gh-129149: Add fast path in PYLONG_FROM_UINT macro for compact integers (#129168)

Add fast path in PyLong_From*() functions for compact integers.

Co-authored-by: Pieter Eendebak <pieter.eendebak@gmail.com>
Co-authored-by: Sergey B Kirpichev <skirpichev@gmail.com>
Co-authored-by: Yan Yanchii <yyanchiy@gmail.com>
Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>
Co-authored-by: Victor Stinner <vstinner@python.org>

Author: 6 people

Misc/NEWS.d/next/Core_and_Builtins/2025-01-22-14-24-44.gh-issue-129149.wAYu43.rst

@@ -0,0 +1,2 @@
+Add fast path for medium-size integers in :c:func:`PyLong_FromUnsignedLong`,
+:c:func:`PyLong_FromUnsignedLongLong` and :c:func:`PyLong_FromSize_t`.

Objects/longobject.c

@@ -365,9 +365,13 @@ PyLong_FromLong(long ival)
         if (IS_SMALL_UINT(ival)) { \
             return get_small_int((sdigit)(ival)); \
         } \
+        if ((ival) <= PyLong_MASK) { \
+            return _PyLong_FromMedium((sdigit)(ival)); \
+        } \
+        /* Do shift in two steps to avoid possible undefined behavior. */ \
+        INT_TYPE t = (ival) >> PyLong_SHIFT >> PyLong_SHIFT; \
         /* Count the number of Python digits. */ \
-        Py_ssize_t ndigits = 0; \
-        INT_TYPE t = (ival); \
+        Py_ssize_t ndigits = 2; \
         while (t) { \
             ++ndigits; \
             t >>= PyLong_SHIFT; \