Bit-twiddling hacks

mdboom · Dec 19, 2023 · 01a12af · 01a12af
1 parent 840006e
commit 01a12af
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Showing 3 changed files with 18 additions and 9 deletions.
diff --git a/Include/cpython/longintrepr.h b/Include/cpython/longintrepr.h
@@ -112,9 +112,14 @@ _PyLong_IsCompact(const PyLongObject* op) {
 static inline Py_ssize_t
 _PyLong_CompactValue(const PyLongObject *op)
 {
+    // Conditionally negate without a branch trick from bit-twiddling hacks.
+    // Avraham Plotnitzky, Alfonso De Gregorio, Sean Eron Anderson.
+    // https://graphics.stanford.edu/~seander/bithacks.html#ConditionalNegate
+
     assert(PyType_HasFeature((op)->ob_base.ob_type, Py_TPFLAGS_LONG_SUBCLASS));
     assert(PyUnstable_Long_IsCompact(op));
-    return ((op->ob_digit[0] & PyLong_IS_NEGATIVE_MASK) ? -1 : 1) * ((Py_ssize_t)op->ob_digit[0] & PyLong_MASK);
+    Py_ssize_t negate = (op->ob_digit[0] & PyLong_IS_NEGATIVE_MASK) != 0;
+    return (((Py_ssize_t)op->ob_digit[0] & PyLong_MASK) ^ -negate) + negate;
 }
 
 #define PyUnstable_Long_CompactValue _PyLong_CompactValue

diff --git a/Include/internal/pycore_long.h b/Include/internal/pycore_long.h
@@ -255,8 +255,9 @@ static inline int
 _PyLong_NonCompactSign(const PyLongObject *op)
 {
     assert(PyLong_Check(op));
-    // assert(!_PyLong_IsCompact(op));
-    return (op->ob_digit[0] & PyLong_IS_NEGATIVE_MASK) ? -1 : 1;
+    assert(!_PyLong_IsCompact(op));
+    Py_ssize_t negate = (op->ob_digit[0] & PyLong_IS_NEGATIVE_MASK) != 0;
+    return ((Py_ssize_t)1 ^ -negate) + negate;
 }
 
 /* Equivalent to _PyLong_DigitCount(op) * _PyLong_NonCompactSign(op) */
@@ -267,9 +268,11 @@ _PyLong_SignedDigitCount(const PyLongObject *op)
     if (op->ob_digit[0] == 0) {
         return 0;
     } else if (_PyLong_IsCompact(op)) {
-        return (op->ob_digit[0] & PyLong_IS_NEGATIVE_MASK) ? -1 : 1;
+        Py_ssize_t negate = (op->ob_digit[0] & PyLong_IS_NEGATIVE_MASK) != 0;
+        return ((Py_ssize_t)1 ^ -negate) + negate;
     } else {
-        return _PyLong_NonCompactDigitCount(op) * _PyLong_NonCompactSign(op);
+        Py_ssize_t negate = (op->ob_digit[0] & PyLong_IS_NEGATIVE_MASK) != 0;
+        return (((Py_ssize_t)op->ob_digit[0] & PyLong_MASK) ^ -negate) + negate;
     }
 }
 
@@ -282,6 +285,7 @@ _PyLong_SetNegative(PyLongObject *op)
 static inline void
 _PyLong_SetSignAndDigitCount(PyLongObject *op, int sign, Py_ssize_t size)
 {
+    // MGDTODO: Make versions of this that can only shrink, or shrink and grow
     assert(size >= 0);
     assert(-1 <= sign && sign <= 1);
     assert(sign != 0 || size == 0);

diff --git a/Objects/longobject.c b/Objects/longobject.c
@@ -208,7 +208,7 @@ long_normalize(PyLongObject *v)
 static inline void
 _PyLong_memset(PyLongObject *ob) {
     if (!_PyLong_IsCompact(ob)) {
-        Py_ssize_t size = _PyLong_DigitCount(ob);
+        Py_ssize_t size = _PyLong_NonCompactDigitCount(ob);
         memset(ob->ob_digit + 1, 0, size * sizeof(digit));
     }
 }
@@ -2900,7 +2900,7 @@ PyLong_FromString(const char *str, char **pend, int base)
 
     /* Set sign and normalize */
     if (!_PyLong_IsZero(z) && sign < 0) {
-        _PyLong_FlipSign(z);
+        _PyLong_SetNegative(z);
     }
     long_normalize(z);
     z = maybe_small_long(z);
@@ -3628,7 +3628,7 @@ _PyLong_Add(PyLongObject *a, PyLongObject *b)
                    That also means z is not an element of
                    small_ints, so negating it in-place is safe. */
                 assert(Py_REFCNT(z) == 1);
-                _PyLong_FlipSign(z);
+                _PyLong_SetNegative(z);
             }
         }
         else
@@ -3666,7 +3666,7 @@ _PyLong_Subtract(PyLongObject *a, PyLongObject *b)
             z = x_add(a, b);
             if (z != NULL) {
                 assert(_PyLong_IsZero(z) || Py_REFCNT(z) == 1);
-                _PyLong_FlipSign(z);
+                _PyLong_SetNegative(z);
             }
         }
     }