* On Tue Dec 20 11:46:24 +0100 2011, Marc Glisse wrote: > 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. > > Hello, > > 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. So I could expect gcc -g -ffast-math -O3 test.c or gcc -g -march=pentium3 -mfpmath=sse -ffast-math -O3 test.c to solve the issue ? Unfortunately, from what I just tested, it does not. However, it does when using gcc -g -msse -mfpmath=sse -O3 -march=native -ffast-math test.c I will no go read the manual and learn about the *exact* meanings of all these options to see if I can understand what exactly is going on under the hood. Thank you, Ico -- :wq ^X^Cy^K^X^C^C^C^C
