Replace outdated optimization with clearer code that compiles better.

rhettinger · rhettinger · commit c629d4c9a2b6 · 2013-08-05T22:24:50.000-07:00
Letting the compiler decide how to optimize the multiply by five
gives it the freedom to make better choices for the best technique
for a given target machine.

For example, GCC on x86_64 produces a little bit better code:

Old-way (3 steps with a data dependency between each step):

    shrq    $5, %r13
    leaq    1(%rbx,%r13), %rax
    leaq    (%rax,%rbx,4), %rbx

New-way (3 steps with no dependency between the first two steps
         which can be run in parallel):

    leaq    (%rbx,%rbx,4), %rax     # i*5
    shrq    $5, %r13                # perturb &gt;&gt;= PERTURB_SHIFT
    leaq    1(%r13,%rax), %rbx      # 1 + perturb + i*5
diff --git a/Objects/setobject.c b/Objects/setobject.c
@@ -118,7 +118,7 @@ set_lookkey(PySetObject *so, PyObject *key, register Py_hash_t hash)
     /* In the loop, key == dummy is by far (factor of 100s) the
        least likely outcome, so test for that last. */
     for (perturb = hash; ; perturb >>= PERTURB_SHIFT) {
-        i = (i << 2) + i + perturb + 1;
+        i = i * 5 + perturb + 1;
         entry = &table[i & mask];
         if (entry->key == NULL) {
             if (freeslot != NULL)
@@ -189,7 +189,7 @@ set_lookkey_unicode(PySetObject *so, PyObject *key, register Py_hash_t hash)
     /* In the loop, key == dummy is by far (factor of 100s) the
        least likely outcome, so test for that last. */
     for (perturb = hash; ; perturb >>= PERTURB_SHIFT) {
-        i = (i << 2) + i + perturb + 1;
+        i = i * 5 + perturb + 1;
         entry = &table[i & mask];
         if (entry->key == NULL)
             return freeslot == NULL ? entry : freeslot;
@@ -258,7 +258,7 @@ set_insert_clean(register PySetObject *so, PyObject *key, Py_hash_t hash)
     i = (size_t)hash & mask;
     entry = &table[i];
     for (perturb = hash; entry->key != NULL; perturb >>= PERTURB_SHIFT) {
-        i = (i << 2) + i + perturb + 1;
+        i = i * 5 + perturb + 1;
         entry = &table[i & mask];
     }
     so->fill++;
