On 06-06-2012 23:39, Steven Schveighoffer wrote:
An interesting situation, the current compiler happily will compile pure
functions that accept shared data.
I believed when we relaxed purity rules, shared data should be taboo for
pure functions, even weak-pure ones. Note that at least at the time, Don
agreed with me: http://forum.dlang.org/post/[email protected]
Now, technically, there's nothing really *horrible* about this, I mean
you can't really have truly shared data inside a strong-pure function.
Any data that's marked as 'shared' will not be shared because a
strong-pure function cannot receive any shared data.
So if you then were to call a weak-pure function that had shared
parameters from a strong-pure function, you simply would be wasting
cycles locking or using a memory-barrier on data that is not truly
shared. I don't really see a compelling reason to have weak-pure
functions accept shared data explicitly.
*Except* that template functions which use IFTI have good reason to be
able to be marked pure.
For example:
void inc(T)(ref T i) pure
{
++i;
}
Now, we have a template function that we know only will affect i, and
the compiler enforces that.
But what happens here?
shared int x;
void main()
{
x.inc();
}
here, T == shared int.
One solution (if shared isn't allowed on pure functions) is, don't mark
inc pure, let it be inferred. But then we are losing the contract to
have the compiler help us enforce purity.
I'll also point out that inc isn't a valid function for data that is
actually shared: ++i is not atomic. So disallowing shared actually helps
us in this regard, by refusing to compile a function that would be
dangerous when used on shared data.
Man, shared is such a mess.
(I'm going to slightly hijack a branch of your thread because I think we
need to address the below concerns before we can make this decision
properly.)
We need to be crystal clear on what we're talking about here. Usually,
people refer to shared as being supposed to insert memory barriers.
Others call operations on shared data atomic.
(And of course, neither is actually implemented in any compiler, and I
doubt they ever will be.)
A memory barrier is what the x86 sfence, lfence, and mfence instructions
represent. They simply make various useful guarantees about ordering of
loads and stores. Nothing else.
Atomic operations are what the lock prefix is used for, for example the
lock add operation, lock cmpxchg, etc. These operate on the most recent
value at whatever memory location is being operated on, i.e. caches are
circumvented.
Memory barriers and atomic operations are not the same thing, and we
should avoid conflating them. Yes, they can be used together to write
low-level, lock-free data structures, but the use of one does not
include the other automatically.
(At this point, I probably don't need to point out how x86-biased and
unportable shared is.....)
So, my question to the community is: What should shared *really* mean?
I don't think that having shared imply memory barriers is going to be
terribly useful to anyone. In fact, I don't know how the compiler would
even determine where to efficiently insert memory barriers. And
*actually*, I think memory barriers is really not what people mean at
*all* when they refer to shared's effect on code generation. I think
what people *really* want is atomic operations.
Steven, in your particular case, I don't agree entirely. The operation
can be atomic quite trivially by implementing inc() like so (for the
shared int case):
void inc(ref shared int i) pure nothrow
{
// just pretend the compiler emitted this
asm
{
mov EDX, i;
lock;
inc [EDX];
}
}
But I may be misunderstanding you. Of course, it gets a little more
complex if you use the result of the ++ operation afterwards, but it's
still not impossible to do atomically. What can *not* be done is doing
the increment and loading the result in one purely atomic instruction
(and I suspect this is what you may have been referring to). It's worth
pointing out that most atomic operations can be implemented with a spin
lock (which is exactly what core.atomic does for most binary
operations), so while it cannot be done with an x86 instruction, it can
be achieved through such a mechanism, and most real world atomic APIs do
this (see InterlockedIncrement in Windows for example).
Further, if shared is going to be useful at all, stuff like this *has*
to be atomic, IMO.
I'm still of the opinion that bringing atomicity and memory barriers
into the type system is a horrible can of worms that we should never
have opened, but now shared is there and we need to make up our minds
already.
The compiler *currently* however, will simply compile this just fine.
I'm strongly leaning towards this being a bug, and needs to be fixed in
the compiler.
Some background of why this got brought up:
https://github.com/D-Programming-Language/druntime/pull/147
Opinions?
-Steve
--
Alex Rønne Petersen
[email protected]
http://lycus.org
