Compiler Design 1 Data Flow Analysis and Computation of SSA
Compiler Design 2 Definitions A basic block is the longest sequence of three-address codes with the following properties. The control flows to the block only through the first three-address code a. The control flows out of the block only through the last three-address code b. a There is no label in the middle of the code. b No three-address code other than the last one can be branch or jump.
Compiler Design 3 Example 1: L2: v1 = i 13: L4:v1 = i 2: v2 = j 14 v2 = j 3: if v1>v2 goto L3 4: v1 = j 15 if v1<>v2 5: v2 = i goto L2 6: v1 = v1 - v2 7: j = v1 8: goto L4 9: L3: v1 = i 10: v2 = j 11: v1 = v1 - v2 12: i = v1
Compiler Design 4 Basic Block - b 1 1: L2: v1 = i 2: v2 = j 3: if v1>v2 goto L3 3a: goto (4)
Compiler Design 5 Basic Block - b 2 4: v1 = j 5: v2 = i 6: v1 = v1 - v2 7: j = v1 8: goto L4
Compiler Design 6 Basic Block - b 3 9: L3: v1 = i 10: v2 = j 11: v1 = v1 - v2 12: i = v1 12a: goto L4
Compiler Design 7 Basic Block - b 4 13: L4:v1 = i 14 v2 = j 15 if v1<>v2 goto L2 15a exit
Compiler Design 8 Definitions A control-flow graph (CFG) is a directed graph G = (V,E), where the nodes are the basic blocks and the edges correspond to the flow of control from one basic block to another. The edge e ij = (v i,v j ) corresponds to the flow of control from the basic block v i to the basic block v j.
Compiler Design 9 Control-Flow Graph Entry b1: L2: v1 = i v2 = j if v1>v2 goto L3 b3: L3: v1=i v2=j v1=v1 v2 i = v1 b2: v1=j v2=i v1=v1 v2 j=v1 goto L4 b4: L4: v1=i v2=j if v1 <> v2 goto L2 Exit
Compiler Design 10 Note We assume that a CFG has a unique entry node and also has a unique exit node. In a CFG corresponding to a function/procedure may have multiple exit and entry points. A compiler can introduce unique initial and final basic blocks and edges to (from) multiple entry (exit) points.
Compiler Design 11 Note A compiler may use 3-address code for each basic block and a CFG for a whole procedure or a program showing flow of control among the basic blocks. Different global optimizations require analysis of dataflow through the CFG.
Compiler Design 12 Definition In a CFG with the unique entry block B 0, a basic block B i dominates a basic block B j if every path from B 0 to B j has B i on it. Dominance is a binary relation on the set of nodes (basic blocks) of a CFG. The relation B i dominates B j is written as B i >>B j. If there is an edge from B i to B j in the CFG, then B i is called a predecessor of B j.
Compiler Design 13 Definition For all node B i, B i >>B i (reflexive). Similarly for all nodes B i,b j,b k, if B i >>B j and B j >>B k, then B i >>B k (transitive). For every node B i, the set Dom(B i ) is the collection of all dominators of B i. For every node B i, the set Pred(B i ) is the collection of all predecessors of B i.
Compiler Design 14 A CFG B0 B1 B2 B3 B4 B5 B6 B7 B8 B9 B10 B11 B12
Compiler Design 15 Examples Node Predecessors Dominators B 0 B 0 B 1 B 0 B 0,B 1 B 2 B 0 B 0,B 2 B 3 B 1 B 0,B 1,B 3 B 8 B 4,B 5 B 0,B 2,B 8 B 9 B 7,B 8 B 0,B 9
Compiler Design 16 Computation The dominator set of any node B i can be computed using the following inductive definition. Dom(B i ) = {B 0 }, if i = 0 {B i } B j Pred(B i ) Dom(B j), if i > 0.
Compiler Design 17 Note This equation corresponds to a forward data-flow analysis. The dominator set of B i depends on the dominator set of its predecessor nodes. Similarly there properties of nodes that can be defined using its successors. This gives rise to backward data-flow analysis.
Compiler Design 18 Algorithm 1 Dom(B 0 ) {B 0 } 2 for i 1 to n 1 3 Dom(B i ) {B 0,,B n 1 } 4 chflg true 5 while chflg = true do 6 chflg false 7 for i 1 to n 1 8 temp {B i } B j Pred(B i ) Dom(B j) 9 if temp Dom(B i ) then 10 Dom(B i ) temp; chflg true
Compiler Design 19 Note This is an example of fixed-point computation. The iteration of the while-loop terminates when the Dom(b) cannot be reduced further. It is the fixed-point of a monotone function defined by the equation.
