Compiling Optimization Betty

After benchmarking x86_64 with the -O3 optimization flag, it's time to test this on ARCH64.
Since I can only work from the command line on the server I needed to come up with a command to replace all Makefiles' -O2 with -O3. The command I found the easiest was the following:

find -name Makefile | xargs sed -i 's/-O2/-O3/' 

The following benchmarks are done on ARCH64, server Betty.

With -O2 flag

10.5 mb file	105 mb file	1.5 gb file
real: 0m0.037s	real: 0m0.345s	real: 0m4.792s
user: 0m0.028s	user: 0m0.323s	user: 0m4.551s
sys: 0m0.001s	sys: 0m0.027s	sys: 0m0.400s

With -O3 flag

10.5 mb file	105 mb file	1.5 gb file
real: 0m0.036s	real: 0m0.343s	real: 0m4.768s
user: 0m0.028s	user: 0m0.323s	user: 0m4.499s
sys: 0m0.001s	sys: 0m0.027s	sys: 0m0.426s

As you can see the -O3 did not do anything on ARCH64. I thought this was very strange and checked the executable file to see if it changed at all. The file did change, it got larger as expected. Yet there is no gain in speed. Comparing the real time of the 1.5gb file again, it wasn't even 1% faster. So for ARCH64 I recommend using -O2 because it doesn't change much in run time and your file is smaller.

For Betty I'll have to find another optimization possibility, although I wouldn't know what the next possibility would be. Probably make something specifically for ARCH64, but this would cost way more time.

Another way to go is add the compiler flag -fopt-info-missed to find missed optimizations and see if I can do something about that. Source: https://gcc.gnu.org/onlinedocs/gccint/Dump-examples.html