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1. If we are two-addressing a commutable instruction and the LHS is not the last use of the variable, see if the instruction is the last use of the RHS. If so, commute the instruction, allowing us to avoid a register-register copy in many cases for common instructions like ADD, OR, AND, etc on X86. 2. If #1 doesn't hold, and if this is an instruction that also existing in 3-address form, promote the instruction to a 3-address instruction to avoid the register-register copy. We can do this for several common instructions in X86, including ADDrr, INC, DEC, etc. This patch implements test/Regression/CodeGen/X86/commute-two-addr.ll, overlap-add.ll, and overlap-shift.ll when I check in the X86 support for it. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@19245 91177308-0d34-0410-b5e6-96231b3b80d8
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