Compiling Optimization x86_64

As mentioned in the previous post I am going to take a look at the compiler flags and how I might be able to optimize them.

The currents flags for the C compiler that are used:

-pthread -g -pg -O2 -MD -D_FORTIFY_SOURCE=2 -Wpointer-arith -Wmissing-declarations -Wmissing-prototypes -Wshadow -Wwrite-strings -Wcast-align -Waggregate-return -Wbad-function-cast -Wcast-qual -Wundef -Wredundant-decls -Wdisabled-optimization -Wfloat-equal -Wmissing-format-attribute -Wmultichar -Wc++-compat -Wmissing-noreturn -funit-at-a-time -Wall -Wstrict-prototypes -Weffc++

As you can see there are a lot of flags to compile this program with the C compiler. What caught my eye was that they're using the O2 flag and not O3, so I saw this as a optimization possibility and went straight in to change it. Once I changed all the flags I did another benchmark of the program to see if it had helped at all. ~

To check if it did anything, I looked at the executable file. It made the file larger with the -O3 optimization. It went from 2.7 mb to 3.5 mb.

Note: Don't forget to take out the -pg flag as well to have a correct benchmark. -pg makes your program slower so then you won't have the correct comparison.

The following benchmarks are for x86_64

With -O2 flag

10.5 mb file	105 mb file	1.5 gb file
real: 0m0.053s	real: 0m0.287s	real: 0m3.458s
user: 0m0.021s	user: 0m0.200s	user: 0m2.793s
sys: 0m0.003s	sys: 0m0.012s	sys: 0m0.184s

With -O3 flag

10.5 mb file	105 mb file	1.5 gb file
real: 0m0.045s	real: 0m0.258s	real: 0m3.071s
user: 0m0.020s	user: 0m0.197s	user: 0m2.756s
sys: 0m0.002s	sys: 0m0.014s	sys: 0m0.177s

By comparing the real time you can see the -O3 flag makes the program 11.2% faster. Which is a pretty nice optimization. So I recommend they replace the -O2 flag with -O3.

Now it's time to benchmark the software on Betty and hope so see an optimization as well.