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Intermediate code generation1

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Intermediate code generation1

  1. 1. Intermediate code generation Shashwat Shriparv dwivedishashwat@gmail.com InfinitySoft
  2. 2. Intermediate Code Generation  Translating source program into an “intermediate language.”  Simple  CPU Independent,  …yet, close in spirit to machine language.  Benefits is 1. Retargeting is facilitated 2. Machine independent Code Optimization can be applied.
  3. 3. Intermediate Code Generation  Intermediate codes are machine independent codes, but they are close to machine instructions.  The given program in a source language is converted to an equivalent program in an intermediate language by the intermediate code generator.  Intermediate language can be many different languages, and the designer of the compiler decides this intermediate language.  syntax trees can be used as an intermediate language.  postfix notation can be used as an intermediate language.  three-address code (Quadruples) can be used as an intermediate language  we will use quadruples to discuss intermediate code generation  quadruples are close to machine instructions, but they are not actual machine instructions.
  4. 4. Types of Intermediate Languages  Graphical Representations.  Consider the assignment a:=b*-c+b*-c: assign a + * * uminus uminusb c c b assign a + * uminus b c
  5. 5. Syntax Dir. Definition to produce syntax trees for Assignment Statements. PRODUCTION Semantic Rule S  id := E { S.nptr = mknode (‘assign’, mkleaf(id, id.entry), E.nptr) } E  E1 + E2 {E.nptr = mknode(‘+’, E1.nptr,E2.nptr) } E  E1 * E2 {E.nptr = mknode(‘*’, E1.nptr,E2.nptr) } E  - E1 {E.nptr = mknode(‘uminus’, E1.nptr) } E  ( E1 ) {E.nptr = E1.nptr } E  id {E.nptr = mkleaf(id, id.entry) }
  6. 6. Three Address Code  Statements of general form x:=y op z  No built-up arithmetic expressions are allowed.  As a result, x:=y + z * w should be represented as t1:=z * w t2:=y + t1 x:=t2  Observe that given the syntax-tree or the dag of the graphical representation we can easily derive a three address code for assignments as above.  In fact three-address code is a linearization of the syntax tree.  Three-address code is useful: related to machine-language/ simple/ optimizable. x,y,z- names,constants or compiler-generated temporaries t1 , t2 – compiler generated temporary names
  7. 7. 3 address code for the syntax tree and the dag a:=b*-c+b*-c: assign a + * * uminus uminusb c c b assign a + * uminus b c Syntax tree Dag
  8. 8. 3-address codes are t1:=- c t2:=b * t1 t5:=t2 + t2 a:=t5 t1:=- c t2:=b * t1 t3:=- c t4:=b * t3 t5:=t2 + t4 a:=t5 Syntax tree Dag
  9. 9. Types of Three-Address Statements. Assignment Statement: x:=y op z Assignment Statement: x:=op z Copy Statement: x:=z Unconditional Jump: goto L Conditional Jump: if x relop y goto L Stack Operations: Push/pop More Advanced Procedure: param x1 param x2 … param xn call p,n Index Assignments: x:=y[ i ] x[ i ]:=y Address and Pointer Assignments: x:=&y x:=*y *x:=y Generated as part of call of proc. p(x1,x2,……,xn)
  10. 10. Syntax-Directed Translation into 3-address code.
  11. 11. Syntax-Directed Translation for 3-address code for assignment statements  Use attributes  E.place to hold the name of the “place” that will hold the value of E  Identifier will be assumed to already have the place attribute defined.  For example, the place attribute will be of the form t0, t1, t2, … for identifiers and v0,v1,v2 etc. for the rest.  E.code to hold the three address code statements that evaluate E (this is the `translation’ attribute).  Use function newtemp that returns a new temporary variable that we can use.  Use function gen to generate a single three address statement given the necessary information (variable names and operations).
