Julia 1.12 Highlights

Julia version 1.12 has finally been released. We want to thank all the contributors to this release and all the testers who helped find regressions and issues in the pre-releases. Without you, this release would not have been possible.

The full list of changes can be found in the NEWS file, but here we'll give a more in-depth overview of some of the release highlights.

New --trim feature

Jeff Bezanson, Cody Tapscott, Gabriel Baraldi

julia now has a new experimental--trim feature, when compiling a system image with this mode julia will trim statically unreachable code leading to significantly better compile times and binary sizes. To use it you also need to pass the --experimental flag when building the system image.

In order to use it, any code that is reachable from the entrypoints must not have any dynamic dispatches otherwise the trimming will be unsafe and it will error during compilation.

The expected way of using it is via the JuliaC.jl package, which provides a CLI and a programmatic API.

For example a simple package with an @main function:

module AppProject

function @main(ARGS)
    println(Core.stdout, "Hello World!")
    return 0
end

end

juliac --output-exe app_test_exe --bundle build --trim=safe --experimental ./AppProject

./build/bin/app_test_exe
Hello World!

ls -lh build/bin/app_test_exe
-rwxr-xr-x@ 1 gabrielbaraldi  staff   1.1M Oct  6 17:22 ./build/bin/app_test_exe*

Redefinition of constants (structs)

Keno Fischer, Tim Holy

Bindings now participate in the "world age" mechanism previously used for methods. This has the effect that constants and structs can be properly redefined. As an example:

# Define a struct and a method on that struct:
julia> struct Foo
          a::Int
       end

julia> g(f::Foo) = f.a^2
g (generic function with 1 method)

julia> g(Foo(2))
4

# Redefine the struct (julia pre-1.12 would error here)
julia> struct Foo
          a::Int
          b::Int
       end

# Note that functions need to be redefined to work on the new `Foo`
julia> g(Foo(1,2))
ERROR: MethodError: no method matching g(::Foo)
The function `g` exists, but no method is defined for this combination of argument types.

Closest candidates are:
  g(::@world(Foo, 39296:39300)) # <- This is syntax for accessing the binding in an older "world"
   @ Main REPL[2]:1

julia> g(f::Foo) = f.a^2 + f.b^2
g (generic function with 2 methods)

julia> g(Foo(2,3))
13

There is also work in progress in Revise.jl to automatically redefine functions on replaced bindings. This should significantly reduce the number of times you have to restart Julia while iterating on some piece of code.

New tracing flags and macros for inspecting what Julia compiles

Ian Butterworth, Nathan Daly

--trace-compile-timing is a new command-line flag that augments --trace-compile by printing how long each compiled method took (in milliseconds) before the corresponding precompile(...) line. This makes it easier to spot costly compilations.

In addition, two macros for ad-hoc tracing without restarting Julia have been added:

• @trace_compile expr runs expr with --trace-compile=stderr --trace-compile-timing enabled, emitting timed precompile(...) entries only for that call.

• @trace_dispatch expr runs expr with --trace-dispatch=stderr enabled, reporting methods that are dynamically dispatched.

Examples

julia> @trace_compile @eval rand(2,2) * rand(2,2)
#=   79.9 ms =# precompile(Tuple{typeof(Base.rand), Int64, Int64})
#=    4.4 ms =# precompile(Tuple{typeof(Base.:(*)), Array{Float64, 2}, Array{Float64, 2}})
2×2 Matrix{Float64}:
 0.302276  0.14341
 0.738941  0.396414

julia> f(x) = x

julia> @trace_dispatch map(f, Any[1,2,3])
precompile(Tuple{Type{Array{Int64, 1}}, UndefInitializer, Tuple{Int64}})
precompile(Tuple{typeof(Base.collect_to_with_first!), Array{Int64, 1}, Int64, Base.Generator{Array{Any, 1}, typeof(Main.f)}, Int64})
3-element Vector{Int64}:
 1
 2
 3

New multi-threading features

One interactive thread by default

Gabriel Baraldi, Ian Butterworth

Julia now starts with one interactive thread by default (in addition to the default thread). This means that by default Julia runs with the threading configuration of 1 default thread, 1 interactive thread.

The interactive thread pool is where the REPL and other interactive operations run. By separating these from the default thread pool (where @spawn and @threads schedule work when no threadpool is specified), the REPL can perform operations like autocomplete queries in parallel with user code execution, leading to a more responsive interactive experience.

