Floating point performance issue

Marc Glisse marc.glisse@inria.fr
Tue Dec 20 10:46:00 GMT 2011

On Tue, 20 Dec 2011, Ico wrote:

> Hello,
> I'm running the program below twice with different command line arguments. The
> argument is used a a floating point scaling factor in the code, but does not
> change the algorithm in any way.  I am baffled by the difference in run time of
> the two runs, since the program flow is not altered by the argument.


you are thinking about the program flow in terms high level code. Most 
float operations simply go through the hardware and complete in equal 
time, but that doesn't include operations on denormals (numbers very close 
to 0) which are emulated and take forever to complete. Notice that 
-ffast-math implies "I don't care about that" and makes it fast.

Marc Glisse

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