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Lecture # 23

Lecture # 23. Chapter 8: Intermediate Code Generation. Intermediate Code Generation. Facilitates retargeting : enables attaching a back end for the new machine to an existing front end Enables machine-independent code optimization. Intermediate code. Target machine code. Front end.

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Lecture # 23

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  1. Lecture # 23 Chapter 8: Intermediate Code Generation

  2. Intermediate Code Generation • Facilitates retargeting: enables attaching a back end for the new machine to an existing front end • Enables machine-independent code optimization Intermediatecode Targetmachinecode Front end Back end

  3. Intermediate Representations • Graphical representations (e.g. AST) • Postfix notation: operations on values stored on operand stack (similar to JVM bytecode) • Three-address code: (e.g. triples and quads)x := y op z • Two-address code:x := op ywhich is the same as x := x op y

  4. Syntax-Directed Translation of Abstract Syntax Trees ProductionS  id :=EE E1+E2E E1*E2E -E1E( E1 ) E id Semantic RuleS.nptr := mknode(‘:=’, mkleaf(id, id.entry), E.nptr)E.nptr := mknode(‘+’, E1.nptr, E2.nptr)E.nptr := mknode(‘*’, E1.nptr, E2.nptr)E.nptr := mknode(‘uminus’, E1.nptr)E.nptr := E1.nptr E.nptr := mkleaf(id, id.entry)

  5. Abstract Syntax Trees E.nptr E.nptr * a * (b + c) E.nptr E.nptr a ( ) E.nptr + E.nptr b c * Pro: easy restructuring of code and/or expressions for intermediate code optimization Cons: memory intensive a + b c

  6. Abstract Syntax Trees versus DAGs a := b * -c + b * -c := := a + a + * * * b uminus b uminus b uminus c c c Tree DAG

  7. Postfix Notation a := b * -c + b * -c a b c uminus * b c uminus * + assign Bytecode (for example) iload 2 // push biload 3 // push cineg // uminusimul // *iload 2 // push biload 3 // push cineg // uminusimul // *iadd // +istore 1 // store a Postfix notation represents operations on a stack Pro: easy to generate Cons: stack operations are more difficult to optimize

  8. Three-Address Code a := b * -c + b * -c t1 := - ct2 := b * t1t3 := - ct4 := b * t3t5 := t2 + t4a := t5 t1 := - ct2 := b * t1t5 := t2 + t2a := t5 Linearized representationof a syntax tree Linearized representationof a syntax DAG

  9. Three-Address Statements • Assignment statements: x:=y op z, x:=op y • Indexed assignments: x:=y[i], x[i] :=y • Pointer assignments: x:=&y, x:=*y, *x:=y • Copy statements: x:=y • Unconditional jumps: goto lab • Conditional jumps: if x relop y goto lab • Function calls: param x… call p, nreturn y

  10. Syntax-Directed Translation into Three-Address Code ProductionsS id := E | whileEdoSE E+E | E*E | -E | (E ) | id | num Synthesized attributes:S.code three-address code for S S.begin label to start of S or nilS.after label to end of S or nilE.code three-address code for E E.place a name holding the value of E gen(E.place ‘:=’ E1.place ‘+’ E2.place) Code generation t3 := t1 + t2

  11. Syntax-Directed Translation into Three-Address Code (cont’d) ProductionsS id := E S whileEdoS1E E1+E2E E1*E2E -E1E (E1 )E id E  num Semantic rules S.code := E.code || gen(id.place ‘:=’ E.place); S.begin := S.after := nil(see next slide) E.place := newtemp();E.code := E1.code || E2.code || gen(E.place ‘:=’ E1.place ‘+’ E2.place)E.place := newtemp();E.code := E1.code || E2.code || gen(E.place ‘:=’ E1.place ‘*’ E2.place) E.place := newtemp();E.code := E1.code || gen(E.place ‘:=’ ‘uminus’ E1.place) E.place := E1.placeE.code := E1.code E.place := id.nameE.code := ‘’E.place := newtemp();E.code := gen(E.place ‘:=’ num.value)

  12. Syntax-Directed Translation into Three-Address Code (cont’d) ProductionS whileEdoS1 S.begin: E.code if E.place=0goto S.after S.code Semantic ruleS.begin := newlabel()S.after := newlabel() S.code := gen(S.begin ‘:’) ||E.code ||gen(‘if’ E.place ‘=‘ ‘0’ ‘goto’ S.after) ||S1.code ||gen(‘goto’ S.begin) ||gen(S.after ‘:’) goto S.begin S.after: …

  13. Example i := 2 * n + kwhile i do i := i - k t1 := 2 t2 := t1 * n t3 := t2 + k i := t3L1: if i = 0 goto L2 t4 := i - k i := t4 goto L1L2:

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