Key behaviors:

• Default: Julia starts with -t1,1 (1 default + 1 interactive thread)

• Explicit -t1: If you explicitly request 1 thread with -t1, Julia will give you exactly that—no additional interactive thread will be added (resulting in -t1,0)

• Multiple threads: -t2 or -tauto will give you the requested default threads plus 1 interactive thread

• Manual control: You can always specify both pools explicitly, e.g., -t4,2 for 4 default and 2 interactive threads

This change improves the out-of-the-box experience while maintaining backwards compatibility for users who explicitly request single-threaded execution.

Threads settings respect CPU affinity

Mosè Giordano

Julia now respects CPU affinity settings, such as those set via cpuset/taskset/cgroups, etc. The same also applies to the default number of BLAS threads, which now follows the same logic. This can also be observed when running Julia inside Docker. Currently, you have

$ docker run --cpus=4 --rm -ti julia:1.11 julia --threads=auto -e '@show Threads.nthreads(); using LinearAlgebra; @show BLAS.get_num_threads()'
Threads.nthreads() = 22
BLAS.get_num_threads() = 11

When starting Julia with --threads=auto, Threads.nthreads() is equal to the total number of CPUs on the system instead of the only 4 CPUs reserved by Docker. Likewise, the number of BLAS threads, which can be obtained with BLAS.get_num_threads() and on x86-64 systems is by default half the number of available cores, is 11 instead of 2. With Julia v1.12 this is fixed, and the number of both Julia and BLAS threads will respect the number of CPUs reserved by Docker:

% docker run --cpus=4 --rm -ti julia:1.12 julia --threads=auto -e '@show Threads.nthreads(); using LinearAlgebra; @show BLAS.get_num_threads()'
Threads.nthreads() = 4
BLAS.get_num_threads() = 2

The new behavior is also important to avoid oversubscription out-of-the-box when running Julia on HPC systems where schedulers set CPU affinity when using shared resources.

OncePerX

Jameson Nash

Certain initialization patterns need to run only once, depending on scope: per process, per thread, or per task. To make this easier and safer, Julia now provides three built-in types:

• OncePerProcess{T}: runs an initializer exactly once per process, returning the same value for all future calls.

• OncePerThread{T}: runs an initializer once for each thread ID. Subsequent calls on the same thread return the same value.

• OncePerTask{T}: runs an initializer once per task, reusing the same value within that task.

These replace common hand-rolled solutions such as using __init__, nthreads(), or task_local_storage() directly.

A simple example of OncePerProcess:

julia> const global_state = Base.OncePerProcess{Vector{UInt32}}() do
           println("Making lazy global value...done.")
           return [Libc.rand()]
       end;

julia> a = global_state();
Making lazy global value...done.

julia> a === global_state()
true

Use cases:

• OncePerProcess: caches, global constants, or initialization that should happen once per Julia process (even across precompilation).

• OncePerThread: per-thread state needed for interoperability with C libraries or specialized threading models.

• OncePerTask: lightweight task-local state without manually managing task_local_storage.

These types provide a safer, composable way to express “initialize once” semantics in concurrent Julia code.

Building Julia and LLVM using the Binary Optimization and Layout Tool (BOLT).

Zentrik

BOLT is a post-link optimizer from LLVM that improves runtime performance by reordering functions and basic blocks, splitting hot and cold code, and folding identical functions. Julia now supports building BOLT-optimized versions of libLLVM, libjulia-internal, and libjulia-codegen.

These optimizations reduce compilation and execution time in common workloads. For example, the all-inference benchmarks improve by about 10%, an LLVM-heavy workload shows a similar ~10% gain, and building corecompiler.ji improves by 13–16% with BOLT. When combined with PGO and LTO, total improvements of up to ~23% have been observed.

To build a BOLT-optimized Julia, run the following commands from contrib/bolt/:

make stage1
make copy_originals
make bolt_instrument
make finish_stage1
make merge_data
make bolt

The optimized binaries will be available in the optimized.build directory. An analogous workflow exists in contrib/pgo-lto-bolt/ for combining BOLT with PGO+LTO.

BOLT currently works only on Linux x86_64 and aarch64, and the resulting .so files must not be stripped. Some readelf warnings may appear during testing but are considered harmless.

The @atomic macro family now supports reference assignment syntax

Marek Kaluba

The @atomic macro family now supports indexing (e.g. m[i], m[i,j]) in addition to field access. This makes it possible to perform atomic fetch, set, modify, swap, compare-and-swap, and set-once directly on array-like references. The macros expand to new APIs: getindex_atomic, setindex_atomic!, modifyindex_atomic!, swapindex_atomic!, replaceindex_atomic!, and setindexonce_atomic!. Vararg and CartesianIndex indexing are supported.