Compiler Design 20 Liveness and Safety The algorithm terminates as in step 8 the size of Dom(B i ) may decrease or remain unchanged. This cannot continue indefinitely. The correctness of the algorithm comes from the correctness of the definition of Dom(B i ). The speed of termination depends on the ordering of the nodes in the CFG.
Compiler Design 21 Live Variable Analysis Using a variable before initialization is a logical error. If a variable x is (i) assigned a value in a 3-address code i, (ii) is used as an operand in a 3-address code j, and (iii) is not redefined in a path p in the CFG from i to j, then x is live at i and all points on path p.
Compiler Design 22 Liveness and Next Use For each 3-address code a b c, we want to know the liveness and next-uses of the variables. It is easy to compute them within a basic block. So the present goal is to determine the liveness and next-use for 3-address codes in a basic block.
Compiler Design 23 Liveness and Next Use in a Basic Block Input: a sequence of 3-address codes of a basic block B. All variables within B are initialized as live on exit from B, but next-use is unknown - (live, )
Compiler Design 24 Liveness and Next Use in a Basic Block The algorithm starts at the last 3-address code of B and works backward. It attaches liveness and next-use information to variables in each assignment statement. These information may be stored in the symbol-table.
Compiler Design 25 i : x = y z 1. Get liveness and next use information (from the symbol table) of x,y,z and attach it to the instruction i. 2. Update symbol-table: x: (not Live, nonextuse) as it is redefined at i. 3. Update symbol-table: y,z: (live, i) - the next use of both y and z is 3-address code i.
Compiler Design 26 Basic Block - b 3 9: L3: v1 = i 10: v2 = j 11: v1 = v1 - v2 12: i = v1 12a: goto L4
Compiler Design 27 Basic Block - b 3 symtab: i:(l, U),j:(L, U),v1:(L,U),v2:(L,U) 12: i = v1 # i: (L,U), v1: (L,U) symtab: i:(d, N),j:(L, U),v1:(L,12),v2:(L,U) 11: v1 = v1 - v2 # v1: (L, 12), v2: (L,U) symtab: i:(d, N),j:(L, U),v1:(L,11),v2:(L,11)
Compiler Design 28 Basic Block - b 3 symtab: i:(d, N),j:(L, U),v1:(L,11),v2:(L,11) 10: v2 = j # v2: (L, 11), j: (L, U) symtab: i:(d, N),j:(L, 10),v1:(L,11),v2:(D,N) 9: L3: v1 = i # v1: (L, 11), i: (D, N) symtab: i:(l, 9),j:(L, 10),v1:(D,N),v2:(D,N)
Compiler Design 29 Note - b 3 Var Beginning End Live Next Use Live Next Use i Yes 9 Yes j Yes 10 Yes v 1 No Yes v 2 No Yes
Compiler Design 30 Live-Variable Analysis on a CFG Given a variable x and a point p in a CFG, it is important to know whether x is live at p i.e. whether the value of x at p will be used at some path in the CFG starting at p. Computation of this information can be formulated as a dataflow equation. But in this case the information is computed opposite to the direction of the control-flow - backward dataflow.
Compiler Design 31 Formulation of Data Flow Equation Given a basic block B, LIn(B) and LOut(B) are the sets of all variables that are live at the entry and exit of the block B. Let ufst(b) be the set of variables whose values are used in B before any definition. Let def(b) be the set of variables that are defined in B.
Compiler Design 32 Formulation of Data Flow Equation Variables live at the exit of B is the union of the variables live at the entry of its successors blocks. LOut(B) = S succ(b) LIn(S). Variables live at the entry of S contains all the variables in ufst(s).
Compiler Design 33 Formulation of Data Flow Equation Variables of def(s) cannot be live at the entry of S unless it is in ufst(b) i.e. used before a definition. Again def(s) LOut(S). So LOut(S) \ def(s) is also contained in LIn(S). So we have LOut(B) = S succ(b) (ufst(s) (LOut(S)\def(S))).
Compiler Design 34 Computation It is necessary to compute def(s) and ufst(s) sets of each basic block. This will be done by examining the 3-address codes of a basic block from the beginning.
