strange allocation in 1.7.0 (machine dependent)

[lmiq]

Followup from: https://discourse.julialang.org/t/where-is-this-one-allocation-coming-from/

Minimal example:

               _
   _       _ _(_)_     |  Documentation: https://docs.julialang.org
  (_)     | (_) (_)    |
   _ _   _| |_  __ _   |  Type "?" for help, "]?" for Pkg help.
  | | | | | | |/ _` |  |
  | | |_| | | | (_| |  |  Version 1.7.0 (2021-11-30)
 _/ |\__'_|_|_|\__'_|  |  Official https://julialang.org/ release
|__/                   |

julia> U(x, y) = x[1];

julia> a=[1.0]; b=(1.0,);

julia> using BenchmarkTools

julia> @btime U($a, $b)
  13.971 ns (1 allocation: 16 bytes)
1.0

The allocation occurs in one of four different computers I have tested (all running Linux), although all used the same Julia version.

In the computer where the allocation occurs, it does not occur in Julia 1.6.0, which I had installed there.

[simeonschaub]

Do you have anything in your startup.jl?

[lmiq]

No, nothing.

More specifically: there no package is installed except BenchmarkTools and its dependencies, I started a fresh julia installation with a new .julia folder.

[amrods]

I tested both 1.7 and 1.6.4 in 2 computers, one running Windows, the other macOS. In all cases the allocation occurs.
1.6.4 Windows:

julia> versioninfo()
Julia Version 1.6.4
Commit 35f0c911f4 (2021-11-19 03:54 UTC)
Platform Info:
  OS: Windows (x86_64-w64-mingw32)
  CPU: Intel(R) Xeon(R) E-2136 CPU @ 3.30GHz
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-11.0.1 (ORCJIT, skylake)

julia> @btime U($a, $b)
  17.635 ns (1 allocation: 16 bytes)
1.0

1.7 Windows:

julia> versioninfo()
Julia Version 1.7.0
Commit 3bf9d17731 (2021-11-30 12:12 UTC)
Platform Info:
  OS: Windows (x86_64-w64-mingw32)
  CPU: Intel(R) Xeon(R) E-2136 CPU @ 3.30GHz
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-12.0.1 (ORCJIT, skylake)

julia> @btime U($a, $b)
  17.818 ns (1 allocation: 16 bytes)
1.0

1.6.4 macOS:

julia> versioninfo()
Julia Version 1.6.4
Commit 35f0c911f4 (2021-11-19 03:54 UTC)
Platform Info:
  OS: macOS (x86_64-apple-darwin19.5.0)
  CPU: Intel(R) Core(TM) i7-8559U CPU @ 2.70GHz
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-11.0.1 (ORCJIT, skylake)

julia> @btime U($a, $b)
  15.952 ns (1 allocation: 16 bytes)
1.0

1.7 macOS:

julia> versioninfo()
Julia Version 1.7.0
Commit 3bf9d17731 (2021-11-30 12:12 UTC)
Platform Info:
  OS: macOS (x86_64-apple-darwin19.5.0)
  CPU: Intel(R) Core(TM) i7-8559U CPU @ 2.70GHz
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-12.0.1 (ORCJIT, skylake)

julia> @btime U($a, $b)
  14.360 ns (1 allocation: 16 bytes)
1.0

[gbaraldi]

For me, I can repro both behaviors on the same machine

@btime U($a, $b)
  17.994 ns (1 allocation: 16 bytes)
1.0

julia> versioninfo()
Julia Version 1.6.4
Commit 35f0c911f4 (2021-11-19 03:54 UTC)
Platform Info:
  OS: macOS (x86_64-apple-darwin19.5.0)
  CPU: Apple M1
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-11.0.1 (ORCJIT, westmere)
Environment:
  JULIA_NUM_PRECOMPILE_TASKS = 4
  JULIA_NUM_THREADS = 4

julia> @btime U($a, $b)
  4.000 ns (0 allocations: 0 bytes)
1.0

julia> versioninfo()
Julia Version 1.7.0
Commit 3bf9d17731 (2021-11-30 12:12 UTC)
Platform Info:
  OS: macOS (x86_64-apple-darwin19.5.0)
  CPU: Apple M1
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-12.0.1 (ORCJIT, westmere)
Environment:
  JULIA_NUM_PRECOMPILE_TASKS = 4
  JULIA_NUM_THREADS = 4

julia> @btime U($a, $b)
  1.291 ns (0 allocations: 0 bytes)
1.0

julia> versioninfo()
Julia Version 1.7.0
Commit 3bf9d17731 (2021-11-30 12:12 UTC)
Platform Info:
  OS: macOS (arm64-apple-darwin21.1.0)
  CPU: Apple M1
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-12.0.1 (ORCJIT, cyclone)
Environment:
  JULIA_NUM_PRECOMPILE_TASKS = 4
  JULIA_NUM_THREADS = 4

[ViralBShah]

Not seeing the allocations on 1.8.