And to illustrate the point more, I have found that -Os often produces faster code than -O3. Not by factor 10, but still clearly measurable.

Getting better performance with -Os over -O is some kind of edge case and people shouldn't generally expect that. -Os has disastrous consequences for C++ algorithms because it refuses to inline functors, so while you may rightly believe that a C++ std::sort is slightly faster than a C qsort, due to superior opportunities to optimize the C++ code, with -Os you'll find that std::sort is an order of magnitude slower. Definitely pays to check the result with a full-scale benchmark.

Interesting claim. I had to try this, and with the Xcode version that I have installed, std::sort with a simple lambda as comparison function gives about the same result with -O3 and -Os. I would not call this a disastrous, but of course opinions differ. Interestingly, qsort is significantly faster with -O3 than with -Os but of course nowhere near std::sort.

Results will vary for small vs. large programs. I've seen catastrophic space optimizations that out-of-lined very small methods like std::vector::at, because the call was one or two bytes smaller than the inline. Lambdas are inlined even with Os because they don't have names or multiple callers and can't be made smaller by out-of-lining. A functor class, or any function with multiple call sites could trigger the problems with Os.

Ok, I'm not continuing with this. Just note that I didn't write that -Os would be always or even usually faster. I could try to come up with an example where -O3 produces a huge loop preamble for loop that's iterated once or just generates enough cache misses to be overall slower, but I don't care enough.

I also have a similar scientific code base which gets 10-15x wall-time improvement just by enabling compiler optimizations.

Not a scientific codebase, but ffmpeg seems to benefit massively from optimization

I use it to encode my DVD and Blu Ray TV/movies to HEVC. Doing so reduces the filesize on DVD's by roughly 80% and Blu Rays by 60%

The downside is the encoding is an incredibly CPU intensive process. Hardware encoders like Nvidia's Nvenc or Intel's QuicSync look absolutely terrible and are a non-starter for archival storage

On a stock Fedora XFCE install, I would get roughly 0.5 FPS for a 1080p Blu Ray file (29.97 FPS at 1920x1080)

A Gentoo Linux installation with a global -O3 -march=native as well as LTO, PGO and Graphite enabled globally boosts it from 0.5 to roughly 1.3 FPS. Still slower than realtime, but an absolutely massive improvement

I suspect -march=native is the only thing doing any work here. The other optimizations are as likely to find compiler bugs as they are to improve things, once you get off well-tested paths.

I think it's the opposite. A good annotated code which can use SIMD instructions accelerates tremendously with -O3. -march=native -mtune=native generally improves things if you can saturate the cores with instructions, which can be measured by running perf record and looking to IPC & instruction retirement numbers.

When I was using Eigen on my code, the biggest performance boost came from -O3. -march and -mtune did minimal improvements on the systems which I've ran benchmarks on.

ffmpeg and associated projects do not rely on autovectorization, which pretty much only works on naive scientific code. They write their SIMD in assembly and don't need to care about compiler settings.

If you're compiling hand tuned assembly, -march and -mtune probably will have more effect when compared to compiling and optimizing C/C++ code.

OTOH, I'd like to underline that heavily optimized scientific code and libraries are neither naive (in terms of algorithmic complexity/implementation) nor straightforward :D

E.g.: This is how Eigen configures its internal vectorization parameters: https://gitlab.com/libeigen/eigen/-/blob/master/Eigen/src/Co...

That's an amazing compression ratio, how is the quality?

ffmpeg recommends a crf of 23 to preserve 1:1 for H.264 -> h.265

I do a probably overkill CRF of 20 just to be safe. But everything looks absolutely perfect, even blown up on my 65 inch (1080p) TV

Can you share your complete parameters, so I can do the same thing for my DVD archives & other stuff?

No hard feelings if you don't want to though :)

Sure! This is the script I wrote for it. I use MP4 containers so I can import them into MAGIX (formally Sony) Vegas but it will iterate over an entire folder of MakeMKV files and convert them https://dpaste.com/DBPA9C59M

Thank you. It's greatly appreciated. :)