Compiler Design 35 Computation of ufst(b) and def(b) Following steps are performed for each basic block B with 1 k 3-adders codes of type assignment e.g. x y z. 1 ufst(b), def(b) 2 for i 1 to k 3 if y def(b), then 4 ufst(b) ufst(b) {y} 5 if z def(b), then 6 ufst(b) ufst(b) {z} 7 def(b) def(b) {x}
Compiler Design 36 Computation of ufst(b) and def(b) For a 3-address code like if x < y goto L there is no change in def(b). But ufst(b) will include {x,y} unless they are already in def(b). Other type of 3-address codes can be handled similarly.
Compiler Design 37 Computation of LOut(B) It is similar to the computation of Dom(B). 1. LOut(B i ) for i = 0,,n 1 are initialized to. 2. All LOut(B i ) s are recomputed until a fixed point is reached. 3. If the CFG corresponds to a function, the formal parameters are in LIn() of the entry block.
Compiler Design 38 Basic Block - b 1 1: L2: v1 = i 2: v2 = j 3: if v1>v2 goto L3 ufst(b 1 ) = {i,j} and def(b 1 ) = {v 1,v 2 }
Compiler Design 39 Basic Block - b 2 4: v1 = j 5: v2 = i 6: v1 = v1 - v2 7: j = v1 8: goto L4 ufst(b 2 ) = {i,j} and def(b 2 ) = {v 1,v 2,j}
Compiler Design 40 Basic Block - b 3 9: L3: v1 = i 10: v2 = j 11: v1 = v1 - v2 12: i = v1 ufst(b 3 ) = {i,j} and def(b 2 ) = {v 1,v 2,i}
Compiler Design 41 Basic Block - b 4 13: L4:v1 = i 14 v2 = j 15 if v1<>v2 goto L2 ufst(b 3 ) = {i,j} and def(b 2 ) = {v 1,v 2 }
Compiler Design 42 Control-Flow Graph
Compiler Design 43 Entry b1: L2: v1 = i v2 = j if v1>v2 goto L3 b3: L3: v1=i v2=j v1=v1 v2 i = v1 b2: v1=j v2=i v1=v1 v2 j=v1 goto L4 b4: L4: v1=i v2=j if v1 <> v2 goto L2 Exit
Compiler Design 44 Computation of LOut(b i ) Iteration & LOut(b i ) b i 0 1 b 1 {i,j} b 2 {i,j} b 3 {i,j} b 4 {i,j}
Compiler Design 45 Note The variables i, j are live at the exit of all four blocks. They may be kept in a register. We can detect uninitialized variable through this analysis. If the set of live-variables at the exit of the dummy entry node of a CFG is non-empty, then all variables of the set are uninitialized.
Compiler Design 46 Control-Flow Graph Ent. ufst = {} def = {} b1 j = 5 ufst = {} def = {j} b2 j = j + i ufst = {i,j} def = {j} Ext. ufst = {} def = {}
Compiler Design 47 Computation of LOut(b i ) Iteration & LOut(b i ) b i 0 1 2 3 Ent. {i} {i} b 1 {i,j} {i,j} {i,j} b 2 Ext. Indicates that i is uninitialized.
Compiler Design 48 Static Single Assignment: SSA SSA is a naming method that encodes both flow of control and flow of data in a program. Each name is defined by an operation at a particular point in the code. So each use of a name has a unique definition. The flow of control is handled by using a selection function φ.
Compiler Design 49 Static Single-Assignment: SSA SSA representation looks similar to three-address code with two main differences. Each name is defined only once, so it is called static single-assignment. If the same variable is defined on different control paths, they are renamed as distinct variables by adding subscript to the base name.
Compiler Design 50 Static Single-Assignment: SSA When two or more flow-paths join, a φ-function is used to select different names of the same variable on these paths. A φ-function also defines a new name. The arguments of φ-functions of a join-block are ordered according to some order of incoming flow-paths.
Compiler Design 51 Three-Address & SSA Codes i = 1 i0 = 1 f = 1 f0 = 1 L2: if i>n goto - if i0 > n goto L1 L2: i1 = φ(i0, i2) f1 = φ(f0, f2) f = f*i f2 = f1*i1 i = i + 1 i2 = i1 + 1 goto L2 if i2 <= n goto L2 L1: i3 = φ(i0, i2) f3 = φ(f0, f2)
Compiler Design 52 Note The φ-function is inserted at the beginning of a basic block where different values of a program variable can reach along different flow-paths. In our example (i0,f0) are copied to (i1,f1) when the control flows from the top. Otherwise, (i2,f2) is copied to (i1,f1).
