Dear OCaml users,

With the progress of the ongoing stabilisation effort for OCaml 5.1.0,
I am happy to announce a second alpha release for OCaml 5.1.0.

This second alpha release contains many noteworthy fixes:

- a long-awaited GC fix
- a Windows ABI fix

as announced in the first alpha but also

- a compiler-libs (parsetree) fix
- a type system compatibility enhancement change
- a restored backed for s390x/IBM Z

The full list of changes is available at the end of this email.

Once most major OCaml tools are updated to the last compiler-libs changes,<= br> we will switch to beta releases. Hopefully, this will happen in the upcomin= g
weeks. The progress on stabilising the ecosystem is tracked on the
opam readiness for 5.1.0 meta-issue at:

https://github.com/ocaml/opam-repository/issues/23669

The release is still planned for around July.

If you find any bugs, please report them here:

https://github.com/ocaml/ocaml/issues

If you are interested by the ongoing list of new features and bug fixes, th= e
updated change log for OCaml 5.1.0 is available at:

https://github.com/ocaml/ocaml/blob/5.1/Changes

Happy hacking,
Florian Angeletti for the OCaml team.

Installation Instructions
-------------------------
The base compiler can be installed as an opam switch with the following com= mands
on opam 2.1:

    opam update
    opam switch create 5.1.0~alpha2

The source code for the alpha is also available at these addresses:

* https://github.com/ocaml/ocaml/archive/5.1.0-alpha2.tar.gz
* https://caml.inria.fr/pub/distrib/ocaml-5.1/ocaml-5.1.0~alpha2.tar.gz

Fine-Tuned Compiler Configuration
---------------------------------

If you want to tweak the configuration of the compiler, you can switch to t= he option variant with:

    opam update
    opam switch create <switch_name> ocaml-variants.5.= 1.0~alpha2+options <option_list>

where `option_list` is a space-separated list of `ocaml-option-*` packages.= For instance, for a flambda and no-flat-float-array switch:

    opam switch create 5.1.0~alpha2+flambda+nffa oca= ml-variants.5.1.0~alpha2+options ocaml-option-flambda ocaml-option-no-f= lat-float-array

All available options can be listed with `opam search ocaml-option`.

Changes Compared To The First Alpha
-----------------------------------

### Runtime System:

- #11589, #11903: Modify the GC pacing code to make sure the GC keeps
   up with allocations in the presence of idle domains.
   (Damien Doligez and Stephen Dolan, report by Florian Angeletti= ,
   review by KC Sivaramakrishnan and Sadiq Jaffer)

* #11865, #11868, #11876: Clarify that the operations of a custom
block must never access the OCaml runtime. The previous
documentation only mentioned the main illicit usages. In particular,=
since OCaml 5.0, it is no longer safe to call
`caml_remove_global_root` or `caml_remove_generational_global_root`<= br> from within the C finalizer of a custom block, or within the
finalization function passed to `caml_alloc_final`. As a workaround,=
such a finalization operation can be registered with `Gc.finalize` instead, which guarantees to run the finalizer at a safe point.
(Report by Timothy Bourke, discussion by Yotam Barnoy, Timothy
Bourke, Sadiq Jaffer, Xavier Leroy, Guillaume Munch-Maccagnoni, and<= br> Gabriel Scherer)

+ #11827, +#12249: Restore prefetching for GC marking
(Fabrice Buoro and Stephen Dolan, review by Gabriel Scherer and Sadi= q Jaffer)

- #12131: Simplify implementation of weak hash sets, fixing a
performance regression. (Nick Barnes, review by Fran=E7ois Bobot, Alain Frisch and Damien Doligez).

- #12231: Support MinGW-w64 11.0 winpthreads library, where the macro
to set up to get flexdll working changed
(David Allsopp and Samuel Hym, light review by Xavier Leroy)

### Type System:

* #12189, #12211: anonymous row variables in explicitly polymorphic type annotation, e.g. `'a. [< X of 'a ] -> 'a`, are now implicitly<= br> universally quantified (in other words, the example above is now rea= d
as `'a 'r. ([< X of 'a `] as 'r) -> 'a`).
(Florian Angeletti and Gabriel Scherer, review by Jacques Garrigue)<= br>
### Code generation and optimizations:

- #11712, #12258, #12261: s390x / IBM Z multicore support:
OCaml & C stack separation; dynamic stack size checks; fiber and=
effects support.
(Aleksei Nikiforov, with help from Vincent Laviron and Xavier Leroy,=
   additional suggestions by Luc Maranget,
   review by the same and KC Sivaramakrishnan)

### Internal/compiler-libs Changes:

+ #12119, +#12188, +#12191: mirror type constraints on value bi= nding in the
parsetree:
the constraint `typ` in `let pat : typ =3D exp` is now directly stor= ed
in the value binding node in the parsetree.
(Florian Angeletti, review by Richard Eisenberg)

### Bug Fixes

- #11846: Mark rbx as destroyed at C call for Win64 (mingw-w64 and Cygwin64= ).
Reserve the shadow store for the ABI in the c_stack_link struct inst= ead of
explictly when calling C functions. This simultaneously reduces the = number of
stack pointer manipulations and also fixes a bug when calling noallo= c
functions where the shadow store was not being reserved.
(David Allsopp, report by Vesa Karvonen, review by Xavier Leroy and<= br>    KC Sivaramakrishnan)

- #12170: fix pthread_geaffinity_np configure check for android
(David Allsopp, review by S=E9bastien Hinderer)

- #12252: Fix shared library build error on RISC-V.
(Edwin T=F6r=F6k, review by Nicol=E1s Ojeda B=E4r and Xavier Leroy)<= br>
- #12255, #12256: Handle large signal numbers correctly (Nick Barnes,
   review by David Allsopp).

- #12277: ARM64, fix a potential assembler error for very large functions b= y
emitting stack reallocation code before the body of the function. (Xavier Leroy, review by KC Sivaramakrishnan)