Languages and Compiler Design II IR Canonicalization

1. Languages and Compiler Design IIIR Canonicalization Material provided by Prof. Jingke Li Stolen with pride and modified by Herb Mayer PSU Spring 2010 rev.: 4/16/2010 CS322

2. Agenda • Definition • Problems with IR Representation • IR Tree Canonicalization • Implementation • Handling CALL Nodes • Handling Other EXP Nodes • Handling STMT Nodes CS322

3. Definition • Definition: In Computer Science, canonicalization is the formal process of converting data with more than one representation from one into another, "standard“ ( or "normal", or canonical) form of representation • Sometimes abbreviated c14n, where 14 represents the number of letters between the ‘c’ and ‘n’. Synonyms are standardization or normalization • Not to be confused with the process in the Catholic Church of declaring a person a saint = canonization  CS322

4. Problems with IR Representation Two areas in our IR tree language may cause representational problems: • Multiple CALL nodes within the same expression may cause problems with call-related register usage when mapped to lower-level code • ESEQ and CALL nodes within expressions may bring side effects to different evaluation orders of sub-expressions, causing problems for low-level optimizations CS322

5. IR Tree Canonicalization To resolve these problems, we could perform tree canonicalization: • Assign each return value immediately into a TEMP; turn a CALL node into an ESEQ node: (CALL (NAME f) ((CONST 1))) => (ESEQ [MOVE (TEMP 100) (CALL (NAME f) ((CONST 1)))] (TEMP 100)) • Pull statements in ESEQ nodes out of an expression tree, effectively eliminating all ESEQ nodes: [ASSIGN (TEMP 101) (BINOP + (CONST 2) (CALL (NAME f) ((CONST 1))))] => [ASSIGN (TEMP 101) (BINOP + (CONST 2) (ESEQ [MOVE (TEMP 100) (CALL (NAME f) ((CONST 1)))] (TEMP 100)))] => [MOVE (TEMP 100) (CALL (NAME f) ((CONST 1)))] [ASSIGN (TEMP 101) (BINOP + (CONST 2) (TEMP 100))] The resulting tree is called a canonical IR tree CS322

6. Implementation Canonicalization can be implemented as a visitor program over IR trees. Visitor touches every expression node, extracts statements out, and up-levels them. public class Canon implements IrVI { public PROG visit( PROG t ) { return new PROG(t.funcs.accept(this)); } //end PROG public FUNClist visit( FUNClist t ) { FUNClist funcs = new FUNClist(); for( int i = 0; i < t.size(); i++ ) { funcs.add(((FUNC) t.elementAt( I )).accept( this )); } //end for return funcs; } //end FUNClist public FUNC visit( FUNC t ) { return new FUNC( t.label, t.varCnt, t.argCnt, (STMTlist) t.stmts.accept(this)); } //end FUNC } //end Canon CS322

7. Handling CALL Nodes New temp saves return value; then replace the node with an ESEQ node: public EXP visit( CALL t ) { EXP args = t.args.accept( this ); STMT s = getStmt(args); TEMP tmp = new TEMP(); STMT s1 = new MOVE( tmp, new CALL( t.func, (EXPlist) getExp( args ))); return new ESEQ( mergeStmts( s, s1 ), tmp ); } //end EXP private STMT getStmt( EXP e ) { if( e instanceof ESEQ) return ((ESEQ) e).stmt; return null; } //end STMT private EXP getExp( EXP e ) { if( e instanceof ESEQ ) return(( ESEQ ) e ).exp; return e; } //end EXPs CS322

8. Handling Other EXP Nodes Assuming components are commutative. public EXP visit( MEM t ) { EXP exp = t.exp.accept( this ); STMT s = getStmt( exp ); if( s != null ) return new ESEQ(s, new MEM( getExp(exp) )); return t; } //end EXP public EXP visit( BINOP t ) { EXP left = t.left.accept( this ); EXP right = t.right.accept( this ); STMT s = mergeStmts(getStmt( left ), getStmt( right )); if( s != null ) return new ESEQ(s, new BINOP( t.op, getExp( left ), getExp( right ))); return t; } //end EXP CS322

9. Handling Other EXP Nodes, Cont’d public EXP visit( EXPlist t ) { STMT s = null; EXPlist args = new EXPlist(); for( int i = 0; i < t.size(); i++ ) { EXP e = ((EXP) t.elementAt(i)).accept( this ); STMT s1 = getStmt(e); if( s1 != null ) s = mergeStmts( s, s1 ); args.add( getExp( e )); } //end for if( s != null ) return new ESEQ(s, args); return t; } //end EXP public EXP visit( ESEQ t ) { STMT stmt = t.stmt.accept(this); EXP exp = t.exp.accept(this); STMT s = mergeStmts(stmt, getStmt(exp)); if( s != null ) return new ESEQ( s, getExp( exp )); return t; } //end EXP CS322

10. Handling STMT Nodes public STMT visit( MOVE t ) { EXP dst = t.dst.accept(this); EXP src = t.src.accept(this); STMT s = mergeStmts( getStmt(dst), getStmt(src) ); if( s != null) return mergeStmts( s, new MOVE(getExp(dst), getExp(src))); return t; } //end STMT public STMT visit( CJUMP t ) { EXP left = t.left.accept( this ); EXP right = t.right.accept( this ); STMT s = mergeStmts(getStmt( left ), getStmt( right )); if( s != null) return mergeStmts(s, new CJUMP( t.op, getExp(left), getExp( right ), t.target )); return t; } //end STMT CS322

11. Handling STMT Nodes, Cont’d public STMT visit(CALLST t) { EXP args = t.args.accept(this); STMT s = getStmt(args); if( s != null ) return mergeStmts( s, new CALLST( t.func, (EXPlist) getExp( args ))); return t; } //end STMT public STMT visit(RETURN t) { EXP exp = t.exp.accept(this); STMT s = getStmt(exp); if( s != null ) return mergeStmts(s, new RETURN( getExp( exp ) )); return t; } //end STMT CS322