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Replace outdated optimization with clearer code that compiles better. 

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++;