CS 112 Lab Exercise: Faster Genome Scanning: Optimization

Compiler Optimization

In the provided Makefile, the C++ compiler uses this command to build genScan:

   g++ genScan.cpp -Wall -ansi -pedantic -std=c++11 -o genScan -fopenmp

• g++ is the name of the GNU C++ compiler--the program we are using to build genScan.
• genScan.cpp is the name of our source program file--a file containing the C++ program we want g++ to translate into the native (binary) language your computer understands.
• -Wall tells g++ to warn us about all potential mistakes it discovers while trying to translate our program.
• -ansi -pedantic -std=c++11 tells g++ to be super-picky about enforcing the language rules of the 2011 C++ language standard.
• -o genScan tells g++ to output the native binary program it produces to a file named genScan.
• -fopenmp tells g++ that our program is using functions from the OpenMP library, so it will look in that library to find those function definitions.

We can change that by tweaking the Makefile. Using your text editor, open Makefile, find the line that looks like this:

   CFLAGS = -Wall -ansi -std=c++11

   CFLAGS = -O2 -Wall -ansi -std=c++11

Compiler optimization is having the compiler spend extra time translating our program into the native binary code, looking for ways to make the binary program faster, smaller, or both.

The GNU compilers support four optimization levels:
-O0 (level zero) performs no optimizations; the compiler just builds the program as fast as possible. This is g++'s default setting if the user doesn't specify a level.
-O1 (level one) performs simple optimizations--improvements that won't slow down the build very much.
-O2 (level two) performs all optimizations that will not increase the size of the binary program.
-O3 (level three) performs all available optimizations.

The change we just made to the Makefile will cause g++ to perform level two optimization instead of level zero, which nearly always speeds up the resulting program.