Hello Eljay, On 10-3-10 下午8:04, John (Eljay) Love-Jensen wrote: > Hi Zheng Da, > >> Can anyone tell me what is the possible reason that can cause time difference? > > To understand the differences, look at the assembly dumps of the routine from the GCC compiler and the Intel compiler. It's not a big program, but not a small one either. I used objdump -S to see the assembly code and two files are almost completely different. But I noticed one thing: the binary file generated by the Intel compiler is much larger than the one from GCC. Another thing I have noticed is that with -O3 enabled, the Intel compiler generate vector instructions but GCC doesn't. But I don't see how this can explain my problem. > > Make sure you are using the right GCC machine flags -mcpu= -mtune= -march= for your platform. (Since you did not provide your command lines for GCC and Intel compilers, I can only offer that as a general suggestion.) I didn't use any of these machine flags. I only used -O3 for GCC and Intel compiler. So the command line I used to compile my program is gcc -fopenmp -DREALTIME -D__LINUX -lm -O3 MORGAN2.c -o MORGAN2-omp icc -openmp -DREALTIME -D__LINUX -O3 MORGAN2.c -o MORGAN2-omp I use Athlon ii x4 620 processor, so I tried -mtune=athlon or -mtune=athlon-4 or -mtune=athlon64 or -mtune=native -march=native when I compiled the code with GCC, but none of them seems to work:-( Best regards, Zheng Da
