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Phlli in a VM node

2022-04-23 05:26:00 【What’smean】

         PHI node , It will choose from different branches i, Because of our IR yes SSA（single static assignment, static single assignment ） Formal . In the control flow diagram , A given variable can come from two different basic blocks （ Two different paths ）, In order to be in SSA Formal LLVM IR This branching situation is expressed in , Need to use phi Instructions .（PHI Instructions are used to implement PHI node . At run time ,PHI The order is based on “ At present block Which one was executed before predecessor( The former ) block” Get the corresponding value .）

         for instance ：
                %i = phi i32 [ 1, %entry ], [ %nextvar, %loop ]

        phi The order must be in basic block Foremost , It's in one basic block in , stay phi No non... Is allowed before the instruction phi Instructions .
         there IR Indicates the variable i The value of may come from two basic blocks ：%entry perhaps %loop. come from %entry
The variable value of the block is 1, and %nextvar Variables will come from %loop.

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