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Splitting critical edges when one of the source edges is an indirectbr is hard in general (because it requires changing the memory the indirectbr reads). But if a block only has a single indirectbr predecessor (which is the common case), we can simulate splitting that edge by splitting the destination block, and retargeting the *direct* branches. This is motivated by the use of computed gotos in python 2.7: PyEval_EvalFrame() ends up using an indirect branch with ~100 successors, and passing a constant to each of those. Since MachineSink can't break indirect critical edges on demand (and doing this in MIR doesn't look feasible), this causes us to emit about ~100 defs of registers containing constants, which we in the predecessor block, where only one of those constants is used in each successor. So, at each computed goto, we needlessly spill about a 100 constants to stack. The end result is that a clang-compiled python interpreter can be about ~2.5x slower on a simple python reduction loop than a gcc-compiled interpreter. Differential Revision: https://reviews.llvm.org/D29916 git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@296149 91177308-0d34-0410-b5e6-96231b3b80d8 |
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