Compiler optimizations

[user923005]

Re: Compiler optimizations

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.

[Chris Dollin]

Re: Compiler optimizations

Randy Howard wrote:

On Wed, 16 Jan 2008 05:24:07 -0600, Joachim Schmitz wrote
(in article <fmkpgn$v36$1@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/

[jacob navia]

Re: Compiler optimizations

Chris Dollin wrote:

Randy Howard wrote:
>

>On Wed, 16 Jan 2008 05:24:07 -0600, Joachim Schmitz wrote
>(in article <fmkpgn$v36$1@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

[Randy Howard]

Re: Compiler optimizations

On Wed, 16 Jan 2008 15:48:56 -0600, user923005 wrote
(in article
<d1de1901-7a6b-4e75-b819-ff7be1539cb9@z1 7g2000hsg.googl egroups.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

[CJ]

Re: Compiler optimizations

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...

[Rui Maciel]

Re: Compiler optimizations

Randy Howard wrote:

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.

Where exactly is there a need to run a single, unique binary across a whole
range of different machines? Why should anyone want that?

Assuming that open source software isn't even being considered here, I don't
see how, in this day and age, a distributor of closed-source, proprietary
software is barred from shipping multiple binaries in the install media or
even making it available over the internet. I mean, in this day and age,
software is being distributed through DVD packs and even through the net.
Where exactly is it impossible to pack a hand full of specialised binaries
in the install media and then pick the more appropriate binary for the
current system?


Rui Maciel

[Rui Maciel]

Re: Compiler optimizations

jacob navia wrote:

Shipping the optimizer with your application?

There was a time where operating systems also came with a compiler. Heck, it
is still happening in this very day. Nowadays there are quite a lot of
operating systems installing compilers in their default configuration. Take
all those linux distributions, for example. Thanks to that, we see
companies like nvidia relying on the system's compiler to install their
software.


Rui Maciel

[Randy Howard]

Re: Compiler optimizations

On Wed, 16 Jan 2008 20:08:54 -0600, Rui Maciel wrote
(in article <478eb8b3$0$176 77$a729d347@new s.telepac.pt>):

Randy Howard wrote:
>

>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.

>
Where exactly is there a need to run a single, unique binary across a whole
range of different machines? Why should anyone want that?

It's the model that pretty much all ISV's use. <insert list of 20,000
canned apps here>

Assuming that open source software isn't even being considered here, I don't
see how, in this day and age, a distributor of closed-source, proprietary
software is barred from shipping multiple binaries in the install media or
even making it available over the internet.

It's not barred from it, technically. It's cost prohibitive to build
and test a lot of versions of the same program just to micro-optimize
for a bunch of slightly different processors, cache sizes, etc.


--
Randy Howard (2reply remove FOOBAR)
"The power of accurate observation is called cynicism by those
who have not got it." - George Bernard Shaw

[Kelsey Bjarnason]

Re: Compiler optimizations

On Wed, 16 Jan 2008 12:24:07 +0100, Joachim Schmitz wrote:

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?

Suppose I write code optimized for, oh, pentiums. There are something
like 197 different flavours of pentiums, with different cache sizes,
pipleine depths, who knows what sort of differences.

How does the optimizer know which of those you're going to run the code
on? Perhaps you'll distribute the app and it will have to run on all of
them. Tomorrow's "mini pentium" for embedded systems might have the full
instruction set, but almost no cache - shall the optimizer predict the
future to tell you might, someday, want to run the binary on this chip?

All it can do is generate the code, as efficiently as possible... and let
_you_, the developer, see whether the code generated really was as
efficient as it should have been, then either accept it, or re-compile
with different optimization settings.

[Gordon Burditt]

Re: Compiler optimizations

>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.

>
>Where exactly is there a need to run a single, unique binary across a whole
>range of different machines? Why should anyone want that?

It simplifies virus-writing a lot.

[Keith Thompson]

Re: [OT] Compiler optimizations

Rui Maciel <rui.maciel@gma il.comwrites:
[snip]

Moreover, please do have in mind that there are quite a few free software
projects, even whole operating system distributions, that distribute
multiple versions of binaries optimised to specific target platforms? The
people behind those projects distribute those binaries on public
repositories open to all and we don't see them bankrupting. How exactly are
the people behind all those free software distributions able to produce
optimised binaries and offer them to everyone interested without charging a
single cent for their work while, according to your statement, the cost for
proprietary software distributors to do an irrelevant fraction of that same
work would be "prohibitiv e", specially when they charge quite a bit over 50
euros for each download/DVD?

For a free software project, the cost of customized builds for
multiple systems is (a) building and testing the software for each
variant target and (b) providing multiple versions on the download
site. A user who downloaded the wrong variant can just go back and
download the right one.

To do the same for a commercial product, you'd have added production
costs (packaging, keeping track of different versions on DVDs, etc.)
-- and a customer who finds he bought the wrong variant is likely to
have more trouble straightening it out; in the worst case, he might
have to pay for the product again.

