* GDC: They map directly to the GCC __sync_* builtins, which have the
semantics you describe (with full sequential consistency).
* LDC: They map to LLVM's load/store instructions with the atomic flag
set and with the given atomic consistency, which have the semantics you
describe.
I don't think there's anything that actually needs to be fixed there.
The language definition should be made clear so as future optimizations
of existing implementations, and future implementations, don't push
things over the limit.
Andrei
