`show_default`, but without type parameters

[goretkin]

Definitions like

julia/base/range.jl

Line 742 in 1e76111

show(io::IO, r::OneTo) = print(io, "Base.OneTo(", r.stop, ")")

could be no more than something like

show(io::IO, r::OneTo) = show_default(io, r; type_parameters=false)

There are many places where that's a default behavior that is wanted. For example, when all the type parameters correspond to fields that when show'd reveal their type. For example

struct Foo{T}
  x::T
end

show(Foo(Base.OneTo(3)))

produces

Foo{Base.OneTo{Int64}}(Base.OneTo(3)),
which is unnecessary.

Also, show_default has great logic to use IOContext to see which names need to be qualified or not:

julia> Base.show_default(stdout, Base.OneTo(3))
Base.OneTo{Int64}(3)
julia> Base.show_default(stdout, Foo(Base.OneTo(3)))
Foo{Base.OneTo{Int64}}(Base.OneTo(3))
julia> using Base: OneTo

julia> Base.show_default(stdout, Base.OneTo(3))
OneTo{Int64}(3)
julia> Base.show_default(stdout, Foo(Base.OneTo(3)))
Foo{OneTo{Int64}}(Base.OneTo(3))

Notice the inconsistency in Foo{OneTo{Int64}}(Base.OneTo(3)).

x-ref: #33260

[goretkin]

Seems suspiciously relevant

julia/base/show.jl

Lines 477 to 483 in 3222aba

	function print_without_params(@nospecialize(x))
	if isa(x,UnionAll)
	b = unwrap_unionall(x)
	return isa(b,DataType) && b.name.wrapper === x
	end
	return false
	end

but doesn't quite fit the bill where it's used.

One idea is a trait-based API, where a type could register "when you show me, my type is obvious", but that would require more careful design since it would likely be public.

[goretkin]

Related: https://github.com/MrVPlusOne/SimpleTypePrint

[timholy]

If a change like this is made, it shouldn't be done without an awareness that at least until recently show_default was a major invalidation risk. One would want to run the tests in #37193, which had to be reverted (see #38461), to make sure this kind of change doesn't cause unintended problems.

[timholy]

Also,

julia> r = Base.OneTo(3)
Base.OneTo(3)

julia> io = IOBuffer();

julia> @btime show($io, $r)
  135.656 ns (2 allocations: 96 bytes)

julia> @btime Base.show_default($io, $r)
  345.577 μs (44 allocations: 21.73 KiB)

A 2000x performance regression is not something to embrace lightly.

[bramtayl]

Is there a way to make show_default more performant?

[goretkin]

A 2000x performance regression is not something to embrace lightly.

I agree. I suspect this is due to "show_default has great logic to use IOContext to see which names need to be qualified or not" and it also has logic to print circular references.

I notice that the definitions prevent specializing compiled code to the type of x:

julia/base/show.jl

Lines 390 to 397 in 814945c

	show(io::IO, @nospecialize(x)) = show_default(io, x)
	show(x) = show(stdout::IO, x)
	# avoid inferring show_default on the type of `x`
	show_default(io::IO, @nospecialize(x)) = _show_default(io, inferencebarrier(x))
	function _show_default(io::IO, @nospecialize(x))

It seems reasonable that specializing on the type would abate that performance penalty, at the cost of introducing a compilation penalty.

[timholy]

It seems reasonable that specializing on the type would abate that performance penalty

When the type is known to inference, yes. When the type is not known to inference (e.g., printing elements of a Vector{Any}) it would have precisely the opposite effect. Currently

julia> using MethodAnalysis

julia> methodinstances(Base._show_default)
3-element Vector{Core.MethodInstance}:
 MethodInstance for _show_default(::IOContext{IOBuffer}, ::Any)
 MethodInstance for _show_default(::IOBuffer, ::Any)
 MethodInstance for _show_default(::IO, ::Any)

so you only have to dispatch on the io argument. If you allow specialization on the type, now you'll go from 3 MethodInstances to hundreds or thousands, and now you have to runtime dispatch on the type argument, and that will be bad for runtime performance. Given that it would also murder latency (it's an expensive method to compile because it's complicated), I think it's very unlikely that this would be a win overall. This is a textbook case for @nospecialize.

[timholy]

I suspect this is due to "show_default has great logic to use IOContext to see which names need to be qualified or not" and it also has logic to print circular references.

Don't guess, profile. 99% of the time is spent on this line:

julia/base/show.jl

Line 399 in a4cd68c

show(io, inferencebarrier(t))

It ends up spending all of its time in calling show_typealias. Most of its time is spent looking up names in modules and deciding whether there's a corresponding isdefined object, whether the name is deprecated, and whether it's const.

A bit of memoization, if that would be safe, might be quite effective here.