Code returned from generated function calls behaves incorrectly

[maleadt]

Caused by #24113 (cc @JeffBezanson), breaks CUDAnative. Repro:

function inner(val::T) where {T}
    println("inner($T)")
end

@generated function outer(val::T) where T
    Core.println("outer($T)")

    ex = quote end

    if T <: Number
        Core.println("-> call inner(val)")
        push!(ex.args, :( inner(val) ))
    elseif T <: Tuple
        Core.println("-> call outer(val[1]) -> inner(val[1])")
        push!(ex.args, :( outer(val[1])) )
    end

    return ex
end

outer(1)
println()
outer((1,))

Before:

outer(Int64)
-> call inner(val)
inner(Int64)

outer(Tuple{Int64})
-> call outer(val[1]) -> inner(val[1])
inner(Int64)

After:

outer(Int64)
-> call inner(val)
inner(Int64)

outer(Tuple{Int64})
-> call outer(val[1]) -> inner(val[1])
inner(Tuple{Int64})
inner(Int64)

ie. the inner function is called on a Tuple{Int64}, while the code returned by outer contains a call to outer(val[1]) where val[1] is a plain integer, and thus should result in a call to inner(val[1]).

Couple of things that change this behavior:

• change to ex = Expr(:block) or return expressions instead of pushing to a block
• not calling/inferring outer(1) first causes infinite recursion

[martinholters]

Looks like the quote end is evaluated only once:

julia> @generated function foo(::Val{N}) where N
           ex = quote end
           println(object_id(ex))
           println(length(ex.args))
           push!(ex.args, :nothing)
           if N > 0
               :(foo(Val(N-1)))
           else
               :(nothing)
           end
       end
foo (generic function with 1 method)

julia> foo(Val(3))
12108886621498232131
1
12108886621498232131
2
12108886621498232131
3
12108886621498232131
4

[martinholters]

Um, I thought one could inspect the lowered generator, but

julia> code_lowered(foo, Tuple{Val{3}}, false)
ERROR: Method is @generated; try `code_lowered` instead.

Julia bug or user bug?

[JeffBezanson]

See if this fixes it:

--- a/base/inference.jl
+++ b/base/inference.jl
@@ -4387,8 +4387,6 @@ function effect_free(@nospecialize(e), src::CodeInfo, mod::Module, allow_vola
             return allow_volatile
         elseif head === :the_exception
             return allow_volatile
-        elseif head === :copyast
-            return true
         else
             return false
         end

[maleadt]

See if this fixes it

Thanks, but no that doesn't seem to fix the original issue, or what @martinholters reduced it to.

[JeffBezanson]

Somehow (likely in the new allocation elimination pass) this transformation is happening:

julia> function f()
       x = quote end
       return x
       end
f (generic function with 1 method)

julia> @code_typed f()
CodeInfo(:(begin
        #= line 3 =#
        return $(QuoteNode(quote
    #= REPL[3]:2 =#
end))
    end)) => Expr

I haven't figured out how yet, though.

[martinholters]

Bisection confirms 4e81b9c as the point where this regressed.

[quinnj]

Anyone have any clues here? I miss generated functions 😢 . Happy to help dig in if anyone can give some pointers.

[maleadt]

I miss generated functions 😢

Where are you running into this issue? Using an Expr(:block) seems to work around it, so maybe it's related to the LineNumberNode that quot end generates.

# example from Jeff
julia> function f()
       x = quote end
       return x
       end
f (generic function with 1 method)

julia> @code_typed f()
CodeInfo(:(begin
        #= line 3 =#
        return $(QuoteNode(quote
    #= REPL[1]:2 =#
end))
    end)) => Expr

# workaround?
julia> function g()
       x = Expr(:block)
       return x
       end
g (generic function with 1 method)

julia> @code_typed g()
CodeInfo(:(begin
        # meta: location boot.jl Type 197
        SSAValue(1) = (Core._expr)(:block)::Expr
        # meta: pop location
        #= line 3 =#
        return SSAValue(1)
    end)) => Expr