Objectives Identify advantages (and disadvantages ?) of optimizing in SSA form Given a CFG in SSA form, perform Global Constant Propagation Dead code elimination Global Value Numbering Conversion of SSA back to CFG form

Constant Propagation Operate on sparse graph (SSA) Incorporate branch folding Meet operations at -nodes

EvalStmt Algorithm EvalStmt( Stmt ) if Stmt is arithmetic or -node evaluate Stmt if result is “lowered” foreach j  Uses (Stmt.Lval())) propagate result if j.block  Visited Work = { j } else if Stmt is a branch foreach possible destination DB of Stmt if Edge(Stmt, DB) not executable mark Edge(Stmt, DB) executable Blocks U= DB

Constant Prop Algorithm Mark all CFG Edges not executable Init node of SSA graph to T Work = Visited = Ø Blocks = {Entry} While Work  Ø  Blocks Ø While Work  Ø Choose Stmt from Work EvalStmt (Stmt) While Blocks  Ø Choose BB from Blocks foreach Stmt in  (BB) if BB  Visited Visited = {BB}

Dead Code Elimination Use SSA to detect dead code Method Remove statement(s) that do not directly or indirectly use data observable outside the procedure Eliminate branches never taken Uses control dependence

EliminateDeadCode () Worklist = Necessary = Ø foreach BB  CFG foreach Stmt  BB if (Stmt defines external data )  (Stmt is and I/O instruction) (Stmt is function call) Necessary U= { Stmt } WorkList U= { Stmt } endif endfor

Eliminate Dead Code Algorithm while Worklist  Ø choose Stmt from Worklist BB = Stmt’s basic block foreach block, CB, upon which BB is control dependent J = CB’s branch statement Necessary U= J Worklist U= J foreach T  Stmt’s operands Def = Stmt’s definition Necessary U= {Def } WorkList U= {Def } foreach BB  CFG foreach Stmt in BB if Stmt  Necessary remove Stmt else if Stmt is a branch  Stmt  Necessary Stmt.target = ipdom(BB)

Global Value Numbering Apply value numbering at function scope Associate a field (for value number) with each temporary. Temps with same value number are = No loops ==> reverse postorder traversal suffices (operands defined before used.)

Strongly Connected Components What about loops ? Value numbers for -nodes ambiguous Assume best case Iterate on the SCC in reverse postorder until no further changes

Processing -nodes If -node has a value number, VN, assign VN to temp defined by -node Consider operands with non-Null value numbers: If two have different values, assign a new value for -node. If all operands have same value number assign that to temp defined by -node

Efficiency Refinements If -node has non-Null value number different from its operands, don’t need a new value number For SCC, use a temp value number table called scratch table. Once that table “stabilizes”, copy to value table.

SSA to CFG -nodes “not executable” directly Must convert SSA back to CFG “Semantics” of -nodes is that they are “executed” simultaneously upon entry to block Insert code for each operand of -node in appropriate predecessor block.