3 ms·

I remember that -flto sometimes adds few %'s to overall speed. If you are doing a lot of floating point math you can check various modes. First try is always -O

I remember that -flto sometimes adds few %'s to overall speed. If you are doing a lot of floating point math you can check various modes. First try is always -Ofast which turns on -ffast-math flag. From the gcc page: -ffast-math Sets -fno-math-errno, -funsafe-math-optimizations, -ffinite-math-only, -fno-rounding-math, -fno-signaling-nans and -fcx-limited-range. This option causes the preprocessor macro __FAST_MATH__ to be defined. This option is not turned on by any -O option besides -Ofast since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that do not require the guarantees of these specifications. I also remember that gcc started producing significantly faster code for my projects at around 4.7 or 4.8 version (can't remember which one).

yoklov 12y ago
`-ffast-math` is a double-edged sword. It can frequently optimize away stuff like NaN checks and the like, breaking code that uses NaN as a missing value. This, specifically, has bitten me before, but there are certainly other areas that it can bite you. OTOH it gives the compiler to do a great deal of algebraic simplifications, including expression reordering. This probably will bring what the compiler can actually do more in line with what you think it should be able to do. Basically, you need to test with it on if you're going to use `-ffast-math`. You might also have good luck with turning on a subset of the flags. For example, IIRC in the project with the NaN checks, using all of them except `-ffinite-math-only` fixed the problem in this case. Some of them are obvious to turn on though. `-fno-math-errno` should be the default for most programs, if you ask me. I've never seen anybody check `errno` to see if their call to, e.g. `sqrt`, was invalid, and I hope I never do.
- emn13 12y ago
  If -ffast-math won't work, a few more to consider: -fassociative-math (generally safe unless you're dealing with math written specifically to take advantage of the details of floating point arithmetic) -fno-signed-zeros - again, though it's possible some code depends on this, it's rather unlikely. -fno-trapping-math - do you actually use traps?

yoklov 12y ago

`-ffast-math` is a double-edged sword. It can frequently optimize away stuff like NaN checks and the like, breaking code that uses NaN as a missing value. This, specifically, has bitten me before, but there are certainly other areas that it can bite you. OTOH it gives the compiler to do a great deal of algebraic simplifications, including expression reordering. This probably will bring what the compiler can actually do more in line with what you think it should be able to do. Basically, you need to test with it on if you're going to use `-ffast-math`. You might also have good luck with turning on a subset of the flags. For example, IIRC in the project with the NaN checks, using all of them except `-ffinite-math-only` fixed the problem in this case. Some of them are obvious to turn on though. `-fno-math-errno` should be the default for most programs, if you ask me. I've never seen anybody check `errno` to see if their call to, e.g. `sqrt`, was invalid, and I hope I never do.

emn13 12y ago
If -ffast-math won't work, a few more to consider: -fassociative-math (generally safe unless you're dealing with math written specifically to take advantage of the details of floating point arithmetic) -fno-signed-zeros - again, though it's possible some code depends on this, it's rather unlikely. -fno-trapping-math - do you actually use traps?