Hello, thank you for developing an excellent library.

This is my first time creating a topic.

I am trying to use `ITensors.jl`

as a general MPS/MPO library for compressing high-dimensional data.

I found some strange behavior in compiling the function `MPS`

.

Here is my code:

```
using Pkg
using ITensors
@show versioninfo()
@show Pkg.status("ITensors")
function run1(ElemType::Type{T}, nbit, data; cutoff=1e-8) where {T}
sites = siteinds("Qubit", nbit)
tensor = ITensor(data, sites)
M = MPS(tensor, sites; cutoff=cutoff)
end
for nbit in 1:10
data = zeros(Float64, 2^nbit)
@show @time run1(Float64, nbit, data; cutoff=1e-1)
end
```

The output on my M1 Macbook Pro without emulation:

```
julia> include("benchmark.jl")
Julia Version 1.8.0
Commit 5544a0fab76 (2022-08-17 13:38 UTC)
Platform Info:
OS: macOS (arm64-apple-darwin21.3.0)
CPU: 8 Ć Apple M1 Pro
WORD_SIZE: 64
LIBM: libopenlibm
LLVM: libLLVM-13.0.1 (ORCJIT, apple-m1)
Threads: 5 on 6 virtual cores
Environment:
JULIA_NUM_THREADS = 5
versioninfo() = nothing
Status `~/.julia/environments/v1.8/Project.toml`
ā [9136182c] ITensors v0.3.19
Info Packages marked with ā have new versions available
Pkg.status("ITensors") = nothing
2.700941 seconds (12.31 M allocations: 725.304 MiB, 8.66% gc time, 99.98% compilation time)
#= /Users/hiroshi/Desktop/benchmark.jl:15 =# @time(run1(Float64, nbit, data; cutoff = 0.1)) = MPS
[1] ((dim=2|id=251|"Qubit,Site,n=1"),)
5.111948 seconds (28.60 M allocations: 1.469 GiB, 6.55% gc time, 99.95% compilation time)
#= /Users/hiroshi/Desktop/benchmark.jl:15 =# @time(run1(Float64, nbit, data; cutoff = 0.1)) = MPS
[1] ((dim=2|id=981|"Qubit,Site,n=1"), (dim=1|id=812|"Link,n=1"))
[2] ((dim=1|id=812|"Link,n=1"), (dim=2|id=849|"Qubit,Site,n=2"))
7.165274 seconds (30.41 M allocations: 1.677 GiB, 6.55% gc time, 99.96% compilation time)
#= /Users/hiroshi/Desktop/benchmark.jl:15 =# @time(run1(Float64, nbit, data; cutoff = 0.1)) = MPS
[1] ((dim=2|id=956|"Qubit,Site,n=1"), (dim=1|id=184|"Link,n=1"))
[2] ((dim=1|id=184|"Link,n=1"), (dim=2|id=143|"Qubit,Site,n=2"), (dim=1|id=318|"Link,n=2"))
[3] ((dim=1|id=318|"Link,n=2"), (dim=2|id=294|"Qubit,Site,n=3"))
6.969422 seconds (22.90 M allocations: 1.440 GiB, 3.69% gc time, 99.97% compilation time)
#= /Users/hiroshi/Desktop/benchmark.jl:15 =# @time(run1(Float64, nbit, data; cutoff = 0.1)) = MPS
[1] ((dim=2|id=285|"Qubit,Site,n=1"), (dim=1|id=473|"Link,n=1"))
[2] ((dim=1|id=473|"Link,n=1"), (dim=2|id=168|"Qubit,Site,n=2"), (dim=1|id=206|"Link,n=2"))
[3] ((dim=1|id=206|"Link,n=2"), (dim=2|id=923|"Qubit,Site,n=3"), (dim=1|id=711|"Link,n=3"))
