Word up!
If there are any gcc users here, maybe you could help me out. I have a
program and I've tried compiling it with -O2 and -O3 optimization
settings.
The wired thing is that it actually runs faster with -O2 than with -O3,
even though -O3 is a higher optimization setting.
Have I found a bug in gcc? Could I be doing something wrong?
Cheers.
Jan 15 '08
29  1909 
On Wed, 16 Jan 2008 05:24:07 -0600, Joachim Schmitz wrote
(in article <fm**********@o nline.de>):
Kelsey Bjarnason wrote:
>On Wed, 16 Jan 2008 00:43:25 +0100, sammy wrote:
>>Word up!
If there are any gcc users here, maybe you could help me out. I have
a program and I've tried compiling it with -O2 and -O3 optimization
settings.
The wired thing is that it actually runs faster with -O2 than with
-O3, even though -O3 is a higher optimization setting.
Have I found a bug in gcc? Could I be doing something wrong?
One point to ponder, which isn't specific to gcc but to optimisation
in general...
Many optimisations consume more space than the less optimised code.
Loop unrolling, for example, can do this. While this _can_ result in
faster code, it can _also_ potentially result in side effects such as
exhausting the cache memory. The net result can be a significant
slowdown.
This sort of thing isn't really a bug; the optimiser has no way to
know what machines the code will run on.
If not the compiler/optimizer, who else?
How can it possibly know which computer(s) you will install and run it
on after it is compiled?
Not every program is something for you to play with for a bit in ~/src
then forget about. ;-)
--
Randy Howard (2reply remove FOOBAR)
"The power of accurate observation is called cynicism by those
who have not got it." - George Bernard Shaw
On Wed, 16 Jan 2008 06:47:46 -0600, Rui Maciel wrote
(in article <47************ ***********@new s.telepac.pt>):
Kelsey Bjarnason wrote:
>This sort of thing isn't really a bug; the optimiser has no way to know
what machines the code will run on.
If the compiler writers think that this is relevant and they need to figure
out a way to know any property of the target machine, couldn't they simply
add an option that enables the user to specify those target properties?
Some provide switches to optimize for different "families" of
processors. The problem is, it doesn't allow for any new hardware that
comes along after the compile is completed or after the compiler was
written. Also, it doesn't allow for a binary to be used on multiple
hardware platforms from the same build while enjoying this special
attention.
--
Randy Howard (2reply remove FOOBAR)
"The power of accurate observation is called cynicism by those
who have not got it." - George Bernard Shaw
On Wed, 16 Jan 2008 09:26:23 -0600, jacob navia wrote
(in article <fm**********@a ioe.org>):
Within lcc-win, I have targeted only ONE optimization strategy:
Code size.
There is nothing that runs faster than a deleted instruction.
<shakes head>
#include "examples of loop unrolling improving performance"
I wonder if everyone using lcc-win today realize just how narrow your
view on optimization is?
--
Randy Howard (2reply remove FOOBAR)
"The power of accurate observation is called cynicism by those
who have not got it." - George Bernard Shaw
Randy Howard wrote:
On Wed, 16 Jan 2008 09:26:23 -0600, jacob navia wrote
(in article <fm**********@a ioe.org>):
>Within lcc-win, I have targeted only ONE optimization strategy:
Code size.
There is nothing that runs faster than a deleted instruction.
<shakes head>
#include "examples of loop unrolling improving performance"
I wonder if everyone using lcc-win today realize just how narrow your
view on optimization is?
Can you explain the results ?
Of course it is narrow. It is a Reduced Optimization Set Compiler
(ROSC).
Jokes aside, obviously for you, the results aren't important but...
what?
--
jacob navia
jacob at jacob point remcomp point fr
logiciels/informatique http://www.cs.virginia.edu/~lcc-win32
Randy Howard wrote:
Some provide switches to optimize for different "families" of
processors. Â*The problem is, it doesn't allow for any new hardware that
comes along after the compile is completed or after the compiler was
written. Â*Also, it doesn't allow for a binary to be used on multiple
hardware platforms from the same build while enjoying this special
attention.
That isn't exactly a compiler problem, is it?
