Stream: helpdesk (published)

Topic: Getting code lowered with only the function type

Mason Protter (Mar 05 2021 at 22:21):

So I know that I can get the lowered code for a method with code_lowered(f, Tuple{...}), but that requires an instance of the function. What if I only have the type? Usually, you can proceed by just doing F.instance, but that won't work for a closure.

Mason Protter (Mar 05 2021 at 22:25):

To be clear, how could this generated function body get the proper code_lowered for this closure?

julia> cl = let x = 1
           y -> x + y
       end
#4 (generic function with 1 method)

julia> @generated function foo(f, x)
           # need the code_lowered for `f(::typeof(x))` here
       end
foo (generic function with 1 method)

given that I can't do

julia> typeof(cl).instance
ERROR: UndefRefError: access to undefined reference

Takafumi Arakaki (tkf) (Mar 05 2021 at 22:29):

It looks like you can do Base.uncompressed_ir(Base.which(Tuple{typeof(identity),Int})) after 1.6.

Mason Protter (Mar 05 2021 at 22:51):

There's gotta be a way that works with all julia versions though, right? Since cassette.jl and IRTools.jl worth through generated functions and worked on 1.0

Takafumi Arakaki (tkf) (Mar 05 2021 at 22:55):

Yeah, I was just too lazy to cook it up. I know you can use Base.uncompressed_ast for backward compatibility. Not sure what's the backward-compatible way to emulate which(::Type), though.

Takafumi Arakaki (tkf) (Mar 05 2021 at 22:57):

since which is a public API, it'd make sense to put it in Compat.jl

Mason Protter (Mar 05 2021 at 23:07):

Ah I see

Takafumi Arakaki (tkf) (Mar 05 2021 at 23:16):

It looks like you can just copy & paste the last few lines of which:

julia> function getmethod(tt)
           m = ccall(:jl_gf_invoke_lookup, Any, (Any, UInt), tt, typemax(UInt))
           if m === nothing
               error("no unique matching method found for the specified argument types")
           end
           return m.func::Method
       end
getmethod (generic function with 1 method)

julia> getmethod(Tuple{typeof(identity),Int})
identity(x) in Base at operators.jl:511

Takafumi Arakaki (tkf) (Mar 05 2021 at 23:16):

(works in 1.0 and 1.5)

Mason Protter (Mar 05 2021 at 23:17):

beautiful, thank you!

Last updated: Oct 02 2023 at 04:34 UTC