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For backends maintaining the CFG during codegen we can now find loops and their nesting level. This is based on the Cmm CFG and dominator analysis. As a result we can estimate edge frequencies a lot better for methods, resulting in far better code layout. Speedup on nofib: ~1.5% Increase in compile times: ~1.9% To make this feasible this commit adds: * Dominator analysis based on the Lengauer-Tarjan Algorithm. * An algorithm estimating global edge frequences from branch probabilities - In CFG.hs A few static branch prediction heuristics: * Expect to take the backedge in loops. * Expect to take the branch NOT exiting a loop. * Expect integer vs constant comparisons to be false. We also treat heap/stack checks special for branch prediction to avoid them being treated as loops.
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