gh-110489: Optimise math.ceil for known exact float (#108801)

This matches a similar optimisation done for math.floor in
https://github.com/python/cpython/pull/21072

2 files changed, 10 insertions, 7 deletions

diff --git a/Misc/NEWS.d/next/Core and Builtins/2023-10-06-22-30-25.gh-issue-110489.rI2n8A.rst b/Misc/NEWS.d/next/Core and Builtins/2023-10-06-22-30-25.gh-issue-110489.rI2n8A.rst
new file mode 100644
index 0000000..6252d00
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and Builtins/2023-10-06-22-30-25.gh-issue-110489.rI2n8A.rst
@@ -0,0 +1 @@
+Optimise :func:`math.ceil` when the input is exactly a float, resulting in about a 10% improvement.
diff --git a/Modules/mathmodule.c b/Modules/mathmodule.c
index 2d896e7..a4d9466 100644
--- a/Modules/mathmodule.c
+++ b/Modules/mathmodule.c
@@ -1125,8 +1125,12 @@ static PyObject *
 math_ceil(PyObject *module, PyObject *number)
 /*[clinic end generated code: output=6c3b8a78bc201c67 input=2725352806399cab]*/
 {
+    double x;
 
-    if (!PyFloat_CheckExact(number)) {
+    if (PyFloat_CheckExact(number)) {
+        x = PyFloat_AS_DOUBLE(number);
+    }
+    else {
         math_module_state *state = get_math_module_state(module);
         PyObject *method = _PyObject_LookupSpecial(number, state->str___ceil__);
         if (method != NULL) {
@@ -1136,11 +1140,10 @@ math_ceil(PyObject *module, PyObject *number)
         }
         if (PyErr_Occurred())
             return NULL;
+        x = PyFloat_AsDouble(number);
+        if (x == -1.0 && PyErr_Occurred())
+            return NULL;
     }
-    double x = PyFloat_AsDouble(number);
-    if (x == -1.0 && PyErr_Occurred())
-        return NULL;
-
     return PyLong_FromDouble(ceil(x));
 }
 
@@ -1196,8 +1199,7 @@ math_floor(PyObject *module, PyObject *number)
     if (PyFloat_CheckExact(number)) {
         x = PyFloat_AS_DOUBLE(number);
     }
-    else
-    {
+    else {
         math_module_state *state = get_math_module_state(module);
         PyObject *method = _PyObject_LookupSpecial(number, state->str___floor__);
         if (method != NULL) {