Creating `@cfunction` from a closure takes a long time in compilation

[tkf]

Edit: it looks like #41827 (which is a bug fix) introduced the performance regression.

Running the script below shows that demo_slow takes about 30 seconds to compile in Julia 1.8-DEV (9a2e763) and 1.7.0-rc1 (9eade61) while it completes in a few tens of milliseconds in Julia 1.7.0-beta4.2 (642719e) and 1.6.

as_cfunction(f) = @cfunction($f, Cvoid, ())

function demo_fast()
    as_cfunction() do
        global GLOBAL = 1
        return
    end
end

function demo_slow()
    x = 0
    as_cfunction() do
        global GLOBAL = x
        return
    end
end

@info "Calling `demo_fast`..."
@time demo_fast()
@info "Calling `demo_slow`..."
t0 = time_ns()
@time demo_slow()
@show (time_ns() - t0) / 1e9

includeing the above script in @profile suggests that Julia spends most of the time in gen_cfun_wrapper (and _ZNK4llvm13AttributeList13hasAttributesEj called from it).

[tkf]

It's a rather drastic regression so I just put in the milestone. But please feel free to remove it if not appropriate.

[vchuravy]

Can you bisect?

[tkf]

Yeah, I think I can

[tkf]

It looks like #41827 introduced the issue.

[JeffBezanson]

Looks like O(n^2) attribute list lookup?

[vtjnash]

Nah, just O(2^32). If there are no attributes, LLVM reports that there are instead 2^32 + 1. They delete this foot-gun in the next LLVM version.

[mwlidar]

Ah, now it becomes clear why my QML.jl based GUI application takes ages to compile and start since I upgraded to julia 1.6.3!!!
Does anyone have an idea how to hack around this until the new LLVM version gets merged? Maybe by adding a fake attribute or something like that?