Add SmallTag type for more compact Dual types

src/config.jl

@@ -20,11 +20,50 @@ end
 
 Tag(::Nothing, ::Type{V}) where {V} = nothing
 
-
 @inline function ≺(::Type{Tag{F1,V1}}, ::Type{Tag{F2,V2}}) where {F1,V1,F2,V2}
     tagcount(Tag{F1,V1}) < tagcount(Tag{F2,V2})
 end
 
+"""
+    HashTag{Hash}
+
+HashTag is similar to a Tag, but carries just a small UInt64 hash, 
+instead of the full type, which makes stacktraces / types easier to 
+read while still providing good resilience to perturbation confusion.
+"""
+struct HashTag{H}
+end
+
+@generated function tagcount(::Type{HashTag{H}}) where {H}
+    :($(Threads.atomic_add!(TAGCOUNT, UInt(1))))
+end
+
+function HashTag(f::F, ::Type{V}) where {F,V}
+    H = if F <: Tuple
+        # no easy way to check Jacobian tag used with Hessians as multiple functions may be used
+        # see checktag(::Type{Tag{FT,VT}}, f::F, x::AbstractArray{V}) where {FT<:Tuple,VT,F,V}
+        nothing
+    else
+        hash(F) ⊻ hash(V)
+    end
+    tagcount(HashTag{H}) # trigger generated function
+    HashTag{H}()
+end
+
+HashTag(::Nothing, ::Type{V}) where {V} = nothing
+
+@inline function ≺(::Type{HashTag{H1}}, ::Type{Tag{F2,V2}}) where {H1,F2,V2}
+    tagcount(HashTag{H1}) < tagcount(Tag{F2,V2})
+end
+
+@inline function ≺(::Type{Tag{F1,V1}}, ::Type{HashTag{H2}}) where {F1,V1,H2}
+    tagcount(Tag{F1,V1}) < tagcount(HashTag{H2})
+end
+
+@inline function ≺(::Type{HashTag{H1}}, ::Type{HashTag{H2}}) where {H1,H2}
+    tagcount(HashTag{H1}) < tagcount(HashTag{H2})
+end
+
 struct InvalidTagException{E,O} <: Exception
 end
 
@@ -36,13 +75,22 @@ checktag(::Type{Tag{FT,VT}}, f::F, x::AbstractArray{V}) where {FT,VT,F,V} =
 
 checktag(::Type{Tag{F,V}}, f::F, x::AbstractArray{V}) where {F,V} = true
 
+# HashTag is a smaller tag, that only confirms the hash
+function checktag(::Type{HashTag{HT}}, f::F, x::AbstractArray{V}) where {HT,F,V}
+    H = hash(F) ⊻ hash(V)
+    if HT == H || HT === nothing
+        true
+    else
+        throw(InvalidTagException{HashTag{H},HashTag{HT}}())
+    end
+end
+
 # no easy way to check Jacobian tag used with Hessians as multiple functions may be used
 checktag(::Type{Tag{FT,VT}}, f::F, x::AbstractArray{V}) where {FT<:Tuple,VT,F,V} = true
 
 # custom tag: you're on your own.
 checktag(z, f, x) = true
 
-
 ##################
 # AbstractConfig #
 ##################
@@ -55,6 +103,14 @@ Base.eltype(cfg::AbstractConfig) = eltype(typeof(cfg))
 
 @inline (chunksize(::AbstractConfig{N})::Int) where {N} = N
 
+function maketag(tagstyle::Union{Symbol,Nothing}, f, X)
+    if HASHTAG_MODE_ENABLED
+        return HashTag(f, X)
+    else
+        return Tag(f, X)
+    end
+end
+
 ####################
 # DerivativeConfig #
 ####################
@@ -108,9 +164,9 @@ vector `x`.
 The returned `GradientConfig` instance contains all the work buffers required by
 `ForwardDiff.gradient` and `ForwardDiff.gradient!`.
 
-If `f` is `nothing` instead of the actual target function, then the returned instance can
-be used with any target function. However, this will reduce ForwardDiff's ability to catch
-and prevent perturbation confusion (see https://github.com/JuliaDiff/ForwardDiff.jl/issues/83).
+If `f` or `tag` is `nothing`, then the returned instance can be used with any target function.
+However, this will reduce ForwardDiff's ability to catch and prevent perturbation confusion
+(see https://github.com/JuliaDiff/ForwardDiff.jl/issues/83).
 
