Re: [LAU] Performance tuning audio apps

On Mon, Aug 17, 2009 at 5:21 AM, Ken Restivo<ken(a)restivo.org> wrote: If you want the fastest possible floating point code, then you probably want something like: -march=core2 -msse -msse2 -mfpmath=sse -ffast-math -fomit-frame-pointer -O3 ... but with caveats. Discussion: Supplying -ffast-math causes the use of non-IEEE-compliant math functions. Among other things, this screws up any code that explicitly deals with infinity or NaN values or signed zeroes, and makes assumptions about properties like associativity for the purposes of optimisation which may not be true in the floating-point world. In other words, it can give you the wrong results. In _most_ cases, audio applications are fine with it, but you need to be aware that it can be problematic. However, -ffast-math in combination with -mfpmath-sse has the very nice quality that it enables denormal flush to zero throughout, thus avoiding denormal slowdowns in filters and the like. It's also much faster for some of the apparently simple operations like floor() that are surprisingly slow in IEEE compliant mode. It might be interesting to know what the authors of the programs you're trying to optimise thought about the use of -ffast-math... Perhaps you could compile them both ways and compare the output. -fomit-frame-pointer is pretty much guaranteed to make things marginally faster but harder to debug. It won't break anything and it won't make any huge improvements. -O3 rather than -O2 because it enables -ftree-vectorize, which does some limited auto-vectorization of loops for things like floating-point copy into SSE operations. This doesn't always do anything (depends on the code, obviously) but sometimes it makes a significant difference, for example it helps when compiling my Rubber Band library. I've never yet seen any problems with the results, but of course there's always an increased risk of running into optimisation bugs the more optimisation you do. You can get interesting (?) debug output about vectorization successes and failures (mostly failures) with e.g. -ftree-vectorizer-verbose=2. I would be slightly suspicious of anyone who recommends -pipe as an optimisation -- it makes no difference to the resulting code, it just makes compiling faster. If you're using a 64-bit distro, then you can omit the options with SSE in them (they're all enabled by default in 64-bit gcc). Chris