Issue #11888: Use system log2() when available
I expect the system libc to use more accurate functions than Python. The GNU
libc uses for example FYL2X and FYL2XP1 hardware instructions on Intel FPU.
diff --git a/Modules/mathmodule.c b/Modules/mathmodule.c
index 14d008a..cebb4ff 100644
--- a/Modules/mathmodule.c
+++ b/Modules/mathmodule.c
@@ -602,6 +602,9 @@
}
if (x > 0.0) {
+#ifdef HAVE_LOG2
+ return log2(x);
+#else
double m;
int e;
m = frexp(x, &e);
@@ -617,6 +620,7 @@
else {
return log(m) / log(2.0) + e;
}
+#endif
}
else if (x == 0.0) {
errno = EDOM;
diff --git a/configure b/configure
index da4c902..6d3f85e 100755
--- a/configure
+++ b/configure
@@ -11864,7 +11864,7 @@
fi
done
-for ac_func in hypot lgamma log1p round tgamma
+for ac_func in hypot lgamma log1p log2 round tgamma
do :
as_ac_var=`$as_echo "ac_cv_func_$ac_func" | $as_tr_sh`
ac_fn_c_check_func "$LINENO" "$ac_func" "$as_ac_var"
diff --git a/configure.in b/configure.in
index 49864f4..c6a9437 100644
--- a/configure.in
+++ b/configure.in
@@ -2512,7 +2512,7 @@
futimens futimes \
gai_strerror getgroups getlogin getloadavg getpeername getpgid getpid \
getpriority getresuid getresgid getpwent getspnam getspent getsid getwd \
- initgroups kill killpg lchmod lchown lockf linkat lstat lutimes mbrtowc mkdirat mkfifo \
+ initgroups kill killpg lchmod lchown lockf log2 linkat lstat lutimes mbrtowc mkdirat mkfifo \
mkfifoat mknod mknodat mktime mremap nice openat pathconf pause plock poll \
posix_fallocate posix_fadvise pread \
pthread_init pthread_kill putenv pwrite readlink readlinkat readv realpath renameat \
@@ -3368,7 +3368,7 @@
LIBS="$LIBS $LIBM"
AC_CHECK_FUNCS([acosh asinh atanh copysign erf erfc expm1 finite gamma])
-AC_CHECK_FUNCS([hypot lgamma log1p round tgamma])
+AC_CHECK_FUNCS([hypot lgamma log1p log2 round tgamma])
AC_CHECK_DECLS([isinf, isnan, isfinite], [], [], [[#include <math.h>]])
# On FreeBSD 6.2, it appears that tanh(-0.) returns 0. instead of
diff --git a/pyconfig.h.in b/pyconfig.h.in
index 89b0c33..55272f1 100644
--- a/pyconfig.h.in
+++ b/pyconfig.h.in
@@ -476,6 +476,9 @@
/* Define to 1 if you have the `log1p' function. */
#undef HAVE_LOG1P
+/* Define to 1 if you have the `log2' function. */
+#undef HAVE_LOG2
+
/* Define this if you have the type long double. */
#undef HAVE_LONG_DOUBLE