  12. 12. Syntax-Dir. Definition for 3-address code PRODUCTION Semantic Rule S  id := E { S.code = E.code||gen(id.place ‘=’ E.place ) } E  E1 + E2 {E.place = newtemp ; E.code = E1.code || E2.code || || gen(E.place‘:=’E1.place‘+’E2.place) } E  E1 * E2 {E.place = newtemp ; E.code = E1.code || E2.code || || gen(E.place‘=’E1.place‘*’E2.place) } E  - E1 {E.place = newtemp ; E.code = E1.code || || gen(E.place ‘=’ ‘uminus’ E1.place) } E  ( E1 ) {E.place = E1.place ; E.code = E1.code} E  id {E.place = id.entry ; E.code = ‘’ } e.g. a := b * - (c+d) ‘||’: string concatenation
  13. 13. while statements  E.g. while statements of the form “while E do S” (interpreted as while the value of E is not 0 do S) PRODUCTION S  while E do S1 Semantic Rule S.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 ‘:’) E.code If E.place = 0 goto S.after S1.code Goto S.begin S.begin: S.after: ………………. To mark the 1st stmt. In code for E stmt. following code S
  14. 14. Implementation of 3 address code Quadruples Triples Indirect triples
  15. 15. Quadruples  A quadruple is a record structure with four fields: op, arg1, arg2, and result  The op field contains an internal code for an operator  Statements with unary operators do not use arg2  Operators like param use neither arg2 nor result  The target label for conditional and unconditional jumps are in result  The contents of fields arg1, arg2, and result are typically pointers to symbol table entries
  16. 16. Implementations of 3-address statements  Quadruples t1:=- c t2:=b * t1 t3:=- c t4:=b * t3 t5:=t2 + t4 a:=t5 op arg1 arg2 result (0) uminus c t1 (1) * b t1 t2 (2) uminus c (3) * b t3 t4 (4) + t2 t4 t5 (5) := t5 a a:=b*-c+b*-c:
  17. 17. Triples  Triples refer to a temporary value by the position of the statement that computes it  Statements can be represented by a record with only three fields: op, arg1, and arg2  Avoids the need to enter temporary names into the symbol table  Contents of arg1 and arg2:  Pointer into symbol table (for programmer defined names)  Pointer into triple structure (for temporaries)
  18. 18. Implementations of 3-address statements, II  Triples t1:=- c t2:=b * t1 t3:=- c t4:=b * t3 t5:=t2 + t4 a:=t5 op arg1 arg2 (0) uminus c (1) * b (0) (2) uminus c (3) * b (2) (4) + (1) (3) (5) assign a (4) a:=b*-c+b*-c:
  19. 19. Implementations of 3-address statements, III  Indirect Triples stmt op arg1 arg2 (14) uminus c (15) * b (14) (16) uminus c (17) * b (16) (18) + (15) (17) (19) assign a (18) stmt (0) (14) (1) (15) (2) (16) (3) (17) (4) (18) (5) (19) t1:=- c t2:=b * t1 t3:=- c t4:=b * t3 t5:=t2 + t4 a:=t5 a:=b*-c+b*-c:
  20. 20. DECLARATIONS  Declarations in a procedure  Langs. like C , Pascal allows declarations in single procedure to be processed as a group  A global variable offset keeps track of the next available relative addresses  Before the Ist declaration is considered, the value of offset is set to 0.  When a new name is seen , name is entered in symbol table with current value as offset , offset incre. by width of data object denoted by name.  Procedure enter(name,type,offset) creates symbol table entry for name , gives it type type ,and rel.addr. offset in its data area  Type , width – denotes no. of memory units taken by objects of that type
  21. 21. SDT to generate ICode for Declarations Using a global variable offset PRODUCTION Semantic Rule P  D { } D  D ; D D  id : T { enter (id.name, T.type, offset); offset:=offset + T.width } T  integer {T.type = integer ; T.width = 4; } T  real {T.type = real ; T.width = 8} T  array [ num ] of T1 {T.type=array(1..num.val,T1.type) T.width = num.val * T1.width} T  ^T1 {T.type = pointer(T1.type);T1.width = 4}
  22. 22. Nested Procedure Declarations  For each procedure we should create a symbol table. mktable(previous) – create a new symbol table where previous is the parent symbol table of this new symbol table enter(symtable,name,type,offset) – create a new entry for a variable in the given symbol table. enterproc(symtable,name,newsymbtable) – create a new entry for the procedure in the symbol table of its parent. addwidth(symtable,width) – puts the total width of all entries in the symbol table into the header of that table.  We will have two stacks:  tblptr – to hold the pointers to the symbol tables  offset – to hold the current offsets in the symbol tables in tblptr stack.
