opt out of CR broadcasting rrule

Author: mcabbott

Project.toml

@@ -1,6 +1,6 @@
 name = "Zygote"
 uuid = "e88e6eb3-aa80-5325-afca-941959d7151f"
-version = "0.6.41"
+version = "0.6.42"
 
 [deps]
 AbstractFFTs = "621f4979-c628-5d54-868e-fcf4e3e8185c"

src/compiler/chainrules.jl

@@ -18,6 +18,9 @@ such that if a suitable rule is defined later, the generated function will recom
 function has_chain_rrule(T)
   config_T, arg_Ts = Iterators.peel(T.parameters)
   configured_rrule_m = meta(Tuple{typeof(rrule), config_T, arg_Ts...})
+  
+  isnothing(configured_rrule_m) && return false, nothing  # too crude, surely
+  
   if _is_rrule_redispatcher(configured_rrule_m.method)
     # The config is not being used:
     # it is being redispatched without config, so we need the method it redispatches to

src/lib/broadcast.jl

@@ -152,16 +152,24 @@ end
 # General Fallback
 # ================
 
-# The fused reverse mode implementation is the most general but currently has
+# The ~~fused~~ reverse mode implementation is the most general but currently has
 # poor performance. It works by flattening the broadcast and mapping the call to
 # `_pullback` over the input.
-
 # However, the core call
 # broadcast(_pullback, (cx,), f, args...)
 # is already 10x slower than a simple broadcast (presumably due to inlining
 # issues, or something similar) and the other operations needed take it to about
 # 100x overhead.
 
+# https://github.com/FluxML/Zygote.jl/pull/1001 tries to use broadcast_forward (using Dual numbers)
+# whenever possible, this was previously used only for CuArrays. It is usually much faster.
+
+# https://github.com/JuliaDiff/ChainRules.jl/pull/644 implements broadcasting.
+# Its generic rule would be applied before the one defined here, with AbstractArrayStyle
+# @adjoint function broadcasted(::AbstractArrayStyle, f::F, args...) where {F}
+# but does not pass all Zygote's tests. So disable it:
+ChainRulesCore.@opt_out rrule(cfg::ZygoteRuleConfig, ::typeof(Broadcast.broadcasted), f::F, args::Vararg{Any,N}) where {F,N}
+
 @generated inclen(::NTuple{N,Any}) where N = Val(N+1)
 
 # Avoid hitting special cases for `Adjoint` etc.