On 04/04/2011 01:23 PM, Stefan Behnel wrote:
Dag Sverre Seljebotn, 04.04.2011 12:17:
CEP up at http://wiki.cython.org/enhancements/prange
"""
Variable handling
Rather than explicit declaration of shared/private variables we rely
on conventions:
* Thread-shared: Variables that are only read and not written in
the loop body are shared across threads. Variables that are only used
in the else block are considered shared as well.
* Thread-private: Variables that are assigned to in the loop body
are thread-private. Obviously, the iteration counter is thread-private
as well.
* Reduction: Variables that only used on the LHS of an inplace
operator, such as s above, are marked as targets for reduction. If the
variable is also used in other ways (LHS of assignment or in an
expression) it does instead turn into a thread-private variable. Note:
This means that if one, e.g., inserts printf(... s) above, s is turned
into a thread-local variable. OTOH, there is simply no way to
correctly emulate the effect printf(... s) would have in a sequential
loop, so such code must be discouraged anyway.
"""
What about simply (ab-)using Python semantics and creating a new inner
scope for the prange loop body? That would basically make the loop
behave like a closure function, but with the looping header at the
'right' place rather than after the closure.
I'm not quite sure what the concrete changes to the CEP this would lead
to (assuming you mean this as a proposal for alternative semantics, and
not an implementation detail).
How would we treat reduction variables? They need to be supported, and
there's nothing in Python semantics to support reduction variables, they
are a rather special case everywhere. I suppose keeping the reduction
clause above, or use the "nonlocal" keyword in the loop body...
Also there's the else:-block, although we could make that part of the
scope. And the "lastprivate" functionality, although that could be
dropped without much loss.
Also, in the example, the local variable declaration of "tmp" outside
of the loop looks somewhat misplaced, although it's precedented by
comprehensions (which also have their own local scope in Cython).
Well, depending on the decision of lastprivate, the declaration would
need to be outside; I really like the idea of moving "cdef", and am
prepared to drop lastprivate for this.
Being explicit about thread-local variables does make things a lot safer
to use.
(One problem is that switching between serial and parallel one needs to
move variable declarations. But that only happens once, and one can use
"nthreads=1" to disable parallel after that.)
An example would then be:
def f(np.ndarray[double] x, double alpha):
cdef double s = 0, globtmp
with nogil:
for i in prange(x.shape[0]):
cdef double tmp # thread-private
tmp = alpha * i # alpha available from global scope
s += x[i] * tmp # still automatic reduction for inplace
operators
# printf(...s) -> now leads to error, since s is not
declared thread-private but is read
else:
# tmp still available here...looks a bit strange, but useful
s += tmp * 10
globtmp = tmp # we save tmp for later
# tmp not available here, globtmp is
return s
Or, we just drop support for the else block on these loops.
Dag Sverre
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