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Summary: For large basic blocks with lots of combinable instructions, the MachineTraceMetrics computations in MachineCombiner can dominate the compile time, as computing the trace information is quadratic in the number of instructions in a BB and it's relevant successors/predecessors. In most cases, knowing the instruction depth should be enough to make combination decisions. As we already iterate over all instructions in a basic block, the instruction depth can be computed incrementally. This reduces the cost of machine-combine drastically in cases where lots of instructions are combined. The major drawback is that AFAIK, computing the critical path length cannot be done incrementally. Therefore we only compute instruction depths incrementally, for basic blocks with more instructions than inc_threshold. The -machine-combiner-inc-threshold option can be used to set the threshold and allows for easier experimenting and checking if using incremental updates for all basic blocks has any impact on the performance. Reviewers: sanjoy, Gerolf, MatzeB, efriedma, fhahn Reviewed By: fhahn Subscribers: kiranchandramohan, javed.absar, efriedma, llvm-commits Differential Revision: https://reviews.llvm.org/D36619 llvm-svn: 312719 |
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