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why so slow? gcc vs vc6


I've written a small numeric program(600 lines) with c under linux. I
compiled it under 64bit linux using 64bit gcc-3.4(without linking
other external library), the program runs very slow.
Then I managed to compile it under windows using VC on a machine which
is exactly the same with mine(amd64 2800, 1g ram). The program runs
very fast, the speed is nearly 1000 times faster. I don't know how to
solve the problem, as my main develop environment is linux.
I used gcc as follows:
gcc-3.4 -g0 -march=k8 -O3 -fomit-frame-pointer -funit-at-a-time
-fpeel-loops -ftracer -funswitch-loops -mfpmath=sse
(do not use -ffast-math, as it will cause output error)
with vc6.0 I just use the default options.

I use clock()/1.e6 to determine the time elapsed in second
under linux, output : 1.39 s
under windows, output: 2.5e-3 s


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