bpo-29882: Add _Py_popcount32() function (GH-20518)

* Rename pycore_byteswap.h to pycore_bitutils.h.
* Move popcount_digit() to pycore_bitutils.h as _Py_popcount32().
* _Py_popcount32() uses GCC and clang builtin function if available.
* Add unit tests to _Py_popcount32().

1 files changed, 41 insertions, 1 deletions

diff --git a/Modules/_testinternalcapi.c b/Modules/_testinternalcapi.c
index 5f217dc..6d5af59 100644
--- a/Modules/_testinternalcapi.c
+++ b/Modules/_testinternalcapi.c
@@ -12,7 +12,7 @@
 #define PY_SSIZE_T_CLEAN
 
 #include "Python.h"
-#include "pycore_byteswap.h"     // _Py_bswap32()
+#include "pycore_bitutils.h"     // _Py_bswap32()
 #include "pycore_initconfig.h"   // _Py_GetConfigsAsDict()
 #include "pycore_hashtable.h"    // _Py_hashtable_new()
 #include "pycore_gc.h"           // PyGC_Head
@@ -63,6 +63,45 @@ test_bswap(PyObject *self, PyObject *Py_UNUSED(args))
 }
 
 
+static int
+check_popcount(uint32_t x, int expected)
+{
+    // Use volatile to prevent the compiler to optimize out the whole test
+    volatile uint32_t u = x;
+    int bits = _Py_popcount32(u);
+    if (bits != expected) {
+        PyErr_Format(PyExc_AssertionError,
+                     "_Py_popcount32(%lu) returns %i, expected %i",
+                     (unsigned long)x, bits, expected);
+        return -1;
+    }
+    return 0;
+}
+
+
+static PyObject*
+test_popcount(PyObject *self, PyObject *Py_UNUSED(args))
+{
+#define CHECK(X, RESULT) \
+    do { \
+        if (check_popcount(X, RESULT) < 0) { \
+            return NULL; \
+        } \
+    } while (0)
+
+    CHECK(0, 0);
+    CHECK(1, 1);
+    CHECK(0x08080808, 4);
+    CHECK(0x10101010, 4);
+    CHECK(0x10204080, 4);
+    CHECK(0xDEADCAFE, 22);
+    CHECK(0xFFFFFFFF, 32);
+    Py_RETURN_NONE;
+
+#undef CHECK
+}
+
+
 #define TO_PTR(ch) ((void*)(uintptr_t)ch)
 #define FROM_PTR(ptr) ((uintptr_t)ptr)
 #define VALUE(key) (1 + ((int)(key) - 'a'))
@@ -157,6 +196,7 @@ static PyMethodDef TestMethods[] = {
     {"get_configs", get_configs, METH_NOARGS},
     {"get_recursion_depth", get_recursion_depth, METH_NOARGS},
     {"test_bswap", test_bswap, METH_NOARGS},
+    {"test_popcount", test_popcount, METH_NOARGS},
     {"test_hashtable", test_hashtable, METH_NOARGS},
     {NULL, NULL} /* sentinel */
 };