Merge pull request #1280 from FluxML/bc/passthrough-threading-ccall

Passthrough safe ccalls in threading code

src/compiler/reverse.jl

@@ -254,6 +254,15 @@ xaccum(ir) = nothing
 xaccum(ir, x) = x
 xaccum(ir, xs...) = push!(ir, xcall(Zygote, :accum, xs...))
 
+function passthrough_expr(ex::Expr)
+    # Metadata we want to preserve
+    isexpr(ex, GlobalRef, :call, :isdefined, :inbounds, :meta, :loopinfo) && return true
+    # ccalls and more that are safe to preserve/required for proper operation:
+    # - jl_set_task_threadpoolid: added in 1.9 for @spawn
+    isexpr(ex, :foreigncall) && unwrapquote(ex.args[1]) in (:jl_set_task_threadpoolid,) && return true
+    return false
+end
+
 function adjoint(pr::Primal)
   ir, sigs = adjointcfg(pr)
   for b in reverse(blocks(pr.ir))
@@ -278,10 +287,9 @@ function adjoint(pr::Primal)
         end
       elseif ex isa Core.PiNode
         grads[ex.val] = grads[v]
-      elseif isexpr(ex, GlobalRef, :call, :isdefined, :inbounds, :meta, :loopinfo)
-      elseif isexpr(ex)
+      elseif isexpr(ex) && !passthrough_expr(ex)
         push!(rb, stmt(xcall(Base, :error, """
-                             Can't differentiate $(ex.head) expression.
+                             Can't differentiate $(ex.head) expression $ex.
                              You might want to check the Zygote limitations documentation.
                              https://fluxml.ai/Zygote.jl/latest/limitations
                              """),