Compiler Design 53 Note All φ-functions at the beginning of a basic block are assumed to be evaluated concurrently. So the ordering does not matter. But it may complicate the implementation of φ-function. The single assignment property simplifies code optimization. As a definition is never killed, the value of a name is available on any path starting from the definition.
Compiler Design 54 Note The φ-function can take any number of arguments. The number of arguments depends on the number of control-flow paths entering a join-block. So it may have more than 3-addresses and there should be mechanism to accommodate it in 3-address data-structure.
Compiler Design 55 Note Some of the arguments of a φ-function may be undefined during its execution. In our example i2 is undefined when the loop is entered first time. But it should not create trouble as φ-function selects the argument corresponding to current control-flow path taken to enter the join-block (where the argument is defined).
Compiler Design 56 Building SSA: A Simple Method For every variable name x used in the code, a φ-function, x φ(y,,y) is inserted at the beginning of each basic block with more than one predecessors. The number of arguments of a φ-function is equal to the number of predecessors of the basic block.
Compiler Design 57 Note The ordering of φ-function inserted at the beginning of a basic block does not matter as they are executed concurrently i.e. they select the input parameters simultaneously and write them simultaneously. But the process may insert many useless φ-functions.
Compiler Design 58 φ-functions Entry b1: L2: i = φ (i,i) b3: j = φ (j,j) u = φ(u,u) v = φ(v,v) u = i v = j if u>v goto L3 L3: u = i v = j u = u v i = u b2: u = j v = i u = u v j = u goto L4 b4: L4: i = φ (i,i) j = φ (j,j) u = φ (u,u) v = φ (v,v) u = i v = j if u <> v goto L2 Exit
Compiler Design 59 After Renaming: SSA Entry b1: L2: i2 = φ (i0,i1) b3: j2 = φ (j0,j1) u2 = φ (u0,u1) v2 = φ (v0,v1) u3 = i2 v3 = j2 if u3 > v3 goto L3 L3: u6 = i2 v5 = j2 u7 = u6 v5 i3 = u7 b2: u4 = j2 b4: L4: i1 = φ(i2,i3) v4 = i2 j1 = φ(j3,j2) u5 = u4 v4 u8 = φ (u5,u7) j3 = u5 v6 = φ (v4,v5) goto L4 u1 = i1 v1 = j1 if u1 <> v1 goto L2 Exit
Compiler Design 60 Note Both in b 1 and b 3, the φ-functions for u and v are useless.
Compiler Design 61 Reaching Definitions For renaming variables for SSA we need to perform a data-flow analysis called reaching definitions. Each φ-function is also a new definition. A definition d of a name v (d : v ) reaches a use i (i : v ) if there is a path from d to i on which v is not redefined.
Compiler Design 62 Reaching Definitions Reaches(n) is the set of definitions reaches the beginning of the basic block n. Gen(n) is the set of set of definitions generated in the basic block n but not killed within it. So they go out of n. Kill(n) is the set of all definitions (globally) killed in the basic block n.
Compiler Design 63 Gen() and Kill() Kill(b1) = {d3,d5,d7,d9,d11, d4, d8,d12} Gen(b1) = {d1,d2} b1: d1: d2: L2: v1 = i v2 = j if v1>v2 goto L3 Gen(b3) = {d8,d9,d10} Kill(b3) = {d1,d2,d3,d4,d5,d7,d11,d12} d7: d8: d9: d10: b3: L3: v1=i v2=j v1=v1 v2 i = v1 Kill(b2) = {d1,d2,d3,d7,d8,d9,d11,d12} Gen(b2) = {d4,d5,d6} b2: d3: v1=j d4: v2=i d5: v1=v1 v2 d6: j=v1 goto L4 d11: d12: b4: L4: v1=i v2=j if v1 <> v2 goto L2 Kill(b4) = {d1,d2,d3,d4,d5,d7,d8,d9} Gen(b4) = {d11,d12}
Compiler Design 64 Reaching Definitions The reaching definition can be formulated as a forward data-flow problem. reaches(n) = m pred(n) gen(m) (reaches(m)\ kill(m)) We start with reaches(n) = for all basic block n.
