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Update NEWS for --enable-mini-gmp

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* NEWS: Update.
This commit is contained in:
Andy Wingo 2021-03-08 22:50:16 +01:00
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45
NEWS
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@ -41,6 +41,51 @@ libraries via also adding the ".libs" subdirectories of
`LTDL_LIBRARY_PATH' elements. See "Foreign Libraries" in the
documentation for a full discussion.
** Fix important incompatibility with GnuTLS
Guile uses the GNU multi-precision (GMP) library to implement
arbitrary-precision integers (bignums) and fractions. Usually Guile is
built to dynamically link to libgmp. In this configuration, any other
user of GMP in the process uses the same libgmp instance, with the same
shared state.
An important piece of shared state is the GMP allocator, responsible for
allocating storage for the digits of large integers. For Guile it's
most efficient to install libgc as the GMP allocator. That way Guile
doesn't need to install finalizers, which have significant overhead, to
free GMP values when Guile bignums are collected. Using libgc to
allocate digits also allows Guile's GC to adequately measure the memory
cost of these values.
However, if the Guile process is linked to some other user of GMP, then
probably the references from the other library to GMP values aren't
visible to the garbage collector. In this case libgc could prematurely
collect values from that other GMP user.
This isn't theoretical, sadly: it happens for Guile-GnuTLS. GnuTLS uses
GMP, and so does Guile. Since Guile 2.0.4, Guile has installed libgc as
the GMP allocator, so since then, Guile-GnuTLS has been buggy.
Therefore, the default is now to not install libgc as the GMP allocator.
This may slow down some uses of bignums.
** New build option: --enable-mini-gmp
For some users, it would be preferable to bundle a private copy of the
GMP bignum library into Guile. Some users would like to avoid the extra
dependency. Others would like to use libgc to manage GMP values, while
not perturbing the GMP allocator for other GMP users.
For these cases, Guile now has an --enable-mini-gmp configure option,
which will use a stripped-down version of GMP, bundled with Guile. This
code doesn't have all the algorithmic optimizations of full GMP, but
implements the same API in a basic way. It can be more optimal in a
Guile context, given that it can use libgc to allocate its data.
Note that a build with --enable-mini-gmp is not ABI-compatible with a
"stock" build, as functions that use GMP types (scm_to_mpz,
scm_from_mpz) are not exported.
** New `read' implementation in Scheme
Guile's `read' procedure has been rewritten in Scheme. Compared to the