I believe something just like that is already implemented in Devectorize.jl's @devec macro. Try: `@devec y[:] = y + a .* x[ind]`.
https://github.com/lindahua/Devectorize.jl On Friday, May 20, 2016 at 11:04:34 AM UTC-4, vav...@uwaterloo.ca wrote: > > I'm not sure that fixed-size array solves the problem. For one thing, the > size of the small array may not be known at compile time. For another, I > don't want the small vectors necessarily to be immutable. > > I decided that what would really solve this problem is indicial notation. > This can be provided by a macro. I have not actually written the macro-- > macro-writing sometimes gives me a headache. But I have already documented > it! See: > > https://github.com/StephenVavasis/Indicial.jl > <https://www.google.com/url?q=https%3A%2F%2Fgithub.com%2FStephenVavasis%2FIndicial.jl&sa=D&sntz=1&usg=AFQjCNHXjC8cNNi7_jdVi1QPzKUvFFeMTQ> > > for documentation of my latest vaporware. If someone out there who enjoys > macro-writing wants to take a crack at this before I do, feel free! > > -- Steve Vavasis > > > > > On Thursday, May 19, 2016 at 9:47:12 PM UTC-4, vav...@uwaterloo.ca wrote: >> >> The two functions test4 and test5 below are equivalent, but test5 is much >> faster than test4. Apparently test4 is carrying out a heap allocation on >> each iteration of the j-loop. Why? In general, which kinds of assignment >> statements of the form <array>=<array> create temporaries, and which don't? >> (In the example below, if the indirect addressing via array i is >> eliminated, then the two functions have comparable performance.) >> >> Thanks, >> Steve Vavasis >> >> function test4(n) >> y = [2.0, 6.0, 3.0] >> i = [1, 2, 3] >> z = [0.0, 0.0, 0.0] >> u = 0.0 >> for j = 1 : n >> z[:] = y[i] >> u += sum(z) >> end >> u >> end >> >> function test5(n) >> y = [2.0, 6.0, 3.0] >> i = [1, 2, 3] >> z = [0.0, 0.0, 0.0] >> u = 0.0 >> for j = 1 : n >> for k = 1 : 3 >> z[k] = y[i[k]] >> end >> u += sum(z) >> end >> u >> end >> >> >> julia> @time Testmv.test4(10000000) >> 1.071396 seconds (20.00 M allocations: 1.192 GB, 7.03% gc time) >> 1.1e8 >> >> julia> @time Testmv.test5(10000000) >> 0.184411 seconds (4.61 k allocations: 198.072 KB) >> 1.1e8 >> >>
