bpo-45548: Remove _math.c workarounds for pre-C99 libm

Makefile.pre.in

@@ -611,10 +611,6 @@ pybuilddir.txt: $(BUILDPYTHON)
 		exit 1 ; \
 	fi
 
-# This is shared by the math and cmath modules
-Modules/_math.o: Modules/_math.c Modules/_math.h
-	$(CC) -c $(CCSHARED) $(PY_CORE_CFLAGS) -o $@ $<
-
 # blake2s is auto-generated from blake2b
 $(srcdir)/Modules/_blake2/blake2s_impl.c: $(srcdir)/Modules/_blake2/blake2b_impl.c $(srcdir)/Modules/_blake2/blake2b2s.py
 	$(PYTHON_FOR_REGEN) $(srcdir)/Modules/_blake2/blake2b2s.py
@@ -625,7 +621,7 @@ $(srcdir)/Modules/_blake2/blake2s_impl.c: $(srcdir)/Modules/_blake2/blake2b_impl
 # -s, --silent or --quiet is always the first char.
 # Under BSD make, MAKEFLAGS might be " -s -v x=y".
 # Ignore macros passed by GNU make, passed after --
-sharedmods: $(BUILDPYTHON) pybuilddir.txt Modules/_math.o
+sharedmods: $(BUILDPYTHON) pybuilddir.txt
 	@case "`echo X $$MAKEFLAGS | sed 's/^X //;s/ -- .*//'`" in \
 	    *\ -s*|s*) quiet="-q";; \
 	    *) quiet="";; \

Misc/NEWS.d/next/Build/2021-10-24-21-49-49.bpo-45548.UWx0UC.rst

@@ -0,0 +1,3 @@
+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.

Modules/Setup

@@ -171,8 +171,8 @@ time timemodule.c
 #array arraymodule.c
 #audioop audioop.c
 #binascii binascii.c
-#cmath cmathmodule.c _math.c  # -lm
-#math mathmodule.c _math.c  # -lm
+#cmath cmathmodule.c  # -lm
+#math mathmodule.c  # -lm
 #pyexpat -I$(srcdir)/Modules/expat  expat/xmlparse.c expat/xmlrole.c expat/xmltok.c pyexpat.c
 #unicodedata  unicodedata.c
 

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.c 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