...after which, the fast path isn't faster anymore, so delete it.

Author: mcabbott

src/rulesets/Base/mapreduce.jl

@@ -276,34 +276,15 @@ function ∇cumprod_dim(vald::Val{dim}, x::AbstractArray, dy=fill!(zero(x),1), y
  end
 
 @inline function ∇cumprod_dim!(dx::AbstractArray, ::Val{dim}, x::AbstractArray, dy, y) where {dim}
-    if any(iszero, x)
-        iters = ntuple(k -> k==dim ? Ref(:) : axes(x,k), ndims(x))
-        for ind in Iterators.product(iters...)
-            @views ∇cumprod!(dx[ind...], x[ind...], dy[ind...], y[ind...])
-        end
-    else
-        step1 = y .* dy # _rscale!!(y, dy)  # is it safe to mutate y?
-        step2 = _reverse!!(_cumsum!!(_reverse!!(step1, dim), dim), dim)
-        dx .+= step2 ./ x
+    iters = ntuple(k -> k==dim ? Ref(:) : axes(x,k), ndims(x))
+    for ind in Iterators.product(iters...)
+        @views ∇cumprod!(dx[ind...], x[ind...], dy[ind...], y[ind...])
     end
     return dx
 end
 
-# _rscale!!(A, β) = A .* β
-# _rscale!!(A::StridedArray, β) = A .*= β
-
-_reverse!!(x, dims=1) = reverse(x; dims=dims)
-if VERSION >= v"1.6"
-    _reverse!!(x::StridedArray, dims=1) = reverse!(x; dims=dims)
-else
-    _reverse!!(x::StridedVector, dims=1) = dims==1 ? reverse!(x) : x
-end
-
-_cumsum!!(x, dims=1) = cumsum(x; dims=dims)
-_cumsum!!(x::StridedArray, dims=1) = cumsum!(x, x; dims=dims)
-
 function ∇cumprod(x::AbstractVector, dy=one(x), y=cumprod(x))
-    T = promote_type(eltype(x), eltype(dy))
+    T = promote_type(eltype(x), eltype(dy))  # really needs to allow dy * y / x
     dx = fill!(similar(x, T, axes(x)), zero(T))  # axes(x) makes MArray on StaticArrays, Array for structured matrices
     ∇cumprod!(dx, x, dy, y)
     return dx