DEFAULT_CHUNK_THRESHOLD is configurable via Preferences.jl (#582)

* DEFAULT_CHUNK_THRESHOLD is configurable via Preferences.jl

* Add docs

docs/src/user/advanced.md

@@ -84,6 +84,12 @@ julia> @time ForwardDiff.gradient!(out, rosenbrock, x, cfg);
   0.281853 seconds (4 allocations: 160 bytes)
 ```
 
+The underlying heuristic will compute a suitable chunk size smaller or equal
+to the `ForwardDiff.DEFAULT_CHUNK_THRESHOLD` constant. As of ForwardDiff
+v0.10.32 and Julia 1.6, this constant can be configured at load time via
+[Preferences.jl](https://github.com/JuliaPackaging/Preferences.jl) by setting the
+`default_chunk_threshold` value.
+
 If your input dimension is constant across calls, you should explicitly select a chunk size
 rather than relying on ForwardDiff's heuristic. There are two reasons for this. The first is
 that ForwardDiff's heuristic depends only on the input dimension, whereas in reality the

src/prelude.jl

@@ -1,7 +1,9 @@
 @static if VERSION >= v"1.6"
     const NANSAFE_MODE_ENABLED = @load_preference("nansafe_mode", false)
+    const DEFAULT_CHUNK_THRESHOLD = @load_preference("default_chunk_threshold", 12)
 else
     const NANSAFE_MODE_ENABLED = false
+    const DEFAULT_CHUNK_THRESHOLD = 12
 end
 
 const AMBIGUOUS_TYPES = (AbstractFloat, Irrational, Integer, Rational, Real, RoundingMode)
@@ -10,8 +12,6 @@ const UNARY_PREDICATES = Symbol[:isinf, :isnan, :isfinite, :iseven, :isodd, :isr
 
 const BINARY_PREDICATES = Symbol[:isequal, :isless, :<, :>, :(==), :(!=), :(<=), :(>=)]
 
-const DEFAULT_CHUNK_THRESHOLD = 12
-
 struct Chunk{N} end
 
 const CHUNKS = [Chunk{i}() for i in 1:DEFAULT_CHUNK_THRESHOLD]