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I have been doing some benchmarking on x86 using llvm 2.9 with the llvm-gcc 4.2 front end. I noticed that the -march and -mtune options make a significant positive difference in x86-32 mode but hardly make any difference in x86-64 mode. The small difference that I am measuring when the target is x86-64 could easily be random variation, while for the x86-32 target I am measuring a huge difference