For example:

mem = AtomicMemory{Int}(undef, 2)

@atomic mem[1] = 2                 # atomic set
x = @atomic mem[1]                 # atomic fetch
@atomic :monotonic mem[1] += 1     # atomic modify with order
old = @atomicswap mem[1] = 4       # atomic swap (returns old)
res = @atomicreplace mem[1] 4 => 10  # (old=4, success=true)
ok  = @atomiconce mem[2] = 7         # set once (Bool)

New option –task-metrics=yes to enable the collection of per-task timing information

Two new per-task metrics can be enabled by starting Julia with --task-metrics=yes or by calling Base.Experimental.task_metrics(true). Enabling or disabling task metrics with Base.Experimental.task_metrics only affects new tasks, not existing ones. The metrics are:

• Base.Experimental.task_running_time_ns(t::Task): the time for which t was actually running. This is currently inclusive of GC time, compilation time, and any spin time.

• Base.Experimental.task_wall_time_ns(t::Task): the time from the scheduler becoming aware of t until t is complete.

New Pkg features

Kristoffer Carlsson

Workspace

A workspace is a set of project files that all share the same manifest. Each project in a workspace can include its own dependencies, compatibility information, and even function as a full package.

When the package manager resolves dependencies, it considers the requirements and compatibility of all the projects in the workspace. The compatible versions identified during this process are recorded in a single manifest file.

A workspace is defined in the base project by giving a list of the projects in it:

[workspace]
projects = ["test", "docs", "benchmarks", "PrivatePackage"]

This structure is particularly beneficial for developers using a monorepo approach, where a large number of unregistered packages may be involved. It is also useful for adding documentation or benchmarks to a package by including additional dependencies beyond those of the package itself. Test-specific dependencies are now recommended to be specified using the workspace approach (a project file in the test directory that is part of the workspace defined by the package project file).

Workspaces can also be nested: a project that itself defines a workspace can also be part of another workspace. In this case, the workspaces are “merged,” with a single manifest being stored alongside the “root project” (the project that is not included in another workspace).

Apps

An app is a Julia package that can be run directly from the terminal, similar to a standalone program. Each app provides an entry point via @main and can define its own default Julia flags and executable name.

When an app is installed, it gets put into .julia/bin and by adding that to your PATH it allows you to launch it by name together with any arguments or options.

A Julia app is defined in the Project.toml file using an [apps] section:

[apps]
reverse = {} # empty dictionary is for additional metadata

with a corresponding entry point in the package module:

# src/MyReverseApp.jl
module MyReverseApp

function (@main)(ARGS)
    for arg in ARGS
        print(stdout, reverse(arg), " ")
    end
end

end # module

After installation, the app can be run directly in the terminal:

$ reverse some input string
emos tupni gnirts

This makes apps useful for building CLI tools or packaging Julia functionality as user-facing executables. Multiple apps can be defined per package by using submodules, and each app can specify default Julia flags (e.g. --threads=4) for performance or debugging.

See the full documentation for more information: https://pkgdocs.julialang.org/dev/apps/

Status highlights when dependencies have different loaded versions

Pkg.status() now highlights when a dependency's loaded version differs from what the current environment would load. This helps identify situations where you may be running code against an outdated or mismatched version of a package—particularly useful when switching between environments or after modifying dependencies.

When a package is already loaded from a different version or path than what the current environment specifies, Pkg will display a yellow [loaded: vX.Y.Z] indicator next to the package name:

This visual cue makes it easier to spot when you need to restart Julia to pick up the correct package versions, reducing debugging time and confusion in iterative development workflows.

Generated LLVM IR now uses pointer types instead of passing pointers as integers

Tim Besard

Ptr{T} now lowers to actual LLVM pointer types in generated IR (i.e. ptr with opaque pointers, or i8*), instead of integers like i64. This simplifies low-level interop: llvmcall no longer needs ptrtoint/inttoptr shims, and many intrinsics can be called via ccall using Ptr directly.

What changes for you

• Inline LLVM (llvmcall): update IR to use ptr/i8* for pointer arguments/returns, and remove redundant ptrtoint/inttoptr casts. Old IR that treats pointers as integers is still accepted but emits a deprecation warning.

• Pointer arithmetic: add_ptr / sub_ptr now operate on real pointers: add_ptr(::Ptr{T}, ::UInt) and sub_ptr(::Ptr{T}, ::UInt) (lowered to GEP).

