Cleanup yielding a small speed boost: before rich comparisons were

introduced, list.sort() was rewritten to use only the "< or not <?"
distinction.  After rich comparisons were introduced, docompare() was
fiddled to translate a Py_LT Boolean result into the old "-1 for <,
0 for ==, 1 for >" flavor of outcome, and the sorting code was left
alone.  This left things more obscure than they should be, and turns
out it also cost measurable cycles.

So:  The old CMPERROR novelty is gone.  docompare() is renamed to islt(),
and now has the same return conditinos as PyObject_RichCompareBool.  The
SETK macro is renamed to ISLT, and is even weirder than before (don't
complain unless you want to maintain the sort code <wink>).

Overall, this yields a 1-2% speedup in the usual (no explicit function
passed to list.sort()) case when sorting arrays of floats (as sortperf.py
does).  The boost is higher for arrays of ints.

diff --git a/Objects/listobject.c b/Objects/listobject.c
index ce78fff9863d804ead9d5da604364263c582f703..f2132b4193b644fcb96e35cd761c9205cb85f142 100644 (file)
--- a/Objects/listobject.c
+++ b/Objects/listobject.c
@@ -758,39 +758,28 @@ reverse_slice(PyObject **lo, PyObject **hi)
 /* New quicksort implementation for arrays of object pointers.
    Thanks to discussions with Tim Peters. */
 
-/* CMPERROR is returned by our comparison function when an error
-   occurred.  This is the largest negative integer (0x80000000 on a
-   32-bit system). */
-#define CMPERROR ( (int) ((unsigned int)1 << (8*sizeof(int) - 1)) )
-
 /* Comparison function.  Takes care of calling a user-supplied
    comparison function (any callable Python object).  Calls the
-   standard comparison function, PyObject_Compare(), if the user-
-   supplied function is NULL. */
+   standard comparison function, PyObject_RichCompareBool(), if the user-
+   supplied function is NULL.
+   Returns <0 on error, >0 if x < y, 0 if x >= y. */
 
 static int
-docompare(PyObject *x, PyObject *y, PyObject *compare)
+islt(PyObject *x, PyObject *y, PyObject *compare)
 {
        PyObject *res;
        PyObject *args;
        int i;
 
-       if (compare == NULL) {
-               /* NOTE: we rely on the fact here that the sorting algorithm
-                  only ever checks whether k<0, i.e., whether x<y.  So we
-                  invoke the rich comparison function with Py_LT ('<'), and
-                  return -1 when it returns true and 0 when it returns
-                  false. */
-               i = PyObject_RichCompareBool(x, y, Py_LT);
-               if (i < 0)
-                       return CMPERROR;
-               else
-                       return -i;
-       }
+       if (compare == NULL)
+               return PyObject_RichCompareBool(x, y, Py_LT);
 
+       /* Call the user's comparison function and translate the 3-way
+        * result into true or false (or error).
+        */
        args = PyTuple_New(2);
        if (args == NULL)
-               return CMPERROR;
+               return -1;
        Py_INCREF(x);
        Py_INCREF(y);
        PyTuple_SET_ITEM(args, 0, x);
@@ -798,20 +787,16 @@ docompare(PyObject *x, PyObject *y, PyObject *compare)
        res = PyObject_Call(compare, args, NULL);
        Py_DECREF(args);
        if (res == NULL)
-               return CMPERROR;
+               return -1;
        if (!PyInt_Check(res)) {
                Py_DECREF(res);
                PyErr_SetString(PyExc_TypeError,
                                "comparison function must return int");
-               return CMPERROR;
+               return -1;
        }
        i = PyInt_AsLong(res);
        Py_DECREF(res);
-       if (i < 0)
-               return -1;
-       if (i > 0)
-               return 1;
-       return 0;
+       return i < 0;
 }
 
 /* MINSIZE is the smallest array that will get a full-blown samplesort
@@ -850,17 +835,21 @@ docompare(PyObject *x, PyObject *y, PyObject *compare)
    exactly in two. */
 #define STACKSIZE 60
 
-
-#define SETK(X,Y) if ((k = docompare(X,Y,compare))==CMPERROR) goto fail
+/* Compare X to Y via islt().  Goto "fail" if the comparison raises an
+   error.  Else "k" is set to true iff X<Y, and an "if (k)" block is
+   started.  It makes more sense in context <wink>.  X and Y are PyObject*s.
+*/
+#define IFLT(X, Y) if ((k = islt(X, Y, compare)) < 0) goto fail;  \
+                  if (k)
 
