-flto making program slower?

[Hei Chan]

I ran into similar issue but I got no response here.

My guess is 
that flto makes your average run time smaller.  But I guess most people 
use it in a hope of making their critical pathes running faster, which 
are benchmarked.





________________________________
From: Uri Moszkowicz <uri@4refs.com>
To: gcc-help <gcc-help@gcc.gnu.org> 
Sent: Wednesday, June 12, 2013 5:48 AM
Subject: -flto making program slower?


Hi,
I'm having trouble with link time optimization in my application. It
is a large application that uses only basic C++ (no exceptions, no
templates, no STL, no floating point, etc). Like many applications,
the source files are compiled separately into object files. Some of
those are combined into shared libraries. The shared libraries are
then statically linked with the remaining object files to produce the
final application, which is about 100MB big. We are using GCC 4.7.2
with a non-GOLD 2.23.1 binutils.

I simply added "-flto" to the GCC command to create object files:
    g++ -Wall -pipe -O3 -flto -fno-strict-aliasing -mtune=generic
--no-exceptions -fPIC -c some.cc

I then added "-flto" to the final link command but not the shared libraries:
  g++ -o exec -Xlinker some1.o some2.o -static some1.a some2.a
-Wl,--wrap,open -flto

I ran a benchmark of tests and the resulting execution time is now
about 7% higher than it was without "-flto" added. Any suggestions for
how to improve this result or why it may have gotten slower?

If it helps, when I add "-fuse-linker-plugin" I get this error:
  g++: error: -fuse-linker-plugin is not supported in this configuration

Thanks!