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The <undef> flag says that a MachineOperand doesn't read its register, or doesn't depend on the previous value of its register. A full register def never depends on the previous register value. A partial register def may depend on the previous value if it is intended to update part of a register. For example: %vreg10:dsub_0<def,undef> = COPY %vreg1 %vreg10:dsub_1<def> = COPY %vreg2 The first copy instruction defines the full %vreg10 register with the bits not covered by dsub_0 defined as <undef>. It is not considered a read of %vreg10. The second copy modifies part of %vreg10 while preserving the rest. It has an implicit read of %vreg10. This patch adds a MachineOperand::readsReg() method to determine if an operand reads its register. Previously, this was modelled by adding a full-register <imp-def> operand to the instruction. This approach makes it possible to determine directly from a MachineOperand if it reads its register. No scanning of MI operands is required. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@141124 91177308-0d34-0410-b5e6-96231b3b80d8
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