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-flto making program slower in gcc/g++

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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

posted Jun 11, 2013 by anonymous

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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.
Let me clarify then. The benchmark consisted of a few dozen runs of the program each with different inputs, with the average time given. For this program, each run typically takes a few hours so there's little risk of sample error.

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