• ccall convenience: passing/returning Ptr{T} maps to LLVM pointer types directly, enabling more intrinsic calls without custom llvmcall glue.

Example (before → after)

; BEFORE (deprecated): integer pointer
define i64 @f(i64 %p) {
  %q = inttoptr i64 %p to i8*
  ; ...
  %r = ptrtoint i8* %q to i64
  ret i64 %r
}

; AFTER: real pointer
define ptr @f(ptr %p) {
  ; ...
  ret ptr %p
}

This change also unlocks minor optimization opportunities in generated code since pointers no longer bounce through integer casts.

Reproducing RNG state in testsets

Mosè Giordano

Many developers may have experience with occasional failures when running tests of their packages which were observed only on remote machines, and wished to be able to reproduce the same run, for debugging purposes. The GitHub Actions workflow julia-actions/julia-runtest recently started printing to the log the full options used to invoke the Julia process which runs the tests, which lets developers use the same compiler options (e.g. bounds checking, code coverage, deprecation warnings, etc.) as the CI runs. However there are occasional failures which don't depend on compiler options, but may depend on the state of the global random number generator (RNG), if for example the input data of the tests is generated with functions like rand and randn, without passing an explicit RNG object, instead relying on the global one. The Test.@testset macro has had for a long time the feature of automatically controlling the global RNG, but until now its state was never displayed. Starting from Julia v1.12, a failure inside a @testset causes the RNG of the outermost test set to be printed to screen, which then you can also set in a new test set to exactly reproduce the same run.

As an example, consider the following test which would fail with a 0.1% probability:

julia> using Test

julia> @testset begin
           @test rand() > 0.001
       end;
test set: Test Failed at REPL[2]:2
  Expression: rand() > 0.001
   Evaluated: 0.00036328334842516963 > 0.001

Stacktrace:
 [1] top-level scope
   @ REPL[2]:2
 [2] macro expansion
   @ ~/.julia/juliaup/julia-1.12.0.x64.linux.gnu/share/julia/stdlib/v1.12/Test/src/Test.jl:1776 [inlined]
 [3] macro expansion
   @ REPL[2]:2 [inlined]
 [4] macro expansion
   @ ~/.julia/juliaup/julia-1.12.0.x64.linux.gnu/share/julia/stdlib/v1.12/Test/src/Test.jl:680 [inlined]
Test Summary: | Fail  Total  Time
test set      |    1      1  1.5s
RNG of the outermost testset: Random.Xoshiro(0xd02e9404e1026b37, 0xca5ae9c15acf6752, 0x976a327d42433534, 0xb5b1305af1734f3a, 0x1c2aa037d6e7d5c7)
ERROR: Some tests did not pass: 0 passed, 1 failed, 0 errored, 0 broken.

Normally, it'd require several attempts to reproduce a similar failure, but now the RNG is printed to screen and you can reproduce the run in a new session by setting the rng option of @testset to the value printed in the failed test:

julia> using Test, Random

julia> @testset rng=Random.Xoshiro(0xd02e9404e1026b37, 0xca5ae9c15acf6752, 0x976a327d42433534, 0xb5b1305af1734f3a, 0x1c2aa037d6e7d5c7) begin
           @test rand() > 0.001
       end;
test set: Test Failed at REPL[2]:2
  Expression: rand() > 0.001
   Evaluated: 0.00036328334842516963 > 0.001

Stacktrace:
 [1] top-level scope
   @ REPL[2]:2
 [2] macro expansion
   @ ~/.julia/juliaup/julia-1.12.0.x64.linux.gnu/share/julia/stdlib/v1.12/Test/src/Test.jl:1776 [inlined]
 [3] macro expansion
   @ REPL[2]:2 [inlined]
 [4] macro expansion
   @ ~/.julia/juliaup/julia-1.12.0.x64.linux.gnu/share/julia/stdlib/v1.12/Test/src/Test.jl:680 [inlined]
Test Summary: | Fail  Total  Time
test set      |    1      1  1.4s
RNG of the outermost testset: Xoshiro(0xd02e9404e1026b37, 0xca5ae9c15acf6752, 0x976a327d42433534, 0xb5b1305af1734f3a, 0x1c2aa037d6e7d5c7)
ERROR: Some tests did not pass: 0 passed, 1 failed, 0 errored, 0 broken.

While there are still many other classes of intermittent failures that aren't captured by the global RNG, being able to reproduce its state inside failing test sets should help debugging more issues during package development.

Acknowledgement

The preparation of this release was partially funded by NASA under award 80NSSC22K1740. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Aeronautics and Space Administration.