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@xxxxxxxxxx> - Web: <https://www.vinc17.net/>
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Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)
