* shared libraries + lto ?
@ 2014-08-01 9:09 Alain Meunier
2014-08-01 10:47 ` Vincent Lefevre
0 siblings, 1 reply; 4+ messages in thread
From: Alain Meunier @ 2014-08-01 9:09 UTC (permalink / raw)
To: gcc-help
Hello,
I would like to know if one can use lto with shared libraries and leverage all the goodness of both worlds ?
My tests show that it works but not sure if lto brang something or not in the game.
gcc -O3 -shared -fPIC my_shared.c -o libmy_shared.so -lto
and linking
gcc -O3 my_app.c -o my_binary -lmy_shared -lto
I would like to keep the ability to have shared libraries. Will gcc make something out of it ?
Thanks
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: shared libraries + lto ?
2014-08-01 9:09 shared libraries + lto ? Alain Meunier
@ 2014-08-01 10:47 ` Vincent Lefevre
2014-08-01 11:59 ` Alain Meunier
0 siblings, 1 reply; 4+ messages in thread
From: Vincent Lefevre @ 2014-08-01 10:47 UTC (permalink / raw)
To: gcc-help
On 2014-08-01 11:09:38 +0200, Alain Meunier wrote:
> I would like to know if one can use lto with shared libraries and
> leverage all the goodness of both worlds ?
>
> My tests show that it works but not sure if lto brang something or
> not in the game.
I did some timings with MPFR + GMP two years ago and I found that it
was useless to use LTO with the shared library (I even wonder whether
this can make sense at all). Here are the results:
Precision 10:
shared static
arg macro arg macro
Default 3.480 3.470 2.670 2.690
LTO paths 4.000 3.980 2.640 2.660
With LTO 4.110 3.970 2.320 2.410
Precision 80:
shared static
arg macro arg macro
Default 5.520 5.470 4.950 5.000
LTO paths 5.510 5.500 4.440 4.470
With LTO 5.540 5.520 4.040 4.120
Precision 300:
shared static
arg macro arg macro
Default 6.770 6.560 5.950 5.960
LTO paths 6.140 5.980 5.060 5.020
With LTO 5.980 5.960 4.280 4.400
Conclusion (on these examples):
* There isn't much difference between a precision given in argument
and a fixed precision given via a macro (known at compile time of
the main program).
* Using a static library instead of a shared library can yield a
speedup of up to 44% (this happens with LTO enabled), i.e. that's
almost twice as fast!
* LTO should be used only with -static (for performance, but also
when considering practical use, it is pointless to use LTO with
shared libraries).
* The LTO speedup ("With LTO" compared to "LTO paths" in static) can
be up to 15% (28% if we compare to the default static library, but
we are not just measuring LTO in this case).
* The LTO speedup compared to traditional linking (shared library
from the vendor, here Debian/unstable) can be up to 37%.
Note: The versions of MPFR in "Default" (Debian packages providing
MPFR 3.1.0-p10) and with LTO paths (MPFR 3.1.1-p2) are not exactly
the same, but the differences consist only of bug fixes, so that the
tested source code should be the same.
--
Vincent Lefèvre <vincent@vinc17.net> - Web: <https://www.vinc17.net/>
100% accessible validated (X)HTML - Blog: <https://www.vinc17.net/blog/>
Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)
^ permalink raw reply [flat|nested] 4+ messages in thread
* RE: shared libraries + lto ?
2014-08-01 10:47 ` Vincent Lefevre
@ 2014-08-01 11:59 ` Alain Meunier
2014-08-03 14:48 ` Vincent Lefevre
0 siblings, 1 reply; 4+ messages in thread
From: Alain Meunier @ 2014-08-01 11:59 UTC (permalink / raw)
To: gcc-help
Thanks Vincent,
I think I will not use shared libraries in this case.
I will stick static ones.
But on the whole net there are many different expressions of correctness when related to lto.
Could you clarify :
say I have a static library libfoo.a:
void function cool(int * restrict a,int * restrict b){
//do something useful
}
I also have libbar.a:
void function eatTheWorld(int * restrict a,int * restrict b){
//do something useful
}
Both compiled with gcc
gcc /*optim. flags here*/ -fPIC foo.c -o libfoo.a -flto
gcc /*optim. flags here*/ -fPIC bar.c -o libbar.a -flto
and a main program my_app.c :
#include here
int main(){
int f1 = 5;
int f2 = 3;
cool(&f1,&f2);
eatTheWorld(&f1,&f2)
return 0;
}
I will compile it with
gcc /*optim. flags here*/ my_app.c -lfoo -lbar -flto
Is that it ? Nothing more ?
