Type Inference Problem with `getproperty`

I apologize in advance for the long post, but I did a good deal of my own analysis first to make sure I was understanding the situation correctly. I wanted to post my findings to save others the trouble of going through the same steps I did.

Getting to the actual problem, I am puzzled by the code generated by an overload of the getproperty function. I created a struct to represent a 2D vector:

struct Vec2
	x::Float64
	y::Float64
end

For implementing math operators, reinterpreting the vector as a tuple is convenient. Likewise, swizzle operators are often useful. Since these are both cheap, data-rearranging operations, getproperty seems like a reasonable way to implement them:

function Base.getproperty(v::Vec2, name::Symbol)
	if name == :tuple
		(v.x, v.y)
	elseif name == :yx
		Vec2(v.y, v.x)
	else
		getfield(v, name)
	end
end

Wrapping one of the getproperty calls in a function, and calling code_native on it reveals the first problem: getproperty is not inlined:

julia> f(v::Vec2) = v.yx;
julia> code_native(f, (Vec2,); syntax=:intel, debuginfo=:none);
        .text
        push    rbp
        mov     rbp, rsp
        push    rsi
        push    rdi
        sub     rsp, 48
        mov     rsi, rcx
        movabs  rax, offset getproperty
        lea     rdi, [rbp - 32]
        mov     r8d, 397788528
        mov     rcx, rdi
        call    rax
        test    dl, dl
        cmovns  rax, rdi
        vmovups xmm0, xmmword ptr [rax]
        vmovups xmmword ptr [rsi], xmm0
        mov     rax, rsi
        add     rsp, 48
        pop     rdi
        pop     rsi
        pop     rbp
        ret
        nop

This is easily fixed by adding the @inline tag to getproperty. After that change, accessing the yx property produces the expected code:

julia> code_native(f, (Vec2,); syntax=:intel, debuginfo=:none);
        .text
        push    rbp
        mov     rbp, rsp
        mov     rax, rcx
        vpermilps       xmm0, xmmword ptr [rdx], 78 # xmm0 = mem[2,3,0,1]
        vmovups xmmword ptr [rax], xmm0
        pop     rbp
        ret
        nop     word ptr cs:[rax + rax]
        nop     dword ptr [rax]

Not only does the compiler succeed in all the expected constant propagation and branch elimination optimizations, but recognizes that the swizzle operation can be implemented using single permute instruction. Impressive. However, the tuple property is still a problem. I won’t bother posting the assembly output, because it is lengthy; suffice it to say that it is very wrong. So, for some reason, wrapping the tuple operation in the getproperty function seems to confuse the optimizer. Using code_warntype reveals a potential problem. I introduce a temporary variable x to make the type of the tuple property clear:

julia> function f(v::Vec2)
           x = v.tuple
           x
       end
julia> code_warntype(f, (Vec2,))
Variables
  #self#::Core.Compiler.Const(f, false)
  v::Vec2
  x::Tuple{Union{Float64, Vec2, Tuple{Any,Any}},Union{Float64, Vec2, Tuple{Any,Any}}}

Body::Tuple{Union{Float64, Vec2, Tuple{Any,Any}},Union{Float64, Vec2, Tuple{Any,Any}}}
1 ─     (x = Base.getproperty(v, :tuple))
└──     return x

Doing the same exercise for the yx property tells a different story:

julia> function f(v::Vec2)
           x = v.yx
           x
       end
julia> code_warntype(f, (Vec2,))
Variables
  #self#::Core.Compiler.Const(f, false)
  v::Vec2
  x::Vec2

Body::Vec2
1 ─     (x = Base.getproperty(v, :yx))
└──     return x

So it looks like this is a type inference problem. So question #1 is: how is it that the type of the yx property can be properly inferred, but not the tuple property?

Moving on, the fact that a Tuple{Any,Any} is part of the type indicates to me that the type inference of (v.x, v.y) is going awry. After some trial and error, I was able to coerce getproperty into doing the right thing:

@inline function Base.getproperty(v::Vec2, name::Symbol)
	if name == :tuple
		#(v.x, v.y) # Original implementation
		#NTuple{2,Float64}(v.x, v.y) # Doesn't work
		(Float64(v.x), Float64(v.y)) # Works!
		#(getfield(v, :x), getfield(v, :y)) # Also works
	elseif name == :yx
		Vec2(v.y, v.x)
	else
		getfield(v, name)
	end
end

With this implementation, I finally get the assembly I expect:

f(v::Vec2) = v.tuple
julia> code_native(f, (Vec2,); syntax=:intel, debuginfo=:none);
        .text
        push    rbp
        mov     rbp, rsp
        mov     rax, rcx
        vmovups xmm0, xmmword ptr [rdx]
        vmovups xmmword ptr [rax], xmm0
        pop     rbp
        ret
        nop     word ptr cs:[rax + rax]
        nop     dword ptr [rax + rax]

Which leads me to question #2 is: why does this particular incantation fix the type inference problem? I would have expected the type inference system to be able to do this transformation on its own.

And finally, question #3 is: is there a better way to write getproperty that avoids this problem?

In case it is relevant, here is my sytem information:

julia> versioninfo()
Julia Version 1.5.3
Commit 788b2c77c1 (2020-11-09 13:37 UTC)
Platform Info:
  OS: Windows (x86_64-w64-mingw32)
  CPU: Intel(R) Core(TM) i7-5930K CPU @ 3.50GHz
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-9.0.1 (ORCJIT, haswell)
4 Likes

The problem is that your getproperty is recursive since v.x and v.y from inside the body will also result in a call to getproperty. Recursion isn’t necessarily (or even usually) a problem for inference, but there’s various heuristic limits on how hard inference tries to go through the recursion, made more complicated here by the fact that there’s also constant propagation needed to get the right answer. A more expert than me could probably tell you the details, but I think its basically just hitting those limits.

The reason :yx works is because the return value in that case is Vec2(...), so it doesn’t matter what’s in the ..., the compiler knows the answer can only be a Vec2, which is a concrete type.

The solution for :tuple is just to use getfield instead and skip the recursion, e.g. this works:

	if name == :tuple
		(getfield(v,:x), getfield(v,:y))
6 Likes

Thanks for your reply; that clears things up to some extent. I read a similar post where recursion was mentioned as a problem with optimizing getproperty, but I was confused since Vec2(v.y, v.x) is also recursive, but works just fine. I guess knowing the return type helps the compiler just enough to cause it to do the right thing; but what’s still puzzling to me is why NTuple{2,Float64}(v.y, v.x) doesn’t work then. This is almost the same as doing Vec2(v.y, v.x) in that the return type is explicit, but still generates the lengthy code.

It seems like the only reliable way to write performant getproperty functions is to always use getfield inside them, rather than accessing fields through the dot operator. This is unfortunate, because many people will make the same mistake I did (since this is the obvious way to implement custom properties), and their performance may suffer for it. Perhaps the documentation for getproperty should be updated to warn people about this problem?

1 Like

You may also try to use if name === :tuple, note the triple equals signs, to compare against singleton symbols. This is supposed to make it easier for the compiler. I haven’t tried it, but it might make the compiler go further here.

Yea, I think that’d be worthwhile. The current docstring even shows an example using getproperty recursively, so further implies that that’s how you should do it (of couse to be fair it will sometimes be performant). I’d encourage you to open a PR improving it if you have the time!

Agreed, I should have used === instead; though I imagine for Symbol, == and === reliably get optimized to the same native code. I did try this for curiosity’s sake, and it didn’t make any difference in the generated native code.

I have submitted a PR adding a documentation warning about this problem here: