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* [Bug libstdc++/30204] New: std::vector operator[] 10x speedup (patch)
@ 2006-12-13 18:01 charles at rebelbase dot com
2006-12-13 18:09 ` [Bug libstdc++/30204] " chris at bubblescope dot net
` (4 more replies)
0 siblings, 5 replies; 6+ messages in thread
From: charles at rebelbase dot com @ 2006-12-13 18:01 UTC (permalink / raw)
To: gcc-bugs
vector::operator[](size_type) in bits/stl_vector.h is currently implemented as
reference
operator[](size_type __n)
{ return *(begin() + __n); }
const_reference
operator[](size_type __n) const
{ return *(begin() + __n); }
A faster implementation would be:
reference
operator[](size_type __n)
{ return _M_impl._M_start[__n]; }
const_reference
operator[](size_type __n) const
{ return _M_impl._M_start[__n]; }
I tried a simple timing test on both implementations,
and the latter appears to be 10x faster:
(11:59:43)(charles xyzzy)(~): cat test.cc
#include <vector>
int main () {
std::vector<int> x (100);
unsigned long l = 0;
const unsigned long iterations = 100000000;
for (unsigned long i=0; i<iterations; ++i)
l += x[50];
return 0;
}
(12:00:14)(charles xyzzy)(~): g++ -o test test.cc -lstdc++
(12:00:22)(charles xyzzy)(~): time ./test
real 0m2.956s
user 0m2.948s
sys 0m0.008s
(12:00:27)(charles xyzzy)(~): cat test2.cc
#include <vector>
int main () {
std::vector<int> x (100);
unsigned long l = 0;
const unsigned long iterations = 100000000;
for (unsigned long i=0; i<iterations; ++i)
l += x._M_impl._M_start[50];
return 0;
}
(12:00:31)(charles xyzzy)(~): g++ -o test2 test2.cc -lstdc++
(12:00:39)(charles xyzzy)(~): time ./test2
real 0m0.228s
user 0m0.228s
sys 0m0.000s
--
Summary: std::vector operator[] 10x speedup (patch)
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: enhancement
Priority: P3
Component: libstdc++
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: charles at rebelbase dot com
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30204
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug libstdc++/30204] std::vector operator[] 10x speedup (patch)
2006-12-13 18:01 [Bug libstdc++/30204] New: std::vector operator[] 10x speedup (patch) charles at rebelbase dot com
@ 2006-12-13 18:09 ` chris at bubblescope dot net
2006-12-14 2:50 ` pinskia at gcc dot gnu dot org
` (3 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: chris at bubblescope dot net @ 2006-12-13 18:09 UTC (permalink / raw)
To: gcc-bugs
------- Comment #1 from chris at bubblescope dot net 2006-12-13 18:08 -------
-O1 is enough to remove all advantages of this patch.
Also, that isn't really a fair timing comparison, as you've removed the
function call altogether (I still expect it to be faster, but possibly not by
10x)
--
chris at bubblescope dot net changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |chris at bubblescope dot net
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30204
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug libstdc++/30204] std::vector operator[] 10x speedup (patch)
2006-12-13 18:01 [Bug libstdc++/30204] New: std::vector operator[] 10x speedup (patch) charles at rebelbase dot com
2006-12-13 18:09 ` [Bug libstdc++/30204] " chris at bubblescope dot net
@ 2006-12-14 2:50 ` pinskia at gcc dot gnu dot org
2006-12-14 10:34 ` pcarlini at suse dot de
` (2 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: pinskia at gcc dot gnu dot org @ 2006-12-14 2:50 UTC (permalink / raw)
To: gcc-bugs
------- Comment #2 from pinskia at gcc dot gnu dot org 2006-12-14 02:49 -------
Can you test with -O1 or above?
As mentioned in comment #1, this should remove all advatages gained from the
"inlined" version.
> Also, that isn't really a fair timing comparison, as you've removed the
> function call altogether (I still expect it to be faster, but possibly not by
> 10x)
For -O0, I would not doubt removing the function calls get a 10x speed up,
which is one of the reasons why -O1 makes this much faster than -O0 :).
--
pinskia at gcc dot gnu dot org changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |WAITING
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30204
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug libstdc++/30204] std::vector operator[] 10x speedup (patch)
2006-12-13 18:01 [Bug libstdc++/30204] New: std::vector operator[] 10x speedup (patch) charles at rebelbase dot com
2006-12-13 18:09 ` [Bug libstdc++/30204] " chris at bubblescope dot net
2006-12-14 2:50 ` pinskia at gcc dot gnu dot org
@ 2006-12-14 10:34 ` pcarlini at suse dot de
2006-12-14 11:20 ` pcarlini at suse dot de
2006-12-14 17:58 ` charles at rebelbase dot com
4 siblings, 0 replies; 6+ messages in thread
From: pcarlini at suse dot de @ 2006-12-14 10:34 UTC (permalink / raw)
To: gcc-bugs
------- Comment #3 from pcarlini at suse dot de 2006-12-14 10:33 -------
Likewise...
--
pcarlini at suse dot de changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|WAITING |RESOLVED
Resolution| |INVALID
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30204
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug libstdc++/30204] std::vector operator[] 10x speedup (patch)
2006-12-13 18:01 [Bug libstdc++/30204] New: std::vector operator[] 10x speedup (patch) charles at rebelbase dot com
` (2 preceding siblings ...)
2006-12-14 10:34 ` pcarlini at suse dot de
@ 2006-12-14 11:20 ` pcarlini at suse dot de
2006-12-14 17:58 ` charles at rebelbase dot com
4 siblings, 0 replies; 6+ messages in thread
From: pcarlini at suse dot de @ 2006-12-14 11:20 UTC (permalink / raw)
To: gcc-bugs
------- Comment #4 from pcarlini at suse dot de 2006-12-14 11:18 -------
By the way, forgot that in mainline and 4.2 branch the issue simply doesn't
exist anymore: for completely different reasons, we are *already* using
directly _M_start, _M_finish, and co.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30204
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug libstdc++/30204] std::vector operator[] 10x speedup (patch)
2006-12-13 18:01 [Bug libstdc++/30204] New: std::vector operator[] 10x speedup (patch) charles at rebelbase dot com
` (3 preceding siblings ...)
2006-12-14 11:20 ` pcarlini at suse dot de
@ 2006-12-14 17:58 ` charles at rebelbase dot com
4 siblings, 0 replies; 6+ messages in thread
From: charles at rebelbase dot com @ 2006-12-14 17:58 UTC (permalink / raw)
To: gcc-bugs
------- Comment #5 from charles at rebelbase dot com 2006-12-14 17:58 -------
(In reply to comment #1)
> -O1 is enough to remove all advantages of this patch.
>
> Also, that isn't really a fair timing comparison, as you've removed the
> function call altogether (I still expect it to be faster, but possibly not by
> 10x)
Yes, you're right. Thanks for looking at this.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30204
^ permalink raw reply [flat|nested] 6+ messages in thread
end of thread, other threads:[~2006-12-14 17:58 UTC | newest]
Thread overview: 6+ messages (download: mbox.gz / follow: Atom feed)
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2006-12-13 18:01 [Bug libstdc++/30204] New: std::vector operator[] 10x speedup (patch) charles at rebelbase dot com
2006-12-13 18:09 ` [Bug libstdc++/30204] " chris at bubblescope dot net
2006-12-14 2:50 ` pinskia at gcc dot gnu dot org
2006-12-14 10:34 ` pcarlini at suse dot de
2006-12-14 11:20 ` pcarlini at suse dot de
2006-12-14 17:58 ` charles at rebelbase dot com
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