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
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, [email protected] 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
>
>