Compiler Design 65 Building SSA: A Simple Method The φ-function ensures that only one definition reaches an use. Variable in each use are renamed according to the definition that reaches it. For each φ-function, names are ordered according to the control-path of their arrival.
Compiler Design 66 Note The algorithm correctly translates a 3-address code to SSA form. But there are many redundant φ-functions e.g. x i φ(x j,x j ) or the name defined may not be live (redefinition of x before use). Redundant φ-functions increases algorithmic cost.
Compiler Design 67 Note There is an algorithm based on the notion of dominator that will not generate useless φ-functions. For every block b i it finds out the blocks that require φ-functions for a definition d in b i.
Compiler Design 68 Note If b i Dom(b j ) and d : x is a definition in b i, then b j does not need a φ-function for x unless it is redefined in some control path that reaches b j. If there is a redefinition of x in some intermediate block b k on a control path between b i and b j, then the redefinition forces a φ-function.
Compiler Design 69 Forcing a φ-function A definition d : x in a block b i forces a φ-function for x in a block b j where more than one control-paths meet (join point), in the following cases. b i Dom(b k ), where b k is a predecessor of b j. b i = b j. We call the set Dom(b j )\{b j } as the set of strict dominators of b j.
Compiler Design 70 Note In our example CFG, a definition in block B 2 forces a φ-function in block B 9. The collection of blocks where the block b i can force a φ-function is called the dominance frontier of b i, DF(b i ). This is the DF(B 2 ) = {B 9 }.
Compiler Design 71 Dominator Tree Given a block b j in a CFG, the block b i DOM(b j )\{b j }, closest to b j, is called the immediate dominator of b j, IDom(b j ). For every block b i of a CFG we draw a directed-edge from IDom(b i ) b i. This forms a tree with the entry-node of the CFG as the root. It is known as dominator tree of the CFG.
Compiler Design 72 Dominator Tree B0 B1 B2 B3 B4 B5 B6 B7 B8 B9 B10 B11 B12
Compiler Design 73 Note The dominator tree of a CFG has all blocks (nodes) of the CFG. For a node b i, IDom(b j ) is the parent of b i in the tree. The Dom(b i ) is the collection of nodes (blocks) on the path from the root (entry node) to b i e.g. IDom(B 10 ) = B 8, Dom(B 10 ) = {B 0,B 2,B 8,B 10 }.
Compiler Design 74 Properties: Node in DF If a node (block) b j is in some DF, then Multiple control-paths meet at b j i.e. b j is a join-point. For each predecessor b k, the node b j DF(b k ). As b j is the first join-point after b k and b j is not dominated by b k. For each predecessor b k, the node b j DF(b l ) for all b l Dom(k)\Dom(j).
Compiler Design 75 Computing Dominance Frontier 1. For each block b j with multiple incoming control paths (join-point) do the following. 2. For each predecessor b k of b j do the following. 3. Walk up the dominator tree until some b i Dom(b j ) is found. 4. For each node b l in the path of b i to b k, except b i, put b j in DF(b l ).
Compiler Design 76 Algorithm 1 for i 0 to n 1 do 2 DF(b i ) 3 for j 0 to n 1 do 4 if join-point(b j ) then 5 for each b k pred(b j ) do 6 temp b k 7 while temp IDom(b j ) do 8 DF(temp) DF(temp) {b j } 9 temp IDom(temp)
Compiler Design 77 Example CFG
Compiler Design 78 B0 B1 B2 B3 B4 B5 B6 B7 B8 B9 B10 B11 B12
Compiler Design 79 Example Blocks/nodes that are join-points (multiple control-paths meet): B 1,B 2,B 7,B 8,B 9,B 11,B 12.
Compiler Design 80 Dominator Tree B0 B1 B2 B3 B4 B5 B6 B7 B8 B9 B10 B11 B12
Compiler Design 81 Example Dominance Frontier: Blocks Predecessors DF B 0 B 1 B 0,B 6 B 1,B 9,B 11 B 2 B 0,B 10 B 2,B 9,B 11 B 3 B 1 B 1,B 7,B 11 B 4 B 2 B 8 B 5 B 2 B 8 B 6 B 3 B 1,B 11
Compiler Design 82 Example Dominance Frontier(cont.): Blocks Predecessors DF B 7 B 3,B 1 B 9 B 8 B 4,B 5 B 2,B 9,B 11 B 9 B 7,B 8 B 12 B 10 B 8 B 2,B 11 B 11 B 6,B 10 B 12 B 12 B 9,B 11
Compiler Design 83 Example The node B 1 has two predecessors B 0,B 6. As B 0 = IDom(B 1 ), the while-loop is not entered. So it does not contribute any node in the dominance frontier. For the other predecessor B 6, the loop is entered with temp B 6,B 3,B 1, so B 1 goes in to DF(B 6 ), DF(B 3 ), DF(B 1 ).
