1. 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

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

3. 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

4. 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}

5. 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

6. 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

7. 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)

8. 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.)

9. 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

10. 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

11. 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.

12. 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.