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Postings from July 2012
Part of this commit:
commit 3edf23ff129b6c5edde184cb3b63953432223591
Author: Andy Lester <andy@petdance.com>
Date: Thu Jan 5 19:44:48 2006 -0600
performance tweaking op.c
Message-ID: <20060106074448.GB3401@petdance.com>
p4raw-id: //depot/perl@26674
was this:
@@ -507,19 +505,25 @@ Perl_op_refcnt_unlock(pTHX)
OP *
Perl_linklist(pTHX_ OP *o)
{
+ OP *first;
if (o->op_next)
return o->op_next;
/* establish postfix order */
- if (cUNOPo->op_first) {
+ first = cUNOPo->op_first;
+ if (first) {
register OP *kid;
- o->op_next = LINKLIST(cUNOPo->op_first);
- for (kid = cUNOPo->op_first; kid; kid = kid->op_sibling) {
- if (kid->op_sibling)
+ o->op_next = LINKLIST(first);
+ kid = first;
+ for (;;) {
+ if (kid->op_sibling) {
kid->op_next = LINKLIST(kid->op_sibling);
- else
+ kid = kid->op_sibling;
+ } else {
kid->op_next = o;
+ break;
+ }
}
}
else
It turns out not to be what it seems. Not only does it add a local variable
'first' that effectively duplicates 'kid' (now refactored on a branch that
I'm working on), the intended "optimisation" actually makes the code larger
(at least with gcc -Os on both x86_64 and ARM), and uses more load and store
instructions.
[To be fair, I've not chased the code flow to work out whether there are
more instructions called. That feels like even more work than this deserves.
Given, for example, that I needed to run nmap to find the IP address of the
Raspberry Pi so that I could log into it to build on ARM :-)]
"Change is inevitable, progress is not"
Not all changes are progress. Maximise progress as a function of change.
(And no, it's usually not obvious what really is progress.)
Nicholas Clark
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