[4] ((dim=1|id=711|"Link,n=3"), (dim=2|id=582|"Qubit,Site,n=4"))
7.823694 seconds (26.74 M allocations: 1.625 GiB, 5.53% gc time, 99.98% compilation time)
#= /Users/hiroshi/Desktop/benchmark.jl:15 =# @time(run1(Float64, nbit, data; cutoff = 0.1)) = MPS
[1] ((dim=2|id=924|"Qubit,Site,n=1"), (dim=1|id=383|"Link,n=1"))
[2] ((dim=1|id=383|"Link,n=1"), (dim=2|id=135|"Qubit,Site,n=2"), (dim=1|id=673|"Link,n=2"))
[3] ((dim=1|id=673|"Link,n=2"), (dim=2|id=579|"Qubit,Site,n=3"), (dim=1|id=412|"Link,n=3"))
[4] ((dim=1|id=412|"Link,n=3"), (dim=2|id=35|"Qubit,Site,n=4"), (dim=1|id=209|"Link,n=4"))
[5] ((dim=1|id=209|"Link,n=4"), (dim=2|id=767|"Qubit,Site,n=5"))
8.291299 seconds (28.73 M allocations: 1.721 GiB, 4.21% gc time, 99.97% compilation time)
#= /Users/hiroshi/Desktop/benchmark.jl:15 =# @time(run1(Float64, nbit, data; cutoff = 0.1)) = MPS
[1] ((dim=2|id=264|"Qubit,Site,n=1"), (dim=1|id=576|"Link,n=1"))
[2] ((dim=1|id=576|"Link,n=1"), (dim=2|id=605|"Qubit,Site,n=2"), (dim=1|id=559|"Link,n=2"))
[3] ((dim=1|id=559|"Link,n=2"), (dim=2|id=947|"Qubit,Site,n=3"), (dim=1|id=623|"Link,n=3"))
[4] ((dim=1|id=623|"Link,n=3"), (dim=2|id=894|"Qubit,Site,n=4"), (dim=1|id=488|"Link,n=4"))
[5] ((dim=1|id=488|"Link,n=4"), (dim=2|id=304|"Qubit,Site,n=5"), (dim=1|id=762|"Link,n=5"))
[6] ((dim=1|id=762|"Link,n=5"), (dim=2|id=954|"Qubit,Site,n=6"))
8.517200 seconds (29.35 M allocations: 1.742 GiB, 5.51% gc time, 99.98% compilation time)
#= /Users/hiroshi/Desktop/benchmark.jl:15 =# @time(run1(Float64, nbit, data; cutoff = 0.1)) = MPS
[1] ((dim=2|id=376|"Qubit,Site,n=1"), (dim=1|id=454|"Link,n=1"))
[2] ((dim=1|id=454|"Link,n=1"), (dim=2|id=122|"Qubit,Site,n=2"), (dim=1|id=227|"Link,n=2"))
[3] ((dim=1|id=227|"Link,n=2"), (dim=2|id=247|"Qubit,Site,n=3"), (dim=1|id=613|"Link,n=3"))
[4] ((dim=1|id=613|"Link,n=3"), (dim=2|id=385|"Qubit,Site,n=4"), (dim=1|id=649|"Link,n=4"))
[5] ((dim=1|id=649|"Link,n=4"), (dim=2|id=601|"Qubit,Site,n=5"), (dim=1|id=88|"Link,n=5"))
[6] ((dim=1|id=88|"Link,n=5"), (dim=2|id=858|"Qubit,Site,n=6"), (dim=1|id=349|"Link,n=6"))
[7] ((dim=1|id=349|"Link,n=6"), (dim=2|id=91|"Qubit,Site,n=7"))
8.938856 seconds (30.28 M allocations: 1.791 GiB, 4.16% gc time, 99.97% compilation time)
#= /Users/hiroshi/Desktop/benchmark.jl:15 =# @time(run1(Float64, nbit, data; cutoff = 0.1)) = MPS
[1] ((dim=2|id=11|"Qubit,Site,n=1"), (dim=1|id=713|"Link,n=1"))
[2] ((dim=1|id=713|"Link,n=1"), (dim=2|id=693|"Qubit,Site,n=2"), (dim=1|id=662|"Link,n=2"))
[3] ((dim=1|id=662|"Link,n=2"), (dim=2|id=165|"Qubit,Site,n=3"), (dim=1|id=547|"Link,n=3"))
[4] ((dim=1|id=547|"Link,n=3"), (dim=2|id=979|"Qubit,Site,n=4"), (dim=1|id=368|"Link,n=4"))