 /* binarysort is the best method for sorting small arrays: it does
    few compares, but can do data movement quadratic in the number of
    elements.
    [lo, hi) is a contiguous slice of a list, and is sorted via
-   binary insertion.
+   binary insertion.  This sort is stable.
    On entry, must have lo <= start <= hi, and that [lo, start) is already
    sorted (pass start == lo if you don't know!).
-   If docompare complains (returns CMPERROR) return -1, else 0.
+   If islt() complains return -1, else 0.
    Even in case of error, the output slice will be some permutation of
    the input (nothing is lost or duplicated).
 */
@@ -869,12 +858,12 @@ static int
 binarysort(PyObject **lo, PyObject **hi, PyObject **start, PyObject *compare)
      /* compare -- comparison function object, or NULL for default */
 {
-       /* assert lo <= start <= hi
-          assert [lo, start) is sorted */
        register int k;
        register PyObject **l, **p, **r;
        register PyObject *pivot;
 
+       assert(lo <= start && start <= hi);
+       /* assert [lo, start) is sorted */
        if (lo == start)
                ++start;
        for (; start < hi; ++start) {
@@ -884,8 +873,7 @@ binarysort(PyObject **lo, PyObject **hi, PyObject **start, PyObject *compare)
                pivot = *r;
                do {
                        p = l + ((r - l) >> 1);
-                       SETK(pivot, *p);
-                       if (k < 0)
+                       IFLT(pivot, *p)
                                r = p;
                        else
                                l = p + 1;
@@ -906,7 +894,7 @@ binarysort(PyObject **lo, PyObject **hi, PyObject **start, PyObject *compare)
 /* samplesortslice is the sorting workhorse.
    [lo, hi) is a contiguous slice of a list, to be sorted in place.
    On entry, must have lo <= hi,
-   If docompare complains (returns CMPERROR) return -1, else 0.
+   If islt() complains return -1, else 0.
    Even in case of error, the output slice will be some permutation of
    the input (nothing is lost or duplicated).
 
@@ -1023,8 +1011,7 @@ samplesortslice(PyObject **lo, PyObject **hi, PyObject *compare)
        */
        /* assert lo < hi */
        for (r = lo+1; r < hi; ++r) {
-               SETK(*r, *(r-1));
-               if (k < 0)
+               IFLT(*r, *(r-1))
                        break;
        }
        /* [lo,r) is sorted, [r,hi) unknown.  Get out cheap if there are
@@ -1036,8 +1023,7 @@ samplesortslice(PyObject **lo, PyObject **hi, PyObject *compare)
           benchmark-driven silliness <wink>. */
        /* assert lo < hi */
        for (r = lo+1; r < hi; ++r) {
-               SETK(*(r-1), *r);
-               if (k < 0)
+               IFLT(*(r-1), *r)
                        break;
        }
        if (hi - r <= MAXMERGE) {
@@ -1192,8 +1178,7 @@ samplesortslice(PyObject **lo, PyObject **hi, PyObject *compare)
                do {
                        /* slide l right, looking for key >= pivot */
                        do {
-                               SETK(*l, pivot);
-                               if (k < 0)
+                               IFLT(*l, pivot)
                                        ++l;
                                else
                                        break;
@@ -1202,8 +1187,7 @@ samplesortslice(PyObject **lo, PyObject **hi, PyObject *compare)
                        /* slide r left, looking for key < pivot */
                        while (l < r) {
                                register PyObject *rval = *r--;
-                               SETK(rval, pivot);
-                               if (k < 0) {
+                               IFLT(rval, pivot) {
                                        /* swap and advance */
                                        r[1] = *l;
                                        *l++ = rval;
@@ -1219,8 +1203,7 @@ samplesortslice(PyObject **lo, PyObject **hi, PyObject *compare)
                   everything to the right of r is >= pivot */
 
                if (l == r) {
-                       SETK(*r, pivot);
-                       if (k < 0)
+                       IFLT(*r, pivot)
                                ++l;
                        else
                                --r;
@@ -1249,8 +1232,7 @@ samplesortslice(PyObject **lo, PyObject **hi, PyObject *compare)
                */
                while (l < hi) {
                        /* pivot <= *l known */
-                       SETK(pivot, *l);
-                       if (k < 0)
+                       IFLT(pivot, *l)
                                break;
                        else
                                /* <= and not < implies == */
@@ -1290,7 +1272,7 @@ samplesortslice(PyObject **lo, PyObject **hi, PyObject *compare)
        return -1;
 }
 
-#undef SETK
+#undef IFLT
 
 static PyTypeObject immutable_list_type;