Optimizing general compiler optimization
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Optimizing General Compiler Optimization. M. Haneda, P.M.W. Knijnenburg, and H.A.G. Wijshoff. Problem: Optimizing optimizations. A compiler usually has many optimization settings (e.g. peephole, delayed-branch, etc) gcc 3.3 has 54 optimization options gcc 4 has over 100 possible settings

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Optimizing general compiler optimization

Optimizing General Compiler Optimization

M. Haneda,

P.M.W. Knijnenburg,

and H.A.G. Wijshoff


Problem optimizing optimizations
Problem: Optimizing optimizations

  • A compiler usually has many optimization settings (e.g. peephole, delayed-branch, etc)

    • gcc 3.3 has 54 optimization options

    • gcc 4 has over 100 possible settings

  • Very little is known about how these options affect each other

  • Compiler writers typically include switches that bundle together many optimization options

    • gcc –O1, -O2, -O3


But can we do better
…but can we do better?

  • It is possible to perform better than these predefined optimization settings, but doing so requires extensive knowledge of the code as well as the available optimization options

  • How do we define one set of options that would work well with a large variety of programs?


Motivation for this paper
Motivation for this paper

  • Since there are too many optimization settings, an exhaustive search would cost too much

    • gcc 3: 2^50 different combinations!

  • We want to define a systematic method to find the optimal settings, with a reduced search space

  • Ideally, we would like to do this with minimal knowledge of what the options actually will do


Big idea
Big Idea

  • We want to find the biggest subsets of compiler options that positively interact with each other

  • Once we obtain these subsets, we will try to combine them together, under the condition that they do not negatively affect each other

  • We will select our ultimate optimal compiler setting from the result of these set combinations


Full vs fractional factorial design
Full vs. Fractional Factorial Design

  • Full Factorial Design: explores the entire search space, with every possible combination

    • Given k options, this will take O(2^k) time

  • Fractional Factorial Design: explores a reduced search space, that is representative of the full search space

    • This can be done using orthogonal arrays


Orthogonal arrays
Orthogonal Arrays

  • An Orthogonal Array is a matrix of 0’s and 1’s.

    • The rows represent the experiments to be performed.

    • The columns represent the factors that the experiment tries to analyze

    • Any option is equally likely to be turned on/off.

    • Given a particular experiment with a particular option turned on, all the other options are still equally likely to be turned on/off



Algorithm step 1
Algorithm – Step 1

  • Finding maximum subsets of positively interacting options

    • Step 1.1: Find a set of options that give the best overall improvement

    • For any single optimization setting i, compute the average speedup for all the settings in the search space in which i is turned on

    • Select M of the highest average improvement settings



Algorithm step 1 cont
Algorithm – Step 1(cont.)

  • Step 1.2: Iteratively add new options to the already obtained sets, to get a maximum set of positively reinforcing optimizations

    • Ex: If using options A and B together produces a more optimal setting than just using A, then add B

    • If using {A, B} and C together produces a more optimal setting than {A, B}, then add C to {A, B}


Algorithm step 2
Algorithm – Step 2

  • Take the sets that we already have and try to combine them together, assuming that they do not negatively influence each other.

  • This is done to maximize the number of settings turned on for each set

  • Example:

    • If {A, B, C} and {D, E} do not counteract each other, then we can combine them into {A, B, C, D, E}

    • Otherwise, leave them separate


Algorithm step 3
Algorithm – Step 3

  • Take the resulting sets from step 2, and select the one with the best overall improvement.

  • The result would be the ideal combination of optimization settings, according to this methodology.




Comparing results
Comparing results

  • The compiler setting obtained by this methodology outpeforms –O1, -O2, and –O3 on almost all the SPECint95 benchmarks

    • -O3 performs better on li (39.2% vs. 38.4%)

    • The new setting delivers the best performance for perl (18.4% vs. 10.5%)


Conclusion
Conclusion

  • The paper introduced a systematic way of combining compiler optimization settings

    • Used a reduced search space, constructed as an orthogonal array

    • Can be done with no knowledge of actual options

    • Can be done independently of architecture

    • Can be applied to a wide variety of applications


Future work
Future work

  • Using the same methodology to find a good optimization setting for a particular domain of applications

  • Applying the methodology to newer versions of the gcc compiler, such as gcc 4.0.1


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