Optimizations

It was time to optimize my raytracer and gprof turned up something unexpected: most of the time was being spent in vec3f constructors. I expected most of the time to be spent checking for intersections. It was time to turn on gcc optimizations.

The result: the constructor calls disappear after passing either a -O or -O1. Best performance was to be had at -O3.

This raytracer is being compiled on a MacBook Pro 17", 2.4GHz Intel Core 2 Duo with 4GB of RAM. gcc:

gcc -v
Using built-in specs.
Target: i686-apple-darwin9
Configured with: /var/tmp/gcc/gcc-5465~16/src/configure --disable-checking -enable-werror --prefix=/usr --mandir=/share/man --enable-languages=c,objc,c++,obj-c++ --program-transform-name=/^[cg][^.-]*$/s/$/-4.0/ --with-gxx-include-dir=/include/c++/4.0.0 --with-slibdir=/usr/lib --build=i686-apple-darwin9 --with-arch=apple --with-tune=generic --host=i686-apple-darwin9 --target=i686-apple-darwin9
Thread model: posix
gcc version 4.0.1 (Apple Inc. build 5465)

Image size: 512x512, 1 plane, two spheres, 1 triangle. Average run times for 5 runs each:

	Average Run Time	Optimization	Debugging and profiling
1.	8.1604598	None	yes
2.	2.0264776	-O	yes
3.	2.036941	-O1	yes
4.	2.2078138	-O2	yes
5.	1.2405112	-O3	yes
6.	1.2331454	-O4	yes
7.	1.1143894	-O3	no
8.	1.1208828	-O6	no
9.	1.1019208	-O4	no

Notes:

1. 1. gprof shows that most of the time is spent in vec3f constructors


2. 2. gprof shows that most of the time is spent in sphere and plane intersections,
   but I still see more calls than expected to vec3f constructors


3. 3. This generates a different binary than the previous test and runs at approximately
   the same speed. Still more vec3f constructor calls than expected.


4. 4. -O2 yields no real change from -O1


5. 5. gprof yields expected results, big speedup from -O1 and -O2


6. 6. Seems to be the same as -O4


7. 7. Gained 1/10th of a second from dropping debugging and profiling


8. 8. -O6 shows no gain over -O3


9. 9. -O4 without debugging and profiling is 1/100th of a second faster than -O3, but that isn't big enough of a change to say that something changed. There could just be less load on this system.