help wanted
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Description
julia> foo(x) = 2.0x
foo (generic function with 1 method)
julia> bar(x) = @fastmath 2.0x
bar (generic function with 1 method)
julia> Zygote.derivative(foo, 1e6)
2.0
julia> Zygote.derivative(bar, 1e6)
ERROR: Non-differentiable function IntrinsicFunction
Stacktrace:
[1] error(::String) at ./error.jl:33
[2] (::Zygote.Pullback{Tuple{Core.IntrinsicFunction,Float64,Float64},Tuple{Core.IntrinsicFunction}})(::Int8) at /home/chriselrod/.julia/packages/Zygote/wYgcz/src/compiler/interface2.jl:0
[3] mul_fast at ./fastmath.jl:163 [inlined]
[4] (::Zygote.Pullback{Tuple{typeof(Base.FastMath.mul_fast),Float64,Float64},Tuple{Zygote.Pullback{Tuple{Core.IntrinsicFunction,Float64,Float64},Tuple{Core.IntrinsicFunction}}}})(::Int8) at /home/chriselrod/.julia/packages/Zygote/wYgcz/src/compiler/interface2.jl:0
[5] bar at ./REPL[55]:1 [inlined]
[6] (::Zygote.Pullback{Tuple{typeof(bar),Float64},Tuple{getfield(Zygote, Symbol("##265#back#163")){getfield(Zygote, Symbol("#back#162")){:mul_fast,Zygote.Context,Module,typeof(Base.FastMath.mul_fast)}},Zygote.Pullback{Tuple{typeof(Base.FastMath.mul_fast),Float64,Float64},Tuple{Zygote.Pullback{Tuple{Core.IntrinsicFunction,Float64,Float64},Tuple{Core.IntrinsicFunction}}}}}})(::Int8) at /home/chriselrod/.julia/packages/Zygote/wYgcz/src/compiler/interface2.jl:0
[7] (::getfield(Zygote, Symbol("##73#74")){Zygote.Pullback{Tuple{typeof(bar),Float64},Tuple{getfield(Zygote, Symbol("##265#back#163")){getfield(Zygote, Symbol("#back#162")){:mul_fast,Zygote.Context,Module,typeof(Base.FastMath.mul_fast)}},Zygote.Pullback{Tuple{typeof(Base.FastMath.mul_fast),Float64,Float64},Tuple{Zygote.Pullback{Tuple{Core.IntrinsicFunction,Float64,Float64},Tuple{Core.IntrinsicFunction}}}}}}})(::Int8) at /home/chriselrod/.julia/packages/Zygote/wYgcz/src/compiler/interface.jl:38
[8] gradient(::Function, ::Float64) at /home/chriselrod/.julia/packages/Zygote/wYgcz/src/compiler/interface.jl:44
[9] derivative(::typeof(bar), ::Float64) at /home/chriselrod/.julia/packages/Zygote/wYgcz/src/compiler/interface.jl:47
[10] top-level scope at REPL[57]:1
@fastmath is convenient for enabling the autovectorizer and contractions (ie, fma instructions) on unrolled expressions. Would be nice to add mul_fast and friends.