`@btime` reports wrong allocation

[chunjiw]

MWE:

using BenchmarkTools

struct S
    x
end
function (s::S)()
    s.x[1]
end

function f()
    s1 = S(rand(10))
    @time s1()
    @btime $s1()
    println(@allocated s1())
    s2 = S(1)
    @time s2()
    @btime $s2()
    @benchmark $s2()
    println(@allocated s2())
end

f()

The output I see is

  0.000001 seconds
  19.435 ns (1 allocation: 16 bytes)
0
  0.000000 seconds
  18.050 ns (0 allocations: 0 bytes)
0

So when the field is a vector, @btime reports wrong allocation, assuming @time and @allocated are right. This may be related to https://github.com/JuliaLang/julia/issues/53694 but I'm not sure.

julia> versioninfo()
Julia Version 1.10.2
Commit bd47eca2c8a (2024-03-01 10:14 UTC)
Build Info:
  Official https://julialang.org/ release
Platform Info:
  OS: Linux (x86_64-linux-gnu)
  CPU: 12 × Intel(R) Core(TM) i7-6850K CPU @ 3.60GHz
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-15.0.7 (ORCJIT, broadwell)
Threads: 1 default, 0 interactive, 1 GC (on 12 virtual cores)