Rui Maciel
On Jan 16, 11:34*am, Randy Howard <randyhow...@FO OverizonBAR.net >
wrote:
On Wed, 16 Jan 2008 05:24:07 -0600, Joachim Schmitz wrote
(in article <fmkpgn$v3...@o nline.de>):
Kelsey Bjarnason wrote:
On Wed, 16 Jan 2008 00:43:25 +0100, sammy wrote:
>Word up!
>If there are any gcc users here, maybe you could help me out. I have
a program and I've tried compiling it with -O2 and -O3 optimization
settings.
>The wired thing is that it actually runs faster with -O2 than with
-O3, even though -O3 is a higher optimization setting.
>Have I found a bug in gcc? Could I be doing something wrong?
One point to ponder, which isn't specific to gcc but to optimisation
in general...
Many optimisations consume more space than the less optimised code.
Loop unrolling, for example, can do this. *While this _can_ result in
faster code, it can _also_ potentially result in side effects such as
exhausting the cache memory. *The net result can be a significant
slowdown.
This sort of thing isn't really a bug; the optimiser has no way to
know what machines the code will run on.
If not the compiler/optimizer, who else?
How can it possibly know which computer(s) you will install and run it
on after it is compiled? *
GCC aside:
-march is supposed to be a promise of that. If you run it on
something else then you won't get the sort of performance you were
hoping for. Many other compilers have this same sort of effect (even
producing code that will only run on certain CPUs in some instances).
Not every program is something for you to play with for a bit in ~/src
then forget about. *;-)
Rats. How deflating.
Randy Howard wrote:
On Wed, 16 Jan 2008 05:24:07 -0600, Joachim Schmitz wrote
(in article <fm**********@o nline.de>):
>>This sort of thing isn't really a bug; the optimiser has no way to
know what machines the code will run on.
If not the compiler/optimizer, who else?
How can it possibly know which computer(s) you will install and run it
on after it is compiled?
The optimizer could run on the target machine, so "this one"
would be the appropriate answer.
[Warning: mere possibility isn't evidence of implementation.]
--
Contains Billion-Year-Old Materials Hedgehog
Otherface: Jena RDF/Owl toolkit http://jena.sourceforge.net/
Chris Dollin wrote:
Randy Howard wrote:
>On Wed, 16 Jan 2008 05:24:07 -0600, Joachim Schmitz wrote
(in article <fm**********@o nline.de>):
>>>This sort of thing isn't really a bug; the optimiser has no way to
know what machines the code will run on.
If not the compiler/optimizer, who else?
How can it possibly know which computer(s) you will install and run it
on after it is compiled?
The optimizer could run on the target machine, so "this one"
would be the appropriate answer.
[Warning: mere possibility isn't evidence of implementation.]
Shipping the optimizer with your application?
--
jacob navia
jacob at jacob point remcomp point fr
logiciels/informatique http://www.cs.virginia.edu/~lcc-win32
On Wed, 16 Jan 2008 15:48:56 -0600, user923005 wrote
(in article
<d1************ *************** *******@z17g200 0hsg.googlegrou ps.com>):
On Jan 16, 11:34*am, Randy Howard <randyhow...@FO OverizonBAR.net >
wrote:
>>
>>>This sort of thing isn't really a bug; the optimiser has no way to
know what machines the code will run on.
If not the compiler/optimizer, who else?
How can it possibly know which computer(s) you will install and run it
on after it is compiled? *
GCC aside:
-march is supposed to be a promise of that. If you run it on
something else then you won't get the sort of performance you were
hoping for. Many other compilers have this same sort of effect (even
producing code that will only run on certain CPUs in some instances).
I think you missed what I was saying there. You can optimize with
something like -march for a specific hardware type, but when you move
that binary to other machines it isn't a promise of anything, it may
not even run properly.
--
Randy Howard (2reply remove FOOBAR)
"The power of accurate observation is called cynicism by those
who have not got it." - George Bernard Shaw
On 16 Jan 2008 at 22:20, jacob navia wrote:
Chris Dollin wrote:
>The optimizer could run on the target machine, so "this one"
would be the appropriate answer.
[Warning: mere possibility isn't evidence of implementation.]
Shipping the optimizer with your application?
Yes, though that rather depends on shipping the source code with your
application too, so it wouldn't be much use for closed-source programs
like lcc-win for example... This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics | |
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