[LLVMdev] -march and -mtune options on x86

[Ghassan Shobaki]

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 on some benchmarks. Does anyone have an explanation for this? Does the llvm back end somehow ignore these options when the target is x86-64?

Thank you in advance!
-Ghassan
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