[LLVMdev] Building for a specific target, corei7

[Varun Agrawal]

Hi,

I am using the LLVM JIT infrastructure (MCJIT). I wanted to see if there are any performance gains as the compiler can detect the target CPU at runtime. But, I didn't see any improvement (I compile with -no-mmx and -no-sse).

I then tried an experiment, where I compiled the program with clang-3.3, with and without specifying the target cpu as "corei7". I was shocked to see that the only difference in the two binaries were related to "Instruction Set Extensions".

Further I tried the same experiment with gcc, and saw that the instructions were shuffled around in the binary. I expected this, because every CPU differs in some way or the other (has different buffer size for out-of-order execution, different cache sizes, etc.).

For clang, I was passing the "-march=corei7" flag.
For gcc, I was passing the "-mcpu=corei7" flag.

Am I passing the correct flags?
Any help, comments or suggestions, would be helpful.

Thanks,
--
Varun Agrawal
PhD Student
Computer Science, Stony Brook University
http://compas.cs.stonybrook.edu/~vagrawal/

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