  23. 23. SDT to generate ICode for Nested Procedures ( P  M D { addwidth(top(tblptr), top(offset)); pop(tblptr); pop(offset) } M   { t:=mktable(null); push(t, tblptr); push(0, offset)} D  D1 ; D2 ... D  proc id ; N D ; S { t:=top(tblpr); addwidth(t,top(offset)); pop(tblptr); pop(offset); enterproc(top(tblptr), id.name, t)} N   {t:=mktable(top(tblptr)); push(t,tblptr); push(0,offset);} D  id : T {enter(top(tblptr), id.name, T.type, top(offset); top(offset):=top(offset) + T.width Example: proc func1; D; proc func2 D; S; S
  24. 24. SDT to generate ICode for assignment statements  Use attributes  E.place to hold the name of the “place” that will hold the value of E  Identifier will be assumed to already have the place attribute defined.  For example, the place attribute will be of the form t0, t1, t2, … for identifiers and v0,v1,v2 etc. for the rest.  E.code to hold the three address code statements that evaluate E (this is the `translation’ attribute).  Use function newtemp that returns a new temporary variable that we can use.  Use function gen to generate a single three address statement given the necessary information (variable names and operations).
  25. 25. Syntax-Dir. Definition for 3-address code PRODUCTION Semantic Rule S  id := E { S.code = E.code||gen(id.place ‘=’ E.place ) } E  E1 + E2 {E.place = newtemp ; E.code = E1.code || E2.code || || gen(E.place‘:=’E1.place‘+’E2.place) } E  E1 * E2 {E.place = newtemp ; E.code = E1.code || E2.code || || gen(E.place‘=’E1.place‘*’E2.place) } E  - E1 {E.place = newtemp ; E.code = E1.code || || gen(E.place ‘=’ ‘uminus’ E1.place) } E  ( E1 ) {E.place = E1.place ; E.code = E1.code} E  id {E.place = id.entry ; E.code = ‘’ } e.g. a := b * - (c+d)
  26. 26. Boolean Expressions  Boolean expressions has 2 purpose  To compute Boolean values  as a conditional expression for statements  Methods of translating boolean expression: (2 methods to represent the value of Boolean expn)  Numerical methods:  True is represented as 1 and false is represented as 0  Nonzero values are considered true and zero values are considered false  By Flow-of-control :  Represent the value of a boolean by the position reached in a program  Often not necessary to evaluate entire expression
  27. 27. SDT for Numerical Representation for booleans  Expressions evaluated left to right using 1 to denote true and 0 to donate false  Example: a or b and not c t1 := not c t2 := b and t1 t3 := a or t2  Another example: a < b 100: if a < b goto 103 101: t : = 0 102: goto 104 103: t : = 1 104: …
  28. 28. Emit & nextstat  emit fn.– places 3-address stmts into an o/p file in the right format  nextstat fn.– gives the index of the next 3 - address stmt in o/p sequence  E.place to hold the name of the “place” that will hold the value of E
  29. 29. Production Semantic Rules E  E1 or E2 E.place := newtemp; emit(E.place ':=' E1.place 'or' E2.place) E  E1 and E2 E.place := newtemp; emit(E.place ':=' E1.place 'and' E2.place) E  not E1 E.place := newtemp; emit(E.place ':=' 'not' E1.place) E  (E1) E.place := E1.place; SDT for Numerical Representation for booleans
  30. 30. Production Semantic Rules E  id1 relop id2 E.place := newtemp; emit('if' id1.place relop.op id2.place 'goto' nextstat+3); emit(E.place ':=' '0'); emit('goto' nextstat+2); emit(E.place ':=' '1'); E  true E.place := newtemp; emit(E.place ':=' '1') E  false E.place := newtemp; emit(E.place ':=' '0') SDT for Numerical Representation for booleans nextstat fn.– gives the index of the next 3 - address stmt in o/p sequence
  31. 31. Example: a<b or c<d and e<f 100: if a < b goto 103 101: t1 := 0 102: goto 104 103: t1 := 1 104: if c < d goto 107 105: t2 := 0 106: goto 108 107: t2 := 1 108: if e < f goto 111 109: t3 := 0 110: goto 112 111: t3 := 1 112: t4 := t2 and t3 113: t5 := t1 or t4
