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This caused PR34596. > [MachineCombiner] Update instruction depths incrementally for large BBs. > > 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 git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@313213 91177308-0d34-0410-b5e6-96231b3b80d8
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