Replace outdated optimization with clearer code that compiles better.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
diff --git a/Objects/setobject.c b/Objects/setobject.c
index ea5a24c..0cea2a8 100644
--- a/Objects/setobject.c
+++ b/Objects/setobject.c
@@ -118,7 +118,7 @@
/* 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 @@
/* 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 @@
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++;