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GCC 4.7 Link-Time Optimization Performance

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  • #16
    Hmm, does Phoronix just ignore posts with links?

    The effect on Firefox are summarized in "Optimizing real world applications with GCC Link Time Optimization" see arXiv:1010.2196v2 . In short you can easily get -O3 performing binary with LTO that has size of -Os binary w/o LTO, so there are huge code size benefits. Performance benefits are smaller on common benchmarks since they have been hand optimized.

    You can also look at gcc.opensuse.org. It has SPEC scores w/o LTO, with LTO and with LTO+FDO.

    In general LTO can have huge performance impact on codebases that was not hand tuned, or are too large to be hand tuned for all workloads, or was developed with LTO enabled compiler (many HPC apps). Optimizations enabled by LTO can also be done by the developers, so LTO is kind of tool making development easier. LTO also enable benchmark compilers to enable various tricks (change datastructures, etc.), but that is not too important in real world.

    For code size the applications are harder to tune: instead of tuning small hot spot you need to optimize whole application. This is why LTO is almost always win code size wide unless you enable a lot of inlining or other code expanding optimizations.

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    • #17
      Thanks Hubica :-)
      That was a very helpful read :-)

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      • #18
        Originally posted by hubicka View Post
        Hmm, does Phoronix just ignore posts with links?
        I think it does that for your first several posts, anti-spam feature.

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