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w11Color - The Role of Program Analysis Program analysis...

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1 The Role of Program Analysis Program analysis must precede many optimizations. – Control flow analysis determines where program execution goes next, what is dependent on branches – Data flow analysis determines how program variables are affected by program sections – Data-dependence analysis determines which data references in a program access the same storage location. (Sometimes data flow analysis is used as a generic term for all these analyses)
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2 Simple Control Flow Analysis A IF (cond) THEN B ELSE C ENDIF D A B C D Control-Flow or Data-flow Graph
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3 Control Flow Analysis Control flow imposes dependences – the execution of a code block must wait until the control conditions have been evaluated. – This is true even if there are no data dependences. Exercise: draw the control flow graph of this program segment d=100 IF (a==b) THEN c=d+e ELSE IF (c==e) THEN d=0 ENDIF b=b*2 DO i=d,e a[i]=0 ENDDO
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4 Data flow analysis takes place over a control flow graph Data flow analysis attempts to track, at compile time, the flow of variable values, pointer values, expression types, etc. At compile time, we don’t know the effect of control flow, and we must approximate the behavior of the program at “join” points in the program Intra-procedural parts of algorithm are either “flow sensitive” or “flow insensitive” depending on how they treat branches, and joins from branches. Inter-procedural algorithms are either “context sensitive” or “context insensitive” depending on how they treat information flowing into (and out of) function calls.
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5 Data flow equations At each point in a program, the effect of data flow is modeled by examining the information (not necessarily variable values) that – comes in to the block/statement, – is gen erated by the block/statement, – is kill ed by the block/statement, – goes out of the block/statement. out[B] = gen[B] (in[B] – kill[B])
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6 out[B] = gen[B] (in[B] – kill[B]) Definition of out , gen , in , kill depends on the analysis we are doing Some analyses go top down through the program (and use the equation above), some analyses go bottom up and use the equation: In(b) = Gen(b) (Out(b) Kill(b))
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7 Global Dataflow Analysis Analysis of how program attributes change across basic blocks Dataflow analysis has many applications. A few examples: – global live variable analysis – uninitialized variable analysis – available expression analysis – busy expression analysis (used along all paths before being killed)
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8 determining Def and Use sets in a basic block is easy. Exercise: determine Def, Use sets for Aside: Finding Local Def, Use c = d + e d = a [ j ] a [ j ] = sb [ i,k ] * p = *q
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9 Sound and unsound analyses A safe or sound analysis is one in which any imprecision reduces the opportunities to do a transformation or optimization, but doesn’t cause incorrect transformations or optimizations to be performed.
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