[5] ((dim=1|id=368|"Link,n=4"), (dim=2|id=968|"Qubit,Site,n=5"), (dim=1|id=933|"Link,n=5"))
[6] ((dim=1|id=933|"Link,n=5"), (dim=2|id=809|"Qubit,Site,n=6"), (dim=1|id=853|"Link,n=6"))
[7] ((dim=1|id=853|"Link,n=6"), (dim=2|id=967|"Qubit,Site,n=7"), (dim=1|id=775|"Link,n=7"))
[8] ((dim=1|id=775|"Link,n=7"), (dim=2|id=88|"Qubit,Site,n=8"))
9.080471 seconds (30.42 M allocations: 1.792 GiB, 4.50% gc time, 99.97% compilation time)
#= /Users/hiroshi/Desktop/benchmark.jl:15 =# @time(run1(Float64, nbit, data; cutoff = 0.1)) = MPS
[1] ((dim=2|id=363|"Qubit,Site,n=1"), (dim=1|id=612|"Link,n=1"))
[2] ((dim=1|id=612|"Link,n=1"), (dim=2|id=415|"Qubit,Site,n=2"), (dim=1|id=963|"Link,n=2"))
[3] ((dim=1|id=963|"Link,n=2"), (dim=2|id=50|"Qubit,Site,n=3"), (dim=1|id=169|"Link,n=3"))
[4] ((dim=1|id=169|"Link,n=3"), (dim=2|id=367|"Qubit,Site,n=4"), (dim=1|id=231|"Link,n=4"))
[5] ((dim=1|id=231|"Link,n=4"), (dim=2|id=103|"Qubit,Site,n=5"), (dim=1|id=151|"Link,n=5"))
[6] ((dim=1|id=151|"Link,n=5"), (dim=2|id=213|"Qubit,Site,n=6"), (dim=1|id=597|"Link,n=6"))
[7] ((dim=1|id=597|"Link,n=6"), (dim=2|id=267|"Qubit,Site,n=7"), (dim=1|id=113|"Link,n=7"))
[8] ((dim=1|id=113|"Link,n=7"), (dim=2|id=952|"Qubit,Site,n=8"), (dim=1|id=741|"Link,n=8"))
[9] ((dim=1|id=741|"Link,n=8"), (dim=2|id=421|"Qubit,Site,n=9"))
9.667407 seconds (31.67 M allocations: 1.853 GiB, 4.98% gc time, 99.97% compilation time)
#= /Users/hiroshi/Desktop/benchmark.jl:15 =# @time(run1(Float64, nbit, data; cutoff = 0.1)) = MPS
[1] ((dim=2|id=365|"Qubit,Site,n=1"), (dim=1|id=867|"Link,n=1"))
[2] ((dim=1|id=867|"Link,n=1"), (dim=2|id=529|"Qubit,Site,n=2"), (dim=1|id=850|"Link,n=2"))
[3] ((dim=1|id=850|"Link,n=2"), (dim=2|id=356|"Qubit,Site,n=3"), (dim=1|id=404|"Link,n=3"))
[4] ((dim=1|id=404|"Link,n=3"), (dim=2|id=753|"Qubit,Site,n=4"), (dim=1|id=450|"Link,n=4"))
[5] ((dim=1|id=450|"Link,n=4"), (dim=2|id=572|"Qubit,Site,n=5"), (dim=1|id=895|"Link,n=5"))
[6] ((dim=1|id=895|"Link,n=5"), (dim=2|id=919|"Qubit,Site,n=6"), (dim=1|id=0|"Link,n=6"))
[7] ((dim=1|id=0|"Link,n=6"), (dim=2|id=80|"Qubit,Site,n=7"), (dim=1|id=847|"Link,n=7"))
[8] ((dim=1|id=847|"Link,n=7"), (dim=2|id=250|"Qubit,Site,n=8"), (dim=1|id=909|"Link,n=8"))
[9] ((dim=1|id=909|"Link,n=8"), (dim=2|id=339|"Qubit,Site,n=9"), (dim=1|id=66|"Link,n=9"))
[10] ((dim=1|id=66|"Link,n=9"), (dim=2|id=439|"Qubit,Site,n=10"))
```

The result of my experiment indicates that a substantial amount of the code is compiled each time when changing the number of sites. This sounds somehow strange to me. Is this an expected behavior?