  32. 32. Flow of control Stmts  S →if E then S1 | if E then S1 else S2|while E do S1  Here E is the boolean expn. to be translated  We assume that 3-address code can be labeled  newlabel returns a symbolic label each time its called.  E is associated with 2 labels 1. E.true – label which controls flow if E is true 2. E.false – label which controls flow if E is false  S.next – is a label that is attached to the first 3 address instruction to be executed after the code for S
  33. 33. 1. Code for if - then E.code S1.code to E.true to E.false ……….. S →If E then S1 Semantic rules E.true := newlabel; E.false := S.next; S1.next := S.next; S.code := E.code || gen(E.true ':') || S1.code E.true: E.false:
  34. 34. 2.Code for if-then-else S  if E then S1 else S2 E.code S1.code to E.true to E.false Semantic rules goto S.next S2.code E.true: E.false: ………..S.next E.true := newlabel; E.false := newlabel; S1.next := S.next; S2.next := S.next; S.code := E.code || gen(E.true ':') || S1.code || gen(‘ goto‘ S.next) || gen ( E.false ‘:’ ) || S2.code
  35. 35. 3. Code for while-do S  while E do S1 E.code S1.code to E.true to E.false Semantic rules S.begin := newlabel; E.true := newlabel; E.false := S.next; S1.next := S.begin; S.code := gen(S.begin ':') || E.code || gen(E.true ':') || S1.code || gen('goto' S.begin) goto S.begin E.true: E.false: ……….. S.begin
  36. 36. Jumping code/Short Circuit code for boolean expression  Boolean Expressions are translated in a sequence of conditional and unconditional jumps to either E.true or E.false.  a < b. The code is of the form: if a < b then goto E.true goto E.false  E1 or E2. If E1 is true then E is true, so E1.true = E.true. Otherwise, E2 must be evaluated, so E1.false is set to the label of the first statement in the code for E2.  E1 and E2. Analogous considerations apply.  not E1. We just interchange the true and false with that for E.
  37. 37. Production Semantic Rules E  E1 or E2 E1.true := E.true; E1.false := newlabel; E2.true := E.true; E2.false := E.false; E.code := E1.code || gen(E1.false ':') || E2.code E  E1 and E2 E1.true := newlabel; E1.false := E.false; E2.true := E.true; E2.false := E.false; E.code := E1.code || gen(E1.true ':') || E2.code Control flow translation of boolean expression We will now see the code produced for the boolean expression E
  38. 38. Production Semantic Rules E  not E1 E1.true := E.false; E1.false := E.true; E.code := E1.code E  (E1) E1.true := E.true; E1.false := E.false; E.code := E1.code E  id1 relop id2 E.code := gen('if' id.place relop.op id2.place 'goto' E.true) || gen('goto' E.false) E  true E.code := gen('goto' E.true) E  false E.code := gen('goto' E.false)
  39. 39. Example while a < b do if c < d then x := y + z else x := y - z
  40. 40. Example Lbegin: if a < b goto L1 goto Lnext L1: if c < d goto L2 goto L3 L2: t1 := y + z x := t1 goto Lbegin L3: t2 := y - z x := t2 goto Lbegin Lnext: while a < b do if c < d then x := y + z else x := y - z
  41. 41. Case Statements  Switch <expression> begin case value : statement case value : statement …….. case value : statement default : statement end
  42. 42. Translation of a case stmt code to evaluate E into t goto test L1: code for S1 goto next … Ln-1: code for Sn-1 goto next Ln: code for Sn goto next test: if t = V1 goto L1 … if t = Vn-1 goto Ln-1 goto Ln next:
  43. 43. Backpatching  Easiest way to implement Syntax directed defn. is to use 2 passes  First, construct syntax tree  Walk through syntax tree in depth-first order, computing translations  May not know the labels to which control must flow at the time a jump is generated  Affect boolean expressions and flow control statements  Leave targets of jumps temporarily unspecified  Add each such statement to a list of goto statements whose labels will be filled in later  This filling in of labels is called back patching How backpatching is implemented in 1 pass….?