Compiler Design 84 Placement of φ-function In the first algorithm (simple) a φ-function was introduced for every variable x at the beginning of each join-block - x φ(x,,x). In the first modification, for every x defined in a block b i, we introduce a φ-function at the beginning of each b j DF(b i ).
Compiler Design 85 Placement of φ-function If the life span of a variable is restricted to a block, no φ-function is required for it. Only the names global to a CFG may require φ-functions. The union of ufst(b), the set of variables whose values are used before any definition in block B, is the set of global names.
Compiler Design 86 Algorithm for Global Names Following algorithm computes global names, glovvar and varblklst(x) for the list of blocks where the variable x is defined.
Compiler Design 87 Algorithm 1 globvar 2 for all variable x, varblklst(x) 3 for each block B 4 ufst(b) 5 for each 3-address code a b c from beginning 6 if b def(b) then globvar globvar {b} 7 if c def(b) then globvar globvar {c} 8 def(b) def(b) {a} 9 varblklst(a) varblklst(a) {B}
Compiler Design 88 Algorithm for Inserting φ-function For each global name x globvar there is a list of blocks (varblklst(x)) where x is defined. For each b i varblklst(x) a φ-function for x is inserted in ever block b j DF(b i ). But then every φ-function in some block b j is also a definition of x and gives rise to new φ-functions in b k DF(b j ).
Compiler Design 89 Algorithm 1 for each x globvar 2 curblklst varblklst(x) 3 for each b i curblklst 4 for each b j DF(b i ) 5 if b j does not have φ(x,,x) insert one 6 curblklst curblklst {b j }
Compiler Design 90 Example It is necessary to put more flesh in the basic blocks of the example CFG. B 0 : a B 1 : b B 2 : a b c d B 3 : d (a > c) : 3,7) (d b) : 4,5) x B 4 : x a 1 B 5 : x b+1 B 6 : y B 7 : a B 8 : b b+1 b b y (y = b) : 1,11 (y = b) : 9,10
Compiler Design 91 Example B 9 : z b+d B 10 : d d+1 B 11 : a b+y a z +d (d < y) : 2,11 b d+x c a+b
Compiler Design 92 globvar and varblklst Global names: a,b,c a,d,x,y but z is local to B 9. List of blocks where a variable is defined are: Var a b c d x y z Blk B 0,B 2 B 0,B 1 B 1 B 2 B 3 B 6 B 9 B 7,B 9 B 6,B 7 B 9 B 3 B 4 B 8 B 11 B 8,B 11 B 10 B 5 a It is pointed out by Ashray Sudhir that c is not a global name.
Compiler Design 93 Placement of φ-function Consider the variable a. It is defined in B 0,B 2,B 7,B 9,B 11. These definitions induces φ-functions for a in the blocks DF(B 0 ) DF(B 2 ) DF(B 7 ) DF(B 9 ) DF(B 11 ) = {B 2,B 9,B 11 } {B 9 } {B 12 } {B 12 } = {B 2,B 9,B 11,B 12 }. As B 12 is exit block, we ignore it.
Compiler Design 94 Placement of φ-function. As {B 2,B 9,B 11 } varblklst(a), no new block is added in the curblklst. After introducing a φ(a,a) blocks are B 2 : a φ(a,a) B 9 : a φ(a,a) B 11 : a φ(a,a) a z b+d a b+y d a z +d b d+x (d b) : 4,5 c a+b
Compiler Design 95 Note. There are redundant φ-functions in B 9 and B 11 as a is not live in these two blocks. The live-variable analysis result may be used to detect that a is not live at the beginning of B 9 and B 11. It can be done at step-5 of φ-function insertion algorithm.
