For statement S, USE(S) contains variables used in S; IN(S) those live at entry; OUT(S) those live at exit. A basic block consists of S1 immediately followed by S2. Which equation is correct?
There is no intervening instruction or alternative successor between the two statements. A value live immediately after S1 is exactly a value live immediately before S2. Hence OUT(S1)=IN(S2), A. Variables used inside S1 need not remain live after it.
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