This article suggests otherwise : http://hubicka.blogspot.fr/2014/04/linktime-optimization-in-gcc-2-firefox.html
No plugin or all this mess ?
I am on Debian testing 64 bits.
The lto best use is still a bit unclear to yield the best performance, at least to me.
----------------------------------------
> Date: Fri, 1 Aug 2014 12:47:16 +0200
> From: vincent+gcc@vinc17.org
> To: gcc-help@gcc.gnu.org
> Subject: Re: shared libraries + lto ?
>
> On 2014-08-01 11:09:38 +0200, Alain Meunier wrote:
>> I would like to know if one can use lto with shared libraries and
>> leverage all the goodness of both worlds ?
>>
>> My tests show that it works but not sure if lto brang something or
>> not in the game.
>
> I did some timings with MPFR + GMP two years ago and I found that it
> was useless to use LTO with the shared library (I even wonder whether
> this can make sense at all). Here are the results:
>
> Precision 10:
> shared static
> arg macro arg macro
> Default 3.480 3.470 2.670 2.690
> LTO paths 4.000 3.980 2.640 2.660
> With LTO 4.110 3.970 2.320 2.410
>
> Precision 80:
> shared static
> arg macro arg macro
> Default 5.520 5.470 4.950 5.000
> LTO paths 5.510 5.500 4.440 4.470
> With LTO 5.540 5.520 4.040 4.120
>
> Precision 300:
> shared static
> arg macro arg macro
> Default 6.770 6.560 5.950 5.960
> LTO paths 6.140 5.980 5.060 5.020
> With LTO 5.980 5.960 4.280 4.400
>
> Conclusion (on these examples):
> * There isn't much difference between a precision given in argument
> and a fixed precision given via a macro (known at compile time of
> the main program).
> * Using a static library instead of a shared library can yield a
> speedup of up to 44% (this happens with LTO enabled), i.e. that's
> almost twice as fast!
> * LTO should be used only with -static (for performance, but also
> when considering practical use, it is pointless to use LTO with
> shared libraries).
> * The LTO speedup ("With LTO" compared to "LTO paths" in static) can
> be up to 15% (28% if we compare to the default static library, but
> we are not just measuring LTO in this case).
> * The LTO speedup compared to traditional linking (shared library
> from the vendor, here Debian/unstable) can be up to 37%.
>
> Note: The versions of MPFR in "Default" (Debian packages providing
> MPFR 3.1.0-p10) and with LTO paths (MPFR 3.1.1-p2) are not exactly
> the same, but the differences consist only of bug fixes, so that the
> tested source code should be the same.
>
> --
> Vincent Lefèvre <vincent@vinc17.net> - Web: <https://www.vinc17.net/>
> 100% accessible validated (X)HTML - Blog: <https://www.vinc17.net/blog/>
> Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: shared libraries + lto ?
2014-08-01 11:59 ` Alain Meunier
@ 2014-08-03 14:48 ` Vincent Lefevre
0 siblings, 0 replies; 4+ messages in thread
From: Vincent Lefevre @ 2014-08-03 14:48 UTC (permalink / raw)
To: gcc-help
On 2014-08-01 13:59:28 +0200, Alain Meunier wrote:
[...]
> Both compiled with gcc
> gcc /*optim. flags here*/ -fPIC foo.c -o libfoo.a -flto
> gcc /*optim. flags here*/ -fPIC bar.c -o libbar.a -flto
>
> and a main program my_app.c :
>
> #include here
> int main(){
> Â Â Â int f1 = 5;
> Â Â Â int f2 = 3;
> Â Â cool(&f1,&f2);
> Â Â eatTheWorld(&f1,&f2)
>
> return 0;
> }
>
> I will compile it with
> gcc /*optim. flags here*/Â my_app.c -lfoo -lbar -flto
>
> Is that it ? Nothing more ?
I compiled everything with "-flto=jobserve -fuse-linker-plugin"
(that was two years ago), but I don't know whether this is still
necessary.
--
Vincent Lefèvre <vincent@vinc17.net> - Web: <https://www.vinc17.net/>
100% accessible validated (X)HTML - Blog: <https://www.vinc17.net/blog/>
Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)
^ permalink raw reply [flat|nested] 4+ messages in thread
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2014-08-01 10:47 ` Vincent Lefevre
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