`return` matters in generated function lowering

[Keno]

Haven't tracked this down fully, but something is up with lowering:

@generated function code_llvm_helper(f, args...)
    NI.code_typed(f.parameters[1], Tuple{args...})[1].first
end
@generated function code_llvm_helper2(f, args...)
    return NI.code_typed(f.parameters[1], Tuple{args...})[1].first
end

julia> typeof(first(methods(code_llvm_helper)).generator.gen(nothing, Val{x->x}, [Bool]))
Expr

julia> typeof(first(methods(code_llvm_helper2)).generator.gen(nothing, Val{x->x}, [Bool]))
CodeInfo

This distinction matters quite a bit of course, since CodeInfo returns get treated specially.
Looks like the extra Expr in the first case is a LineNumberNode.

[vtjnash]

The result of code_typed isn’t valid as the return object for generated functions (we’re just sloppy about detecting this). Do you have an example with code_lowered?

[Keno]

Haven't tried, but it's a lowering transform that's being misapplied here, so I suspect the same will work.

[Keno]

Also, it would be nice if code_typed was valid for this, so I could feed my inference experiments to the code generator. It already does mostly work (prints an error, but then does compile the right code).

[vtjnash]

It's not valid because inference is an iterative algorithm, and this would be calling it recursively. It's also the wrong sort of return object (inferred code contains expressions that can't be passed to inference).

[Keno]

Well, I'm calling a different Inference's code_typed so from Inference's point of view I'm not calling it recursively.

[oxinabox]

aslo came up in NHDaly/StagedFunctions.jl#8

[NHDaly]

Copied from that issue:

Apparently using an actual return code_info line, instead of just having the code_info as the final expression, prevents Julia lowering pass from wrapping the return value in an Expr(:block, ...), meaning it can be used by Julia to build the generated function.

Here is an example showing that difference:

With return:

julia> Meta.@lower @generated function f() return 2 end
...
    $(Expr(:meta, :nospecialize, :(@_2)))
    return 2
    Core._expr(:block, $(QuoteNode(:(#= REPL[107]:1 =#))), nothing)
    return %3
...

Without return:

julia> Meta.@lower @generated function f() 2 end
...
    $(Expr(:meta, :nospecialize, :(@_2)))
    Core._expr(:block, $(QuoteNode(:(#= REPL[108]:1 =#))), 2)
    return %2
...

As far as I can tell, it's been like this since Julia 0.7.

It looks to me like the lowering was attempting to wrap the return value in a QuoteNode (potentially to prevent returning a CodeInfo to have the above effect?), but that there's a bug that prevents that from happening if you return the expression instead of leaving it as the last line?