Stack Allocation

Stack allocated arrays are great, as are mutable arrays.

StrideArrays.jl tries it's hardest to provide you with both. As you may have noted from the RNG and broadcasting pages, we were creating mutable StrideArrays without suffering memory allocations, just like with the immutable StaticArrays.SArray type. The mutable StaticArrays.MArray, on the other hand, would have allocated:

julia> @benchmark sum(exp.(@StrideArray randn(8,10))) # StrideArrays
  memory estimate:  0 bytes
  allocs estimate:  0
  minimum time:     127.557 ns (0.00% GC)
  median time:      127.986 ns (0.00% GC)
  mean time:        128.116 ns (0.00% GC)
  maximum time:     165.890 ns (0.00% GC)
  samples:          10000
  evals/sample:     888

julia> @benchmark sum(exp.(@MMatrix randn(8,10))) # StaticArrays
  memory estimate:  672 bytes
  allocs estimate:  1
  minimum time:     703.599 ns (0.00% GC)
  median time:      862.130 ns (0.00% GC)
  mean time:        887.160 ns (4.56% GC)
  maximum time:     136.675 μs (99.29% GC)
  samples:          10000
  evals/sample:     142

This is achieved thanks to a convenient macro, StrideArrays.@gc_preserve. When the macro is applied to a function call, it GC.@preserves all the arrays, and substitutes them with PtrArrays. This will safely preserve the array's memory during the call, while promising that the array won't escape, so that it may be stack allocated. Otherwise, passing mutable structs to non-inlined functions currently forces heap allocation. Many functions are overloaded for StrideArrays to provide a @gc_preserve barrier, so that calling them will not force heap allocation. However, doing this systematically is still a work in progress, so please file an issue if you encounter a function commonly used on arrays, especially if already defined in StrideArrays.jl, in which this is not the case.

When writing code making use of statically sized StrideArrays, you can use @gc_preserve in your own code when you can promise the array won't escape to make use of mutable stack allocated arrays. Note that @gc_preserve should also work on MArrays.