I'm sure there are ways to work around these issues. I'm also sure
there are better places for this discussion.

--
Keith Thompson (The_Other_Keit h) <kst-u@mib.org>
Nokia
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

[Rui Maciel]

Re: Compiler optimizations

Gordon Burditt wrote:

How much do the tech support calls from people who don't know what
to download cost?  If you're distributing an OS, this might not be
much of a problem, but if you're distributing an application, good
luck.  Some people don't know how to tell if their desktop is a PC
or a Mac, much less whether it's got an Intel vs. AMD processor,
or whether it's dual-core.  And a lot of people know they are
running Windows, but don't recognize such terms as "XP", "Vista",
"Windows 98", etc.

The tech support reference is a bit absurd. We see free software supporting
a vast set of architectures but they don't suffer from any support issue.
Moreover, there is even absolutely no need for the user to know those
details, as even microsoft's windows line of operating systems benefits
from automatic package deployment. It's just a matter of checking the
relevant system properties and deploy the right binary. So what stops
anyone from shipping specialized binaries?


Rui Maciel

[Randy Howard]

Re: Compiler optimizations

On Sun, 20 Jan 2008 16:32:18 -0600, Rui Maciel wrote
(in article <4793cc0d$0$165 82$a729d347@new s.telepac.pt>):

Gordon Burditt wrote:
>

>How much do the tech support calls from people who don't know what
>to download cost?  If you're distributing an OS, this might not be
>much of a problem, but if you're distributing an application, good
>luck.  Some people don't know how to tell if their desktop is a PC
>or a Mac, much less whether it's got an Intel vs. AMD processor,
>or whether it's dual-core.  And a lot of people know they are
>running Windows, but don't recognize such terms as "XP", "Vista",
>"Windows 98", etc.

>
The tech support reference is a bit absurd. We see free software supporting
a vast set of architectures but they don't suffer from any support issue.

Right, because for the bulk of the open source community, no formal
support is available at all. It's a non-issue. The rest of the world
does not behave identically though.

Moreover, there is even absolutely no need for the user to know those
details, as even microsoft's windows line of operating systems benefits
from automatic package deployment.

When it works. It still doesn't make it free to develop 10 packages
instead of 1, test them, package them, then test the deployment via
package managers, in 10X the number of cases.

It's just a matter of checking the
relevant system properties and deploy the right binary.

It's not that simple.

So what stops anyone from shipping specialized binaries?

Reality.


--
Randy Howard (2reply remove FOOBAR)
"The power of accurate observation is called cynicism by those
who have not got it." - George Bernard Shaw

[jacob navia]

Re: Compiler optimizations

Rui Maciel wrote:

The tech support reference is a bit absurd. We see free software supporting
a vast set of architectures but they don't suffer from any support issue.
Moreover, there is even absolutely no need for the user to know those
details, as even microsoft's windows line of operating systems benefits
from automatic package deployment. It's just a matter of checking the
relevant system properties and deploy the right binary. So what stops
anyone from shipping specialized binaries?
>

The show stopper is the fact that you would have to TEST each
of the binaries before you ship it.


--
jacob navia
jacob at jacob point remcomp point fr
logiciels/informatique

http://www.cs.virginia.edu/~lcc-win32

[Stephen Sprunk]

Re: Compiler optimizations

"Rui Maciel" <rui.maciel@gma il.comwrote in message
news:4793cc0d$0 $16582$a729d347 @news.telepac.p t...

Gordon Burditt wrote:

>How much do the tech support calls from people who don't know what
>to download cost? If you're distributing an OS, this might not be
>much of a problem, but if you're distributing an application, good
>luck. Some people don't know how to tell if their desktop is a PC
>or a Mac, much less whether it's got an Intel vs. AMD processor,
>or whether it's dual-core. And a lot of people know they are
>running Windows, but don't recognize such terms as "XP", "Vista",
>"Windows 98", etc.

>
The tech support reference is a bit absurd. We see free software
supporting
a vast set of architectures but they don't suffer from any support issue.
Moreover, there is even absolutely no need for the user to know those
details, as even microsoft's windows line of operating systems benefits
from automatic package deployment. It's just a matter of checking the
relevant system properties and deploy the right binary. So what stops
anyone from shipping specialized binaries?

Support costs. Even if you can automatically install the correct binary on
each machine, you still have to test all of the individual binaries in QA,
and you have to troubleshoot the correct ones when customers find problems.
This is why many companies are still shipping binaries that are compiled for
original Pentiums with low optimization settings and debug symbols -- it's
easier to support, and it works, if not optimally, on every system their
customers own.

The FOSS crowd has it a bit easier since they don't have to support
anything, so each user can compile the software however they want. Notice
that those for-profit companies that do support FOSS often require customers
run the binaries _they_ compiled.

S

--
Stephen Sprunk "God does not play dice." --Albert Einstein
CCIE #3723 "God is an inveterate gambler, and He throws the
K5SSS dice at every possible opportunity." --Stephen Hawking