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When register allocator's stage is RS_Spill, we choose spill over using the CSR for the first time, if the spill cost is lower than CSRCost. When register allocator's stage is < RS_Split, we choose pre-splitting over using the CSR for the first time, if the cost of splitting is lower than CSRCost. CSRCost is set with command-line option "regalloc-csr-first-time-cost". The default value is 0 to generate the same codes as before this commit. With a value of 15 (1 << 14 is the entry frequency), I measured performance gain of 3% on 253.perlbmk and 1.7% on 197.parser, with instrumented PGO, on an arm device. rdar://16162005 git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204690 91177308-0d34-0410-b5e6-96231b3b80d8
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