 This constructor does not store/modify `x`.
 """
@@ -145,9 +201,9 @@ The returned `JacobianConfig` instance contains all the work buffers required by
 `ForwardDiff.jacobian` and `ForwardDiff.jacobian!` when the target function takes the form
 `f(x)`.
 
-If `f` is `nothing` instead of the actual target function, then the returned instance can
-be used with any target function. However, this will reduce ForwardDiff's ability to catch
-and prevent perturbation confusion (see https://github.com/JuliaDiff/ForwardDiff.jl/issues/83).
+If `f` or `tag` is `nothing`, then the returned instance can be used with any target function.
+However, this will reduce ForwardDiff's ability to catch and prevent perturbation confusion
+(see https://github.com/JuliaDiff/ForwardDiff.jl/issues/83).
 
 This constructor does not store/modify `x`.
 """
@@ -170,9 +226,9 @@ The returned `JacobianConfig` instance contains all the work buffers required by
 `ForwardDiff.jacobian` and `ForwardDiff.jacobian!` when the target function takes the form
 `f!(y, x)`.
 
-If `f!` is `nothing` instead of the actual target function, then the returned instance can
-be used with any target function. However, this will reduce ForwardDiff's ability to catch
-and prevent perturbation confusion (see https://github.com/JuliaDiff/ForwardDiff.jl/issues/83).
+If `f!` or `tag` is `nothing`, then the returned instance can be used with any target function.
+However, this will reduce ForwardDiff's ability to catch and prevent perturbation confusion
+(see https://github.com/JuliaDiff/ForwardDiff.jl/issues/83).
 
 This constructor does not store/modify `y` or `x`.
 """
@@ -212,9 +268,9 @@ configured for the case where the `result` argument is an `AbstractArray`. If
 it is a `DiffResult`, the `HessianConfig` should instead be constructed via
 `ForwardDiff.HessianConfig(f, result, x, chunk)`.
 
-If `f` is `nothing` instead of the actual target function, then the returned instance can
-be used with any target function. However, this will reduce ForwardDiff's ability to catch
-and prevent perturbation confusion (see https://github.com/JuliaDiff/ForwardDiff.jl/issues/83).
+If `f` or `tag` is `nothing`, then the returned instance can be used with any target function.
+However, this will reduce ForwardDiff's ability to catch and prevent perturbation confusion
+(see https://github.com/JuliaDiff/ForwardDiff.jl/issues/83).
 
 This constructor does not store/modify `x`.
 """
@@ -236,9 +292,9 @@ type/shape of the input vector `x`.
 The returned `HessianConfig` instance contains all the work buffers required by
 `ForwardDiff.hessian!` for the case where the `result` argument is an `DiffResult`.
 
-If `f` is `nothing` instead of the actual target function, then the returned instance can
-be used with any target function. However, this will reduce ForwardDiff's ability to catch
-and prevent perturbation confusion (see https://github.com/JuliaDiff/ForwardDiff.jl/issues/83).
+If `f` or `tag` is `nothing`, then the returned instance can be used with any target function.
+However, this will reduce ForwardDiff's ability to catch and prevent perturbation confusion
+(see https://github.com/JuliaDiff/ForwardDiff.jl/issues/83).
 
 This constructor does not store/modify `x`.
 """

src/prelude.jl

@@ -1,6 +1,11 @@
 const NANSAFE_MODE_ENABLED = @load_preference("nansafe_mode", false)
 const DEFAULT_CHUNK_THRESHOLD = @load_preference("default_chunk_threshold", 12)
 
+# On ≤1.10, the hash of a type cannot be computed at compile-time,
+# making `HashTag(...)` type-unstable, so `Tag(...)` is left as
+# as the default.
+const HASHTAG_MODE_ENABLED = @load_preference("hashtag_mode", VERSION ≥ v"1.11")
+
 const AMBIGUOUS_TYPES = (AbstractFloat, Irrational, Integer, Rational, Real, RoundingMode)
 
 const UNARY_PREDICATES = Symbol[:isinf, :isnan, :isfinite, :iseven, :isodd, :isreal, :isinteger]