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bpo-45569: Change PYLONG_BITS_IN_DIGIT default to 30 #30497

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Merged
merged 8 commits into from
Jan 14, 2022
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bpo-45569: Change PYLONG_BITS_IN_DIGIT default to 30 #30497

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76924d1
Change PYLONG_BITS_IN_DIGIT default to 30
mdickinson Jan 9, 2022
515c0e8
Tweak comment wording
mdickinson Jan 9, 2022
8a761bf
Merge remote-tracking branch 'upstream/main' into default-to-30-bit-p…
mdickinson Jan 13, 2022
2fdd174
Add what's new entry
mdickinson Jan 13, 2022
d537d22
Update configure docs
mdickinson Jan 13, 2022
4e927c7
Update configure.ac
mdickinson Jan 13, 2022
f7e9033
Replace pyconfig.h with PC/pyconfig.h
mdickinson Jan 13, 2022
34dcb2c
Make corresponding change in configure
mdickinson Jan 13, 2022
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3 changes: 1 addition & 2 deletions Doc/using/configure.rst
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Expand Up @@ -35,8 +35,7 @@ General Options

Define the size in bits of Python :class:`int` digits: 15 or 30 bits.

By default, the number of bits is selected depending on ``sizeof(void*)``:
30 bits if ``void*`` size is 64-bit or larger, 15 bits otherwise.
By default, the digit size is 30.

Define the ``PYLONG_BITS_IN_DIGIT`` to ``15`` or ``30``.

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10 changes: 10 additions & 0 deletions Doc/whatsnew/3.11.rst
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Expand Up @@ -622,6 +622,16 @@ Build Changes
like Pyodide.
(Contributed by Christian Heimes and Ethan Smith in :issue:`40280`.)

* CPython will now use 30-bit digits by default for the Python :class:`int`
implementation. Previously, the default was to use 30-bit digits on platforms
with ``SIZEOF_VOID_P >= 8``, and 15-bit digits otherwise. It's still possible
to explicitly request use of 15-bit digits via either the
``--enable-big-digits`` option to the configure script or (for Windows) the
``PYLONG_BITS_IN_DIGIT`` variable in ``PC/pyconfig.h``, but this option may
be removed at some point in the future. (Contributed by Mark Dickinson in
:issue:`45569`.)


C API Changes
=============

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16 changes: 4 additions & 12 deletions Include/pyport.h
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Expand Up @@ -85,20 +85,12 @@ Used in: Py_SAFE_DOWNCAST
#define PY_INT32_T int32_t
#define PY_INT64_T int64_t

/* If PYLONG_BITS_IN_DIGIT is not defined then we'll use 30-bit digits if all
the necessary integer types are available, and we're on a 64-bit platform
(as determined by SIZEOF_VOID_P); otherwise we use 15-bit digits.

From pyodide: WASM has 32 bit pointers but has native 64 bit arithmetic
so it is more efficient to use 30 bit digits.
/* PYLONG_BITS_IN_DIGIT describes the number of bits per "digit" (limb) in the
* PyLongObject implementation (longintrepr.h). It's currently either 30 or 15,
* defaulting to 30. The 15-bit digit option may be removed in the future.
*/

#ifndef PYLONG_BITS_IN_DIGIT
#if SIZEOF_VOID_P >= 8 || defined(__wasm__)
# define PYLONG_BITS_IN_DIGIT 30
#else
# define PYLONG_BITS_IN_DIGIT 15
#endif
#define PYLONG_BITS_IN_DIGIT 30
#endif

/* uintptr_t is the C9X name for an unsigned integral type such that a
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5 changes: 5 additions & 0 deletions Misc/NEWS.d/next/Build/2022-01-09-11-24-54.bpo-45569.zCIENy.rst
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@@ -0,0 +1,5 @@
The build now defaults to using 30-bit digits for Python integers. Previously
either 15-bit or 30-bit digits would be selected, depending on the platform.
15-bit digits may still be selected using the ``--enable-big-digits=15`` option
to the ``configure`` script, or by defining ``PYLONG_BITS_IN_DIGIT`` in
``pyconfig.h``.
Comment on lines +4 to +5
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pyconfig.h is auto-generated and not user-editable. I suggest to remove the qualifier.

Suggested change
to the ``configure`` script, or by defining ``PYLONG_BITS_IN_DIGIT`` in
``pyconfig.h``.
to the ``configure`` script, or by defining ``PYLONG_BITS_IN_DIGIT``.

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True; I was thinking of the Windows setup. Maybe replacing pyconfig.h with PC/pyconfig.h would be enough?

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OTOH, probably not worth worrying about explicitly specifying how to change things for Windows here.

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Clarified (I hope).

2 changes: 1 addition & 1 deletion configure
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Expand Up @@ -1728,7 +1728,7 @@ Optional Features:
Doc/library/socket.rst (default is yes if supported)
--enable-big-digits[=15|30]
use big digits (30 or 15 bits) for Python longs
(default is system-dependent)]
(default is 30)]
--disable-test-modules don't build nor install test modules

Optional Packages:
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2 changes: 1 addition & 1 deletion configure.ac
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Expand Up @@ -5039,7 +5039,7 @@ AC_CHECK_DECLS([RTLD_LAZY, RTLD_NOW, RTLD_GLOBAL, RTLD_LOCAL, RTLD_NODELETE, RTL
# determine what size digit to use for Python's longs
AC_MSG_CHECKING([digit size for Python's longs])
AC_ARG_ENABLE(big-digits,
AS_HELP_STRING([--enable-big-digits@<:@=15|30@:>@],[use big digits (30 or 15 bits) for Python longs (default is system-dependent)]]),
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You have to update configure, too. It's a trivial change, so manually editing is easier.

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Yes, I've spent the last hour trying to get autoreconf to not generate spurious changes. :-( I'll go for the manual edit option.

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Done.

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Yes, I've spent the last hour trying to get autoreconf to not generate spurious changes. :-( I'll go for the manual edit option.

I have you covered: https://quay.io/repository/tiran/cpython_autoconf

AS_HELP_STRING([--enable-big-digits@<:@=15|30@:>@],[use big digits (30 or 15 bits) for Python longs (default is 30)]]),
[case $enable_big_digits in
yes)
enable_big_digits=30 ;;
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