Add clarifying comment

Author: penelopeysm

src/threadsafe.jl

@@ -9,6 +9,15 @@ struct ThreadSafeVarInfo{V<:AbstractVarInfo,L} <: AbstractVarInfo
     logps::L
 end
 function ThreadSafeVarInfo(vi::AbstractVarInfo)
+    # In ThreadSafeVarInfo we use threadid() to index into the array of logp
+    # fields. This is not good practice --- see
+    # https://github.com/TuringLang/DynamicPPL.jl/issues/924 for a full
+    # explanation --- but it has worked okay so far.
+    # The use of nthreads()*2 here ensures that threadid() doesn't exceed
+    # the length of the logps array. Ideally, we would use maxthreadid(),
+    # but Mooncake can't differentiate through that. Empirically, nthreads()*2
+    # seems to provide an upper bound to maxthreadid(), so we use that here.
+    # See https://github.com/TuringLang/DynamicPPL.jl/pull/936
     return ThreadSafeVarInfo(
         vi, [Ref(zero(getlogp(vi))) for _ in 1:(Threads.nthreads() * 2)]
     )