[PATCH] [LoopUnrollRuntime] Avoid high-cost trip count computation.

[hfinkel at anl.gov]

You'll need to add a field in the UnrollingPreferences structure to control this. The makes a lot of sense on X86, where we only unroll small loops to fill the LSD's dispatch buffer (and they're still pretty small even after unrolling). On other targets we unroll to much larger sizes and the cost of a division is amortized over many more instructions and iterations.


http://reviews.llvm.org/D8994

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