Two things: 1. What happens without the branch if N is not a multiple of blockDim? Without the branch, you will get a segmentation error. 2. Branches are not as expensive as you think. Memory reads and writes are the most expensive things.
- bryan On Tue, Sep 27, 2011 at 8:08 AM, ericyosho <ericyo...@gmail.com> wrote: > I'm not sure if it is the right place, but since it is so elementary, > I just appreciate some explanation. > So in every CUDA tutorial example, e.g., to double each element in an > array, in kernel function, we have the following lines: > > int idx = // calculate a unique value for each thread > if (idx < N) // N is the number of elements of an array > a[idx] *= 2; > > "if branch" is a rather expensive operation, why do we want each > thread to go for this check? > Since on each device, only one kernel function is allowed to evaluate > at a time, why don't we let each thread double its own associated > value, and afterwards we simply copy N elements back to the host. > Basically, we just omit the "if" check, and go for the "double values" > line unconditionally. > > It seems this approach is more straightforward. > Do I miss anything? > > Best, > Zhe Yao > -------------- > Department of Electrical and Computer Engineering > McGill University > Montreal, QC, Canada > H3A 2A7 > > zhe....@mail.mcgill.ca > > _______________________________________________ > PyCUDA mailing list > PyCUDA@tiker.net > http://lists.tiker.net/listinfo/pycuda >
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