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Description
The following code demonstrates a performance issue where a function that returns a Union{Nothing,Tuple{String}} heap-allocates the returned tuple:
julia> versioninfo()
Julia Version 1.12.0-DEV.1571
Commit 671cd5e1db7 (2024-11-07 02:50 UTC)
Build Info:
Official https://julialang.org release
Platform Info:
OS: macOS (arm64-apple-darwin22.4.0)
CPU: 12 × Apple M2 Max
WORD_SIZE: 64
LLVM: libLLVM-18.1.7 (ORCJIT, apple-m2)
Threads: 1 default, 0 interactive, 1 GC (on 8 virtual cores)
Environment:
JULIA_SSL_CA_ROOTS_PATH =
julia> using BenchmarkTools
julia> zot(s) = s=="" ? nothing : (s,)
zot (generic function with 2 methods)
julia> foo(s) = s=="" ? nothing : s
foo (generic function with 2 methods)
julia> @btime zot($("a"))
6.750 ns (1 allocation: 16 bytes)
("a",)
julia> @btime foo($("a"))
4.625 ns (0 allocations: 0 bytes)
"a"
julia> @btime zot($(""))
2.250 ns (0 allocations: 0 bytes)And this is unique to Tuple{String}. With Tuple{Int}, we don't see it:
julia> bar(x) = x==0 ? nothing : (x,)
bar (generic function with 2 methods)
julia> @btime bar(10)
1.125 ns (0 allocations: 0 bytes)
(10,)Also, I see this on 1.12, 1.10, as well as all the way back to 1.0 on the various versions I tested. So not a recent regression.