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This has been disabled for a while, and it is not a feature we want to support. Copies between physical and virtual registers are eliminated by good hinting support in the register allocator. Joining virtual and physical registers is really a form of register allocation, and the coalescer is not properly equipped to do that. In particular, it cannot backtrack coalescing decisions, and sometimes that would cause it to create programs that were impossible to register allocate, by exhausting a small register class. It was also very difficult to keep track of the live ranges of aliasing registers when extending the live range of a physreg. By disabling physreg joining, we can let fixed physreg live ranges remain constant throughout the register allocator super-pass. One type of physreg joining remains: A virtual register that has a single value which is a copy of a reserved register can be merged into the reserved physreg. This always lowers register pressure, and since we don't compute live ranges for reserved registers, there are no problems with aliases. llvm-svn: 157055 |
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