  44. 44. Lists of Labels  Imagine that we are generating quadruples into a quadruple array.  Labels are indices into this array  To manipulate this list of labels we use 3 fns.  makelist(i)  Creates a new list containing only i, and index into the array of quadruples  Returns a pointer to the new list  merge(p1, p2)  Concatenates two lists of labels  Returns a pointer to the new list  backpatch(p, i) – inserts i as target label for each statement on the list pointed to by p
  45. 45. Boolean Expressions and Markers E  E1 or M E2 | E1 and M E2 | not E1 | (E1) | id1 relop id2 | true | false M  ε
  46. 46. The New Marker , M  Translation scheme suitable for producing quadruples during bottom-up pass  The new marker has an associated semantic action which Picks up, at appropriate times, the index of the next quadruple to be generated  M.quad := nextquad  Nonterminal E will have two new synthesized attributes:  E.truelist contains a list of statements that jump when expression is true  E.falselist contains a list of statements that jump when expression is false
  47. 47. Example: E  E1 and M E2  If E1 is false:  Then E is also false  So statements on E1.falselist become part of E.falselist  If E1 is true:  Still need to test E2  Target for statements on E1.truelist must be the beginning of code generated for E2  Target is obtained using the marker M
  48. 48. New Syntax-Directed Definition (1) Production Semantic Rules E  E1 or M E2 backpatch(E1.falselist, M.quad); E.truelist := merge(E1.truelist, E2.truelist); E.falselist := E2.falstlist E  E1 and M E2 backpatch(E1.truelist, M.quad); E.truelist := E2.truelist; E.falselist := merge(E1.falselist, E2.falselist) E  not E1 E.truelist := E1.falselist; E.falselist := E1.truelist E  (E1) E.truelist := E1.truelist; E.falselist := E1.falselist
  49. 49. New Syntax-Directed Definition (2) Production Semantic Rules E  id1 relop id2 E.truelist := makelist(nextquad); E.falselist := makelist(nextquad+1); emit('if' id1.place relop.op id2.place 'goto _'); emit('goto _') E  true E.truelist := makelist(nextquad); emit('goto _') E  false E.falselist := makelist(nextquad); emit('goto _') M  ε M.quad := nextquad
  50. 50. Example Revisited (1)  Reconsider: a<b or c<d and e<f  First, a<b will be reduced, generating: 100: if a < b goto _ 101: goto _  Next, the marker M in E  E1 or M E2 will be reduced, and M.quad will be set to 102  Next, c<d will be reduced, generating: 102: if c < d goto _ 103: goto _
  51. 51. Example Revisited (2)  Next, the marker M in E  E1 and M E2 will be reduced, and M.quad will be set to 104  Next, e<f will be reduced, generating: 104: if e < f goto _ 105: goto _  Next, we reduce by E  E1 and M E2  Semantic action calls backpatch({102}, 104)  E1.truelist contains only 102  Line 102 now reads: if c <d goto 104
  52. 52. Example Revisited (3)  Next, we reduce by E  E1 or M E2  Semantic action calls backpatch({101}, 102)  E1.falselist contains only 101  Line 101 now reads: goto 102  Statements generated so far: 100: if a < b goto _ 101: goto 102 102: if c < d goto 104 103: goto _ 104: if e < f goto _ 105: goto _  Remaining goto instructions will have their addresses filled in later
  53. 53. Annotated Parse Tree
  54. 54. Procedure Calls  Grammar S -> call id ( Elist ) Elist -> Elist , E Elist -> E  Semantic Actions 1. S -> call id (Elist) for each item p in queue do { gen(‘param’ p) gen(‘call’ id.place) }
  55. 55. 2. Elist -> Elist , E {append E.place to the end of queue} 3. Elist -> E { initialize queue to contain only E.place} e.g. P (x1,x2,x3,…………….xn) param x1 param x2 …………. …………. param xn call P
  56. 56. Shashwat Shriparv dwivedishashwat@gmail.com InfinitySoft

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