How to check if Julia has been built with MMTk?

[MilesCranmer]

I followed the README instructions but I don't see anything indicating MMTk has actually been included in the Julia build. How should I verify this? (I'm on Julia 1.11, maybe this is why)

[udesou]

Running versioninfo() inside the REPL should tell you which GC is being used. For example, you should get:

Julia Version 1.13.0-DEV.791
Commit c985dba510d (2025-06-29 18:09 UTC)
Build Info:
  Official https://julialang.org release
Platform Info:
  OS: Linux (x86_64-linux-gnu)
  CPU: 128 × AMD EPYC 7502 32-Core Processor
  WORD_SIZE: 64
  LLVM: libLLVM-20.1.2 (ORCJIT, znver2)
  GC: Built with MMTk Julia 0.30.3 (4933fb4), using MMTk 0.30.0 (051bc74, BUILTIN_ENV_LOGGER, DEFAULT, IMMIX_NON_MOVING, IMMIX_SMALLER_BLOCK, OBJECT_PINNING, VM_SPACE)
Threads: 1 default, 1 interactive, 1 GC (on 128 virtual cores)
Environment:
  JULIA_CPU_TARGET = generic;sandybridge,-xsaveopt,clone_all;haswell,-rdrnd,base(1);x86-64-v4,-rdrnd,base(1)
  JULIA_CPU_THREADS = 128
  JULIA_INSTALL_DIR = julia-c985dba510
  JULIA_IMAGE_THREADS = 128
  JULIA_VERSION = 1.13.0-DEV
  JULIA_BINARYDIST_FILENAME = julia-c985dba510-linux64
  JULIA_DEPOT_PATH = /cache/julia-buildkite-plugin/depots/53de1ea5-6dd3-4008-b976-161c9f34b0ad

But indeed, building Julia with MMTk is supported in 1.12+, in particular, anything after this commit.