The Performance Between GCC Optimization Levels
Phoronix: The Performance Between GCC Optimization Levels
For those that have never benchmarked the performance differences between GCC's different optimization levels, here are some recent test results comparing the performance differences when using an AMD FX-8150 processor with GCC 4.7.2.
When it's a bit more stable, I would like to see some benchmarks with the new -Og option in the 4.8 dev series.
These are a big improvement on previous benchmarks done by Phoronix. They would be better if tests that set -march=native tests in addition to the optimization level settings were also done.
It would be useful if we could see the effect that cache has on optimization levels. Small caches are generally thought to favor lower optimization levels. In particular, -Os and -O2.
I agree, especially w/ gentoo. GCC & binutils need to be optimized and errata pulled out to see the true benefits.
Originally Posted by ryao
Indeed, some optimizations will work better in combination with -march settings. The cache issue will show in benchmarks with highly parallelized workloads (such as web serving or databases with many clients).
Regarding the article, it is interesting how the selection of benchmarks emphasizes floating-point heavy code, since this is what benefits a lot from -O3 (and -Ofast, but this may cause calculations result to be different from what you expect).
For some historic reference, Linux Magazine already ran a comparison of -Os, -O2 and -O3 on Gentoo vs. Ubuntu a while back: http://www.linux-mag.com/id/7574/
-O2 and -O3 can actually produce massive binary output size increases for not much gain over -Os.
When you compile Mozilla software with -O3, you will get much larger binary size, which actually can make it take longer to load, and make the resulting program take up more space in RAM. I think Mozilla recommends -O2, but I've seen where some distributions use -Os, which doesn't make the binaries much smaller, but can hurt Firefox's score on things like Sunspider or Google's V8 benchmark. (-O3 doesn't help it enough to be worth the cost in load times and additional RAM usage)
Obviously some things benefit so much from -O3 that it becomes worth the tradeoff in longer load times and higher RAM consumption. You can't take that for granted, though.
Yes, there is such a thing as being too aggressive with optimization level. Unfortunately, it's hard to always know when you've gone too far because it varies from program to program. Just use Fedora and be happy. They usually do OK with things like this.
-O2 -march=native is generally considered to be optimal outside of special cases.
Originally Posted by DaemonFC
Firefox used to use -Os, but when they updated their builds to use modern GCC they also switched to using -O3. They do limit the total amount of code inlining which can be done, though, which keeps the binary size from getting too large. And they turned on pgo as well.
Originally Posted by DaemonFC
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