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Replace outdated optimization with clearer code that compiles better.

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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 >>= PERTURB_SHIFT
    leaq    1(%r13,%rax), %rbx      # 1 + perturb + i*5
This commit is contained in:
Raymond Hettinger 2013-08-05 22:24:50 -07:00
parent 9e3d27b574
commit c629d4c9a2
1 changed files with 3 additions and 3 deletions
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6
Objects/setobject.c
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@ -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++;