bpo-45548: Remove _math.c workarounds for pre-C99 libm (GH-29179)

The :mod:`math` and :mod:`cmath` implementation now require a C99 compatible
``libm`` and no longer ship with workarounds for missing acosh, asinh,
expm1, and log1p functions.

The changeset also removes ``_math.c`` and moves the last remaining
workaround into ``_math.h``. This simplifies static builds with
``Modules/Setup`` and resolves symbol conflicts.

Co-authored-by: Mark Dickinson <mdickinson@enthought.com>
Co-authored-by: Brett Cannon <brett@python.org>
Signed-off-by: Christian Heimes <christian@python.org>

1 files changed, 20 insertions, 37 deletions

diff --git a/Modules/_math.h b/Modules/_math.h
index 398b7e8..4a6bc22 100644
--- a/Modules/_math.h
+++ b/Modules/_math.h
@@ -1,41 +1,24 @@
-#ifdef HAVE_ACOSH
-#  define m_acosh acosh
-#else
-/* if the system doesn't have acosh, use the substitute
-   function defined in Modules/_math.c. */
-double _Py_acosh(double x);
-#  define m_acosh _Py_acosh
-#endif
+/* log1p(x) = log(1+x).  The log1p function is designed to avoid the
+   significant loss of precision that arises from direct evaluation when x is
+   small. Use the substitute from _math.h on all platforms: it includes
+   workarounds for buggy handling of zeros.
+ */
 
-#ifdef HAVE_ASINH
-#  define m_asinh asinh
-#else
-/* if the system doesn't have asinh, use the substitute
-   function defined in Modules/_math.c. */
-double _Py_asinh(double x);
-#  define m_asinh _Py_asinh
-#endif
+static double
+_Py_log1p(double x)
+{
+    /* Some platforms supply a log1p function but don't respect the sign of
+       zero:  log1p(-0.0) gives 0.0 instead of the correct result of -0.0.
 
-#ifdef HAVE_ATANH
-#  define m_atanh atanh
-#else
-/* if the system doesn't have atanh, use the substitute
-   function defined in Modules/_math.c. */
-double _Py_atanh(double x);
-#define m_atanh _Py_atanh
-#endif
+       To save fiddling with configure tests and platform checks, we handle the
+       special case of zero input directly on all platforms.
+    */
+    if (x == 0.0) {
+        return x;
+    }
+    else {
+        return log1p(x);
+    }
+}
 
-#ifdef HAVE_EXPM1
-#  define m_expm1 expm1
-#else
-/* if the system doesn't have expm1, use the substitute
-   function defined in Modules/_math.c. */
-double _Py_expm1(double x);
-#define m_expm1 _Py_expm1
-#endif
-
-double _Py_log1p(double x);
-
-/* Use the substitute from _math.c on all platforms:
-   it includes workarounds for buggy handling of zeros. */
 #define m_log1p _Py_log1p