shared libraries + lto ?

[Alain Meunier]

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)