First release candidate of OCaml 5.1.1

Two months (and half) after the release of OCaml 5.1.0, we have discovered three
significant regressions in OCaml 5.1.0, affecting the packaging of executables,
the typechecking of OCaml programs, and the performance of numerical codes.

Since those regressions affect many users and could have lasting effects, we
have decided to cut a patch release of OCaml 5.1 with fixes for those issues
next week.

To give time for opam packages to test this patch version, we
have just published a first release candidate for OCaml 5.1.1 .
The full list of changes in this release candidate is available below.

As a major exception to our policy for patch releases, OCaml 5.1.1 will
contain one breaking change in the standard library: the Compression flag has
been removed from the Marshal module.
This drastic measure was taken because supporting zstd compression in the
standard library made zstd a dependency of all OCaml executables. Since the
compiler should not impose its dependency on end-users, the support for
compressed marshaling has been moved to a compiler internal library in 5.1.1.
This internally library might be released as an independent library in later
releases of OCaml but not in OCaml 5.1.1 to give us time to polish the library
integration and packaging.

Installation Instructions

The base compiler can be installed as an opam switch with the following commands on opam 2.1 and later:

opam update
opam switch create 5.1.1~rc1

The source code for the release candidate is also directly available on:

Fine-Tuned Compiler Configuration

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

opam update
opam switch create <switch_name> ocaml-variants.5.1.1~rc1+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 5.1.1~rc1+flambda+nffa ocaml-variants.5.1.1~rc1+options ocaml-option-flambda ocaml-option-no-flat-float-array

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

Changes in OCaml 5.1.1

Standard Library Dependency Fix:

  • (breaking change) #12562, #12734, #12783: Remove the Marshal.Compression flag to the
    Marshal.to_* functions introduced in 5.1 by #12006, as it cannot
    be implemented without risking to link -lzstd with all
    ocamlopt-generated executables. The compilers are still able to use
    ZSTD compression for compilation artefacts.
    (Xavier Leroy and David Allsopp, report by Kate Deplaix, review by
    Nicolás Ojeda Bär, Kate Deplaix, and Damien Doligez).

Type System Bug Fix:

  • #12623, fix the computation of variance composition
    (Florian Angeletti, report by Vesa Karvonen, review by Gabriel Scherer)

GC Performance Regression Fixes

  • #12590, #12595: Move caml_collect_gc_stats_sample in
    caml_empty_minor_heap_promote before barrier arrival.
    (B. Szilvasy, review by Gabriel Scherer)

  • #12318: GC: simplify the meaning of custom_minor_max_size: blocks with
    out-of-heap memory above this limit are now allocated directly in
    the major heap.
    (Damien Doligez, report by Stephen Dolan, review by Gabriel Scherer)

  • #12439: Finalize and collect dead custom blocks during minor collection
    (Damien Doligez, review by Xavier Leroy, Gabriel Scherer and KC

  • #12491, #12493, #12500, #12754: Do not change GC pace when creating
    sub-arrays of bigarrays
    (Xavier Leroy, report by Ido Yariv, analysis by Gabriel Scherer,

Miscellaneous Bug Fixes

  • #12581, #12609: Fix error on uses of packed modules outside their pack
    to correctly handle nested packs
    (Vincent Laviron, report by Javier Chávarri, review by Pierre Chambart)

  • #12757: Fix ocamlnat (native toplevel) by registering frametables correctly
    (Stephen Dolan, Nick Barnes and Mark Shinwell,
    review by Vincent Laviron and SĂ©bastien Hinderer)

  • #12645, #12649 fix error messages for cyclic type definitions in presence of
    the -short-paths flag.
    (Florian Angeletti, report by Vesa Karvonen, review by Gabriel Scherer)