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About pristine regsiters: Pristine registers "hold a value that is useless to the current function, but that must be preserved - they are callee saved registers that have not been saved." This concept saves compile time as it frees the prologue/epilogue inserter from adding every such register to every basic blocks live-in list. However the current code in getPristineRegs is formulated in a complicated way: Inside the function prologue and epilogue all callee saves are considered pristine, while in the rest of the code only the non-saved ones are considered pristine. This requires logic to differentiate between prologue/epilogue and the rest and in the presence of shrink-wrapping this even becomes complicated/expensive. It's also unnecessary because the prologue epilogue inserters already mark callee-save registers that are saved/restores properly in the respective blocks in the prologue/epilogue (see updateLiveness() in PrologueEpilogueInserter.cpp). So only declaring non-saved/restored callee saved registers as pristine just works. Differential Revision: http://reviews.llvm.org/D10101 llvm-svn: 238524