slow fallback array `copyto!` in Julia 1.7.0

[johnnychen94]

It looks like a potential type inference issue

using BenchmarkTools
X = rand(ComplexF32, 64, 64);

dst = reinterpret(reshape, Float32, X);
src = copy(dst);

@btime copyto!($dst, $src);
# 1.6.4: 42.120 μs (0 allocations: 0 bytes)
# 1.7.0: 800.737 μs (32768 allocations: 1.12 MiB)
# 1.8.0-DEV.1090: 829.629 μs (32768 allocations: 1.12 MiB)

Profiling the code shows that it's hitting the fallback dual-iterator implementation version and a slow Tuple iterator

julia/base/abstractarray.jl

Lines 1041 to 1047 in d16f480

	# Dual-iterator implementation
	ret = iterate(iterdest)
	@inbounds for a in src
	idx, state = ret
	dest[idx] = a
	ret = iterate(iterdest, state)
	end

julia/base/tuple.jl

Line 88 in d16f480

indexed_iterate(t::Tuple, i::Int, state=1) = (@inline; (getfield(t, i), i+1))

Downstream JuliaImages issues: JuliaImages/Images.jl#992 JuliaImages/ImageCore.jl#174 (cc: @etibarg @timholy)

[N5N3]

@code_warntype gives no warning.
But @code_typed shows that:

9 ── %70  = %new(Base.SCartesianIndex2{2}, 1, %65)::Base.SCartesianIndex2{2}
│    %71  = Core.tuple(1, %65, %66)::Tuple{Int64, Int64, Int64}
│    %72  = Core.tuple(%70, %71)::Tuple{Base.SCartesianIndex2{2}, Tuple{Int64, Int64, Int64}}
└───        goto #10
10 ┄ %74  = φ (#9 => %70)::Union{Base.SCartesianIndex2{2}, Tuple{Int64, Int64, Int64}}
│    %75  = φ (#9 => %71)::Union{Base.SCartesianIndex2{2}, Tuple{Int64, Int64, Int64}}
│    %76  = φ (#8 => %68, #9 => %72)::Union{Nothing, Tuple{Base.SCartesianIndex2{2}, Tuple{Int64, Int64, Int64}}}
│    %77  = Base.bitcast(UInt64, 1)::UInt64
│    %78  = Base.sub_int(%77, 0x0000000000000001)::UInt64
│    %79  = Base.arraylen(src)::Int64
│    %80  = Base.sle_int(0, %79)::Bool
│    %81  = Base.bitcast(UInt64, %79)::UInt64
│    %82  = Base.ult_int(%78, %81)::Bool
│    %83  = Base.and_int(%80, %82)::Bool
└───        goto #12 if not %83

Looks like we messes up at %74 and %75

[timholy]

git bisect blames 1f21f2d

[aviatesk]

Thanks for the bisect. Will be fixed by #43347.