Compiler Design 96 Placement of φ-function. So blocks B 2,B 9,B 11 are B 2 : a φ(a,a) B 9 : z b+d B 11 : a b+y a a z +d b d+x d c a+b (d b) : 4,5
Compiler Design 97 Placement of φ-function Consider the variable b. It is defined in B 0,B 1,B 6,B 7,B 8,B 11. φ-functions for b are introduced in DF(B 0 ) DF(B 1 ) DF(B 6 ) DF(B 7 ) DF(B 8 ) DF(B 11 ) = {B 1,B 9,B 11 } {B 1,B 11 } {B 9 } {B 2,B 9,B 11 } {B 12 } = {B 1,B 2,B 9,B 11,B 12 }.
Compiler Design 98 Placement of φ-function The block B 2 is added in the curblklst. DF(B 2 ) = {B 2,B 9,B 11 } are added in the list of blocks where b φ(b,b) are introduced. Final set of blocks are {B 1,B 2,B 9,B 11 }.
Compiler Design 99 Placement of φ-function Blocks B 1,B 2,B 9,B 11 after insertion of b φ(b,b) are B 1 : b φ(b,b) B 2 : b φ(b,b) B 9 : b φ(b,b) b a z b+d c d a z +d (a > c) : 3,7 (d b) : 4,5 c a+b B 11 : b φ(b,b) a b+y b d+x
Compiler Design 100 Placement of φ-function The variable c introduces φ-function in B 1,B 9,B 11, all are useless, but the given algorithm cannot detect that. The variable d introduces φ-function in B 1,B 2,B 7,B 9,B 11. But it is redundant in B 1,B 7 that our algorithm cannot detect.
Compiler Design 101 Placement of φ-function The variable x introduces φ-function in B 1,B 2,B 7,B 8,B 9,B 11. Again they are redundant in B 1 and B 7. The variable y introduces φ-function in B 1,B 2,B 9,B 11 out of which B 1,B 2 and B 9 are redundant.
Compiler Design 102 Renaming of Variables Each global name is treated as a base of name in each new definition of it e.g. x. For each definition it is differentiated using subscripts e.g. x 0,x 1,. One algorithm for renaming traverse the dominator tree in pre-order. It works as follows.
Compiler Design 103 Renaming of Variables Each variable defined by the φ-function at the beginning of the block (b) are renamed first. After that the 3-address codes of b are visited in order. If the 3-address code is x y z and the current indices of x,y,z are i,j,k respectively, then after renaming it will be x i+1 y j z k and the current indices are modified to i+1,j,k.
Compiler Design 104 Renaming of Variables After renaming the variables in the 3-address codes of the block b, appropriate arguments of the φ-functions in the CFG-successors of b are renamed. Then the renaming is recursively called on each children block of b in the dominator tree.
Compiler Design 105 Renaming of Variables On return from the recursive of renaming on block b, the state of name indices is restored to the value that existed before entering the block. It is necessary to maintain a stack of indices for all global names.
Compiler Design 106 Data Structure For each global name we use a separate stack. This requires a second pass over the block (b) at the end of the recursive call to restore the status. This reduces the total memory requirement for the stack. We also may use a counter that holds the current indices of global names.
Compiler Design 107 Algorithm 1 for each x globvar 2 cind[x] 0 3 stack[x] 4 rename(b 0 )
Compiler Design 108 sub(x) Returns the current subscript for the global name x, pushes it in the stack of x and increments it. sub(x) 1 i cind[x] 2 push(stack[x], i) 3 cind cind +1 4 return i
Compiler Design 109 rename(b) 1. For each φ-function x φ( ) of block b rewrite x by x sub(x). 2. For each 3-address code in order where y is an operand change it to y i, where i = top(stack[y]). 3. If x is a target of a 3-address code i.e. x, it is replaced by x sub(x).
Compiler Design 110 rename(b) 4. For each successor block of b in the CFG, initialize the appropriate parameters of φ-functions. 5. Recursively call rename() on the dominator tree children of b. 6. For each definition of x in block b, pop(stack[x]).
