tweaks

src/rulesets/Base/mapreduce.jl

@@ -158,7 +158,7 @@ for mimum in (:minimum, :maximum)
             imax = findfirst(mask)
         else
             y = $mimum(f, xs; dims=dims)
-            mask = y .== f.(xs)
+            mask = y .== f.(xs)  # this is N^2 more calls to f, that's a lot!
             mask .= (mask .== cumsum(mask; dims=dims) .== true)
             imax = findall(mask)
         end
@@ -174,7 +174,7 @@ for mimum in (:minimum, :maximum)
             elseif Base.issingletontype(F)
                 # Then we need not accumulate the gradient with respect to `f`.
                 dfs = NoTangent()
-                # On a matrix we called `f` N^2 times, now call it N more with `rrule_via_ad`:
+                # On a matrix we called `f` 2*N^2 times, now call it N more with `rrule_via_ad`:
                 dxmax = map(view(xs, imax), unthunk(dys)) do x, dy
                     _, bk = rrule_via_ad(config, f, x)
                     df, dx = bk(dy)
@@ -184,7 +184,7 @@ for mimum in (:minimum, :maximum)
                 # This could perhaps accumulate df more smartly...
                 call(g, x) = g(x)
                 backs = map(x -> last(rrule_via_ad(config, f, x)), view(xs, imax))
-                dfs_and_dxs = map(unthunk∘last∘call, backs, unthunk(dy))
+                dfs_and_dxs = map(unthunk∘last∘call, backs, unthunk(dys))
                 dfs = sum(first, dfs_and_dxs)
                 dxmax = map(unthunk∘last, dfs_and_dxs)
             end