Hello -
I have been told that for extra efficiency we should always compile
code with frame pointer ommited (-fomit-frame-pointer), also -pipe.
What is the reason for this? Why does it make a better executable? Are
there situations where it doesn't work or isn't the best thing to do?
Also I don't understand why not always make Position Independent Code
(-fPIC)?
Thanks.
Dec 12 '07
14  1847 
On 14 Dec 2007 at 18:22, Stephen Sprunk wrote:
"Brian" <tr**@nospam.co mwrote in message
news:sl******** ***********@nos pam.invalid...
>>>>I have been told that for extra efficiency we should always compile
code with frame pointer ommited (-fomit-frame-pointer)
Not using a frame pointer makes an extra register available for other
uses, which speeds things up on processors with few registers (in
particular, on x86 processors). Using one can make debugging easier,
though I think gdb can do most things without one.
Wouldn't it be better then if the frame pointer was ommited by default
unless debugging (-g) is switched on?
Some debugging can be done without symbols, and it's easier with a frame
pointer.
But if you are planning to debug, why wouldn't you include symbols (-g)?
>>>also -pipe.
This gcc option just uses pipes between compiler stages instead of
temporary files. It might speed up compilation a little but probably
not a lot on current systems.
So why isn't this enabled by default?
Because a pipe means multiple compiler stages would be in memory and
consuming CPU at the same time. That may be fine for some systems, but it
isn't for others.
OK, but nowadays any computer will have huge CPU and RAM resources, so
why not make -pipe the default and let people turn it off if they're
compiling on a stone-age machine?
>>>>Also I don't understand why not always make Position Independent Code
(-fPIC)?
Position independent code is likely to be somewhat slower on most
systems, just because of extra indirections. It's needed for shared
libraries.
Isn't it good to be able to relocate all code, not just library code?
Non-library code is relocated at link time; it doesn't need to be
position-independent (or relocated) at runtime.
But if it doesn't cost anything to have the extra flexibility, then why
not?
Best
Brian
Richard Tobin wrote:
Brian <tr**@nospam.co mwrote:
>>Position independent code is likely to be somewhat slower on most
systems, just because of extra indirections. It's needed for shared
libraries.
>Isn't it good to be able to relocate all code, not just library code?
Why? Who needs to relocate code in complete programs?
The code in compiled-but-not-linked files (.o files in unix) is
relocatable in the sense that it doesn't yet have the addresses filled
in, but that's a quite separate issue: when it's linked, a new version
is made with the addresses filled in. The code in shared libraries
needs to be runnable at different addresses without copying it (so it
can be shared).
The voice of someone who has never seen machines without relocation
registers. The innocence of youth.
--
Chuck F (cbfalconer at maineline dot net)
<http://cbfalconer.home .att.net>
Try the download section.
--
Posted via a free Usenet account from http://www.teranews.com
"Brian" <tr**@nospam.co mwrote in message
news:sl******** ***********@nos pam.invalid...
On 14 Dec 2007 at 18:22, Stephen Sprunk wrote:
>"Brian" <tr**@nospam.co mwrote in message
news:sl******* ************@no spam.invalid...
>>>>>I have been told that for extra efficiency we should always compile
>code with frame pointer ommited (-fomit-frame-pointer)
Not using a frame pointer makes an extra register available for other
uses, which speeds things up on processors with few registers (in
particular , on x86 processors). Using one can make debugging easier,
though I think gdb can do most things without one.
Wouldn't it be better then if the frame pointer was ommited by default
unless debugging (-g) is switched on?
Some debugging can be done without symbols, and it's easier with a frame
pointer.
But if you are planning to debug, why wouldn't you include symbols (-g)?
I'm usually not _planning_ to debug. Bugs are things that take one by
surprise, because if they weren't, one would have already fixed the problem.
When we ship products to customers, they do not contain symbols; if the code
crashes, we have to debug the code without them -- or dig up the separate
symbol files we don't ship.
Of course, when adding a new function or doing unit tests, all the
appropriate flags are set to make debugging as easy as possible, but some
always manage to hide until customers get their hands on the "finished"
product.
>>>>also -pipe.
This gcc option just uses pipes between compiler stages instead of
temporary files. It might speed up compilation a little but probably
not a lot on current systems.
So why isn't this enabled by default?
Because a pipe means multiple compiler stages would be in memory and
consuming CPU at the same time. That may be fine for some systems, but
it
isn't for others.
OK, but nowadays any computer will have huge CPU and RAM resources, so
why not make -pipe the default and let people turn it off if they're
compiling on a stone-age machine?
*shrug* Take that up with the GCC folks.
>>>>>Also I don't understand why not always make Position Independent Code
>(-fPIC)?
Position independent code is likely to be somewhat slower on most
systems, just because of extra indirections. It's needed for shared
libraries.
Isn't it good to be able to relocate all code, not just library code?
Non-library code is relocated at link time; it doesn't need to be
position-independent (or relocated) at runtime.
But if it doesn't cost anything to have the extra flexibility, then why
not?
On most systems I'm familiar with, PIC code is larger and slower. If you
don't need to suffer that penalty, you don't turn it on.
On the systems where there is no penalty for PIC code, it's on by default
(IIRC).
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
On 14 Dec, 18:50, rich...@cogsci. ed.ac.uk (Richard Tobin) wrote:
In article <slrnfm5g26.hn7 .nos...@nospam. invalid>,
Brian <t...@nospam.co mwrote:
Position independent code is likely to be somewhat slower on most
systems, just because of extra indirections. It's needed for shared
libraries.
Isn't it good to be able to relocate all code, not just library code?
Why? Who needs to relocate code in complete programs?
machines with pageing but without virtual memory.
Old Macs needed to relocate code on-the-fly.
Didn't Crays not have virtual memory?
The code in compiled-but-not-linked files (.o files in unix) is
relocatable in the sense that it doesn't yet have the addresses filled
in, but that's a quite separate issue: when it's linked, a new version
is made with the addresses filled in. The code in shared libraries
needs to be runnable at different addresses without copying it (so it
can be shared).
--
Nick Keighley
In article <fa************ *************** *******@b1g2000 pra.googlegroup s.com>,
Nick Keighley <ni************ ******@hotmail. comwrote:
>Position independent code is likely to be somewhat slower on most
systems, just because of extra indirections. It's needed for shared
libraries.
>Isn't it good to be able to relocate all code, not just library code?
>Why? Who needs to relocate code in complete programs?
>machines with pageing but without virtual memory.
Old Macs needed to relocate code on-the-fly.
Didn't Crays not have virtual memory?
In other words, almost nobody. Unless you have peculiar requirements,
there's no advantage to position independent code outside libraries.
-- Richard
--
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