Compiler Design 111 Example In our example we have six global names - globvar = {a,b,c,d,x,y}. Initial cind and stack[x] are u cind stack[u] a 0 b 0 c 0 d 0 x 0 y 0
Compiler Design 112 rename(b 0 ) Old B 0 : a New B 0 a 0 b b 0 u cind stack[u] a 1 a 0 Modified b 1 b 0 c 0 d 0 x 0 y 0
Compiler Design 113 CFG Successor of B 0 B 1 : b φ(b,b 0 ) B 2 : a φ(a 0,a) c φ(c, ) b φ(b 0,b) d φ(d, ) d φ(,d) x φ(x, ) x φ(,x) y φ(y, ) y φ(,y) b a c d (a > c) : 3,7 (d b) : 4,5
Compiler Design 114 rename(b 1 ): DT Successor of B 0 B 1 : b 1 φ(b,b 0 ) c 0 φ(c, ) d 0 φ(d, ) x 0 φ(x, ) y 0 φ(y, ) b 2 c 1 (a 0 > c 1 ) : 3,7
Compiler Design 115 Modified cind and stack[u] u cind stack[u] a 1 a 0 b 3 b 0 b 1 b 2 c 2 c 0 c 1 d 1 d 0 x 1 x 0 y 1 y 0
Compiler Design 116 CFG Successor of B 1 B 3 : d B 7 : d φ(d,d 0 ) x x φ(x,x 0 ) a b
Compiler Design 117 rename(b 3 ): DT Successor of B 1 B 3 : d 1 x 1
Compiler Design 118 Modified cind and stack[u] u cind stack[u] a 1 a 0 b 3 b 0 b 1 b 2 c 2 c 0 c 1 d 2 d 0 d 1 x 2 x 0 x 1 y 1 y 0
Compiler Design 119 CFG Successor of B 3 B 6 : y B 7 : d φ(d 1,d 0 ) b x φ(x 1,x 0 ) a b
Compiler Design 120 rename(b 6 ): DT Successor of B 3 B 6 : y 1 b 3
Compiler Design 121 Modified cind and stack[u] u cind stack[u] a 1 a 0 b 4 b 0 b 1 b 2 b 3 c 2 c 0 c 1 d 2 d 0 d 1 x 2 x 0 x 1 y 2 y 0 y 1
Compiler Design 122 CFG Successor of B 6 B 1 : b 1 φ(b 3,b 0 ) B 11 : a φ(a 0,a) c 0 φ(c 1, ) b φ(b 3,b) d 0 φ(d 1, ) c φ(c 1,c) x 0 φ(x 1, ) d φ(d 1,d) y 0 φ(y 1, ) x φ(x 1,x) b 2 y φ(y 1,y) c 1 a b+y (a 0 > c 1 ) : 3,7 b d+x
Compiler Design 123 Return from rename(b 6 ) There is no children of B 6 in the dominator tree. So the stack entries corresponding to B 6 are popped. Again B 6 is the only child of B 3 in the dominator tree. So the stack entries corresponding to B 3 are also popped.
Compiler Design 124 stack[u] after pop-b 6 u cind stack[u] a 1 a 0 b 4 b 0 b 1 b 2 c 2 c 0 c 1 d 2 d 0 d 1 x 2 x 0 x 1 y 2 y 0 b 3 and y 1 removed.
Compiler Design 125 stack[u] after pop-b 3 u cind stack[u] a 1 a 0 b 4 b 0 b 1 b 2 c 2 c 0 c 1 d 2 d 0 x 2 x 0 y 2 y 0 d 1 and x 1 removed.
Compiler Design 126 Note Next step is to rename B 7. But It is necessary to translate SSA form to 3-address code.
Compiler Design 127 Removing φ-function Let in a block b there is x i φ(x j,x k ). The value is x j when the control-path to b is from the block b u ; and it is x k when the control-path to b is from the block b v. x i φ(x j,x k ) in block b is replaced by inserting x i x j in b u and x i x k in b v (at the end).
Compiler Design 128 Example: B 1 and Two Predecessors B 1 : b 1 φ(b 3,b 0 ) B 6 : y 1 B 0 : a 0 c 0 φ(c 1, ) b 3 b 0 d 0 φ(d 1, ) x 0 φ(x 1, ) y 0 φ(y 1, ) b 2 c 1 (a 0 > c 1 ) : 3,7
Compiler Design 129 Example: Replacing φ from B 1 B 1 : b 2 B 6 : y 1 B 0 : a 0 c 1 b 3 b 0 (a 0 > c 1 ) : 3,7 b 1 b 3 b 1 b 0 c 0 c 1 d 0 d 1 x 0 x 1 y 0 y 1
Compiler Design 130 References [KDCLT] Engineering a Compiler by Kieth D. Cooper & Linda Troczon, 2nd ed., Morgan Kaufmann Pub., 2012